小林 達夫(コバヤシ タツオ) |

理学研究院 物理学部門 非線形物理学分野 |

教授 |

Last Updated :2023/11/02

- 超対称性 超弦理論 素粒子論 質量行列 素粒子(理論) フレーバー対称性 超対称性の破れ 湯川結合 フレーバー 素 弦理論 余剰次元 素粒子の質量 くり込み群 標準模型 FCNC 超対称の破れ モジュライ ニュートリノ 超重力理論 大統一理論 世代構造 E_6大統一理論 オービフォルド 超弦模型 ニュートリノ振動 暗黒物質 超対称弦 レプジェネシス Leptogenesis

- Yoshihiko Abe, Tetsutaro Higaki, Fumiya Kaneko, Tatsuo Kobayashi, Hajime OtsukaJournal of High Energy Physics 2023 6 2023年07月We study slow-roll inflation model controlled by the modular flavor symmetry. In the model, the modulus field plays a role of inflaton and the introduction of the stabilizer field coupled to a modular form in the superpotential produces the inflaton potential. In order to generate the flat direction for the slow-roll inflation, we consider the Kähler potential corrected by the modular form. It is noted that the modulus field perpendicular to the inflaton direction is stabilized during the inflation. The model turns out to be consistent with the current observations and behaves similarly to the α-attractor models in some parameter spaces. The inflaton rolls down to the CP-symmetric vacuum at the end of inflation.
- Yoshihiko Abe, Tetsutaro Higaki, Fumiya Kaneko, Tatsuo Kobayashi, Hajime OtsukaJournal of High Energy Physics 2023 6 2023年06月27日Abstract We study slow-roll inflation model controlled by the modular flavor symmetry. In the model, the modulus field plays a role of inflaton and the introduction of the stabilizer field coupled to a modular form in the superpotential produces the inflaton potential. In order to generate the flat direction for the slow-roll inflation, we consider the Kähler potential corrected by the modular form. It is noted that the modulus field perpendicular to the inflaton direction is stabilized during the inflation. The model turns out to be consistent with the current observations and behaves similarly to the α-attractor models in some parameter spaces. The inflaton rolls down to the CP-symmetric vacuum at the end of inflation.
- Yoshihiko Abe, Tetsutaro Higaki, Fumiya Kaneko, Tatsuo Kobayashi, Hajime OtsukaJOURNAL OF HIGH ENERGY PHYSICS 6 2023年06月We study slow-roll inflation model controlled by the modular flavor symmetry. In the model, the modulus field plays a role of inflaton and the introduction of the stabilizer field coupled to a modular form in the superpotential produces the inflaton potential. In order to generate the flat direction for the slow-roll inflation, we consider the Kahler potential corrected by the modular form. It is noted that the modulus field perpendicular to the inflaton direction is stabilized during the inflation. The model turns out to be consistent with the current observations and behaves similarly to the & alpha;-attractor models in some parameter spaces. The inflaton rolls down to the CP-symmetric vacuum at the end of inflation.
- Tatsuo Kobayashi, Hajime Otsuka, Makoto Sakamoto, Maki Takeuchi, Yoshiyuki Tatsuta, Hikaru UchidaPHYSICAL REVIEW D 107 7 2023年04月We investigate blow-up manifolds of T2=7LN(N = 2; 3; 4; 6) orbifolds with magnetic flux. First, we construct the blow-up manifolds and zero-mode wave functions on them more precisely. In particular, through an appropriate singular gauge transformation, winding numbers of wave functions on T2=7LN can be replaced with localized curvature and localized flux at orbifold fixed points. In addition, since the blow-up manifolds have no singularities, we apply the Atiyah-Singer index theorem to them; the chiral zero-mode number is given by the total magnetic flux. It can be also applied for T2=7LN orbifolds through the blow-up process, and then we find that it is consistent with the zero-mode counting formula in M. Sakamoto et al. [Zero-mode counting formula and zeros in orbifold compactifications, Phys. Rev. D 102, 025008 (2020).]. Furthermore, the Atiyah-Singer index theorem shows that an additional degree of freedom of localized flux gives new chiral zero modes. We study their wave functions and then we find that they correspond to localized modes at the orbifold singular points. We also calculate their Yukawa couplings.
- Tatsuo Kobayashi, Satsuki Nishimura, Hajime Otsuka, Morimitsu Tanimoto, Kei YamamotoPROGRESS OF THEORETICAL AND EXPERIMENTAL PHYSICS 2023 4 2023年04月We classify a supersymmetric extension of the Standard Model by discrete symmetries originating from finite modular symmetries Gamma(N). Since all the couplings in supersymmetric theories of finite modular symmetries Gamma(N) are described by holomorphic modular forms with even modular weights, renormalizable and non-renormalizable operators such as baryon- and/or lepton-number violating operators are severely constrained. From the modular transformation of matter multiplets with modular weight 1/M, we find Z(2M) symmetries, including the generalized baryon and lepton parities, R-parity, Z(3) baryon triality and Z(6) proton hexality. Such Z(2M) symmetries are enlarged to Z(2M) (sic) Z(2)(CP) symmetries together with the CP transformation.
- Shota Kikuchi, Tatsuo Kobayashi, Hajime Otsuka, Morimitsu Tanimoto, Hikaru Uchida, Kei YamamotoPhysical Review D 2022年08月01日We study a scenario to derive four-dimensional modular flavor symmetric models from higher dimensional theory by assuming the compactification consistent with the modular symmetry. In our scenario, wavefunctions in extra dimensional compact space are modular forms. That leads to constraints on combinations between modular weights and $\Gamma_N$ ($\Gamma_N')$ representations of matter fields. We also present illustrating examples.
- Tatsuo Kobayashi, Hajime Otsuka, Morimitsu Tanimoto, Kei YamamotoJournal of High Energy Physics 2022年08月We study the lepton flavor violation (LFV), the leptonic magnetic moments $(g-2)_{\mu,\,e} $ and the electric dipole moment (EDM) of the electron in the Standard-Model Effective Field Theory with the $\Gamma_N$ modular flavor symmetry. We employ the stringy Ansatz on coupling structure that 4-point couplings of matter fields are written by a product of 3-point couplings of matter fields. We take the level 3 finite modular group, $\Gamma_3$ for the flavor symmetry, and discuss the dipole operators at nearby fixed point $\tau=i$, where observed lepton masses and mixing angles are well reproduced. Suppose the anomaly of the anomalous magnetic moment of the muon to be evidence of the new physics (NP), we have related it with $(g-2)_{e}$, LFV decays, and the electron EDM. It is found that the NP contribution to $(g-2)_{e}$ is proportional to the lepton masses squared likewise the naive scaling. We also discuss the correlations among the LFV processes $\mu\to e\gamma$, $\tau\to \mu\gamma$ and $\tau\to e\gamma$, which are testable in the future. The electron EDM requires the tiny imaginary part of the relevant Wilson coefficient in the basis of real positive charged lepton masses, which is related to the $\mu\to e\gamma$ transition in our framework.
- Tatsuo Kobayashi, Hajime Otsuka, Morimitsu Tanimoto, Kei YamamotoPhysical Review D 105 5 2022年03月01日We study the modular symmetric standard model effective field theory. We employ the stringy Ansatz on coupling structure that 4-point couplings y(4) of matter fields are written by a product of 3-point couplings y(3) of matter fields, i.e., y(4)=y(3)y(3). In this framework, we discuss the flavor structure of bilinear fermion operators and 4-fermion operators, where the holomorphic and antiholomorphic modular forms appear. From the Ansatz, the A4 modular-invariant semileptonic four-fermion operator [ĒRΓER][D¯RΓDR] does not lead to the flavor changing (FC) processes since this operator would be constructed in terms of gauge couplings g as y(3)∼g. The chirality flipped bilinear operator [D¯RΓDL] also does not lead FC if the mediated mode corresponds to the Higgs boson Hd. In this case, the flavor structure of this operator is the exactly same as the mass matrix. On the other hand, if the flavor structure of the operator is not the exactly same as the mass matrix, the situation would change drastically. Then, we obtain the nontrivial relations of FC transitions at nearby fixed points τ=i,ω,i∞, which are testable in the future. As an application, we discuss the relations of the lepton flavor violation processes μ→eγ,τ→μγ and τ→eγ at nearby τe=i, where the successful lepton mass matrix was obtained. We also study the flavor changing 4-quark operators in the A4 modular symmetry of quarks. As a result, the minimal flavor violation could be realized by taking relevant specific parameter sets of order one.
- Keiya Ishiguro, Tatsuo Kobayashi, Hajime OtsukaJournal of High Energy Physics 2022 1 2022年01月We examine a common origin of four-dimensional flavor, CP, and U(1)R symmetries in the context of heterotic string theory with standard embedding. We find that flavor and U(1)R symmetries are unified into the Sp(2h + 2, ℂ) modular symmetries of Calabi-Yau threefolds with h being the number of moduli fields. Together with the ℤ2CP CP symmetry, they are enhanced to GSp(2h + 2, ℂ) ≃ Sp(2h + 2, ℂ) ⋊ ℤ2CP generalized symplectic modular symmetry. We exemplify the S3, S4, T′, S9 non-Abelian flavor symmetries on explicit toroidal orbifolds with and without resolutions and ℤ2, S4 flavor symmetries on three-parameter examples of Calabi-Yau threefolds. Thus, non-trivial flavor symmetries appear in not only the exact orbifold limit but also a certain class of Calabi-Yau three-folds. These flavor symmetries are further enlarged to non-Abelian discrete groups by the CP symmetry.
- Keiya Ishiguro, Tatsuo Kobayashi, Hajime OtsukaNuclear Physics B 973 2021年12月We explore the geometrical origin of CP and the spontaneous CP violation in Calabi-Yau compactifications. We find that the CP symmetry is identified with an outer automorphism of the symplectic modular group in the large complex structure regime of Calabi-Yau threefolds, thereby enlarging the symplectic modular group to their semidirect product group. The spontaneous CP violation is realized by the introduction of fluxes, whose effective action is invariant under CP as well as the discrete Z2 symmetry or Z4 R-symmetry. We explicitly demonstrate the spontaneous CP violation on a specific Calabi-Yau threefold.
- Keiya Ishiguro, Tatsuo Kobayashi, Hajime OtsukaJournal of High Energy Physics 2021 7 2021年07月We study the impacts of matter field Kähler metric on physical Yukawa couplings in string compactifications. Since the Kähler metric is non-trivial in general, the kinetic mixing of matter fields opens a new avenue for realizing a hierarchical structure of physical Yukawa couplings, even when holomorphic Yukawa couplings have the trivial structure. The hierarchical Yukawa couplings are demonstrated by couplings of pure untwisted modes on toroidal orbifolds and their resolutions in the context of heterotic string theory with standard embedding. Also, we study the hierarchical couplings among untwisted and twisted modes on resolved orbifolds.
- Keiya Ishiguro, Tatsuo Kobayashi, Hajime OtsukaJournal of High Energy Physics 2021 3 2021年03月We study the moduli stabilization from the viewpoint of modular flavor symmetries. We systematically analyze stabilized moduli values in possible configurations of flux compactifications, investigating probabilities of moduli values and showing which moduli values are favorable from our moduli stabilization. Then, we examine their implications on modular symmetric flavor models. It is found that distributions of complex structure modulus τ determining the flavor structure are clustered at a fixed point with the residual ℤ3 symmetry in the SL(2, ℤ) fundamental region. Also, they are clustered at other specific points such as intersecting points between |τ|2 = k/2 and Re τ = 0,±1/4,±1/2, although their probabilities are less than the ℤ3 fixed point. In general, CP-breaking vacua in the complex structure modulus are statistically disfavored in the string landscape. Among CP-breaking vacua, the values Re τ = ±1/4 are most favorable in particular when the axio-dilaton S is stabilized at Re S = ±1/4. That shows a strong correlation between CP phases originated from string moduli.
- Shota Kikuchi, Tatsuo Kobayashi, Hajime Otsuka, Shintaro Takada, Hikaru UchidaJournal of High Energy Physics 2020 11 2020年11月We study the modular symmetry of zero-modes on T12×T22 and orbifold compactifications with magnetic fluxes, M1, M2, where modulus parameters are identified. This identification breaks the modular symmetry of T12×T22, SL(2, ℤ)1× SL(2, ℤ)2 to SL(2, ℤ) ≡ Γ. Each of the wavefunctions on T12×T22 and orbifolds behaves as the modular forms of weight 1 for the principal congruence subgroup Γ(N), N being 2 times the least common multiple of M1 and M2. Then, zero-modes transform each other under the modular symmetry as multiplets of double covering groups of ΓN such as the double cover of S4.
- Tatsuo Kobayashi, Hajime OtsukaPhysics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics 807 2020年08月10日We explore the scenario where both the strong CP and Cabbibo-Kobayashi-Maskawa (CKM) phases are determined by the same axion field. Such a scenario is naturally realized in string compactifications. We find that there exists parameter region to realize the tiny strong CP phase and observed CKM phase in magnetized D-brane models.
- Hiroyuki Abe, Tatsuo Kobayashi, Shohei Uemura, Junji YamamotoPhysical Review D 102 4 2020年08月05日
- Tatsuo Kobayashi, Hajime OtsukaPhysical Review D 102 2 2020年07月15日We systematically investigate the possibilities of realizing the spontaneous CP violation in Type IIB flux compactifications on toroidal orientifolds. Our detailed analysis for a generic choice of three-form fluxes leads to the presence of flat directions, where CP-breaking and -conserving vacua are degenerate, indicating that flux compactifications are not sufficient to realize the spontaneous CP violation. Furthermore, the four-dimensional CP can be embedded into the duality symmetries, namely modular symmetries for a particular choice of fluxes.
- Tatsuo Kobayashi, Hajime OtsukaPhysical Review D 101 10 2020年05月15日We classify discrete modular symmetries in the effective action of Type IIB string on toroidal orientifolds with three-form fluxes, emphasizing on T6/Z2 and T6/(Z2×Z2′) orientifold backgrounds. In the three-form flux background, the modular group is spontaneously broken down to its congruence subgroup whose pattern is severely constrained by a quantization of fluxes and tadpole cancellation conditions. We explicitly demonstrate that the congruence subgroups appearing in the effective action arise on magnetized D-branes wrapping certain cycles of tori.
- Tatsuo Kobayashi, Hajime Otsuka, Hikaru UchidaJournal of High Energy Physics 2020 3 2020年03月01日We discuss the flavor structure in resolution of magnetized T2/ℤ2 orbifold models. The flavor structure of Yukawa couplings among chiral zero-modes is sensitive to radii of the resolved fixed points of T2/ℤ2 orbifold. We show in concrete models that the realistic mass hierarchy and mixing angles of the quark sector can be realized at certain values of blow-up radii, where the number of parameters in the blow-up model can be smaller than toroidal orbifold one.
- Tatsuo Kobayashi, Osamu Seto, Shintaro Takada, Takuya H. Tatsuishi, Shohei Uemura, Junji YamamotoProgress of Theoretical and Experimental Physics 2019 9 2019年09月26日© 2019 The Author(s). We study Kähler moduli stabilization in type IIB superstring theory. We propose a new moduli stabilization mechanism by the supersymmetry breaking chiral superfield which is coupled to Kähler moduli in the Kähler potential. We also study its uplifting of the Large Volume Scenario (LVS). In both cases, the form of the superpotential is crucial for moduli stabilization. We confirm that our uplifting mechanism does not destabilize the vacuum of the LVS drastically.
- Hiroyuki Abe, Tatsuo Kobayashi, Shohei Uemura, Junji YamamotoPhysical Review D 100 6 065020-1 - 065020-10 2019年09月 [査読有り][通常論文]
- Tatsuo Kobayashi, Hajime Otsuka, Hikaru UchidaJournal of High Energy Physics 2019 8 2019年08月01日We propose matter wavefunctions on resolutions of T2/ℤN singularities with constant magnetic fluxes. In the blow-down limit, the obtained wavefunctions of chiral zero-modes result in those on the magnetized T2/ℤN orbifold models, but the wavefunctions of ℤN -invariant zero-modes receive the blow-up effects around fixed points of T2/ℤN orbifolds. Such blow-up effects change the selection rules and Yukawa couplings among the chiral zero-modes as well as the modular symmetry, in contrast to those on the magnetized T2/ℤN orbifold models.
- Masaki Honda, Tatsuo Kobayashi, Hajime OtsukaPHYSICAL REVIEW D 100 2 2018年12月08日We study a four-dimensional low-energy effective field theory derived from extra dimensional field theories with general gauge backgrounds. We find that products among fermionic zero-modes and lightest scalar modes are expanded by other fermionic zero-modes and lightest scalar modes. Using this aspect, we show that higher-order couplings among the fermionic zero-modes and the lightest scalar mode can be decomposed into three-point couplings. This selection rule originating from a structure of the Dirac-type operators indicates the operator product expansion the underlying conformal field theory.
- Hiroyuki Abe, Tatsuo Kobayashi, Shintaro Takada, Shio Tamba, Takuya H. TatsuishiPhysical Review D 98 106017 106017-1 - 106017-8 2018年11月 [査読有り][通常論文]
- 上村 尚平, 小林 達夫日本物理学会講演概要集 71 40 - 40 一般社団法人日本物理学会 2016年
- 秋田 謙介, 小林 達夫, 及川 茜, 大塚 啓日本物理学会講演概要集 71 32 - 32 一般社団法人 日本物理学会 2016年
私たちは超弦理論の有効理論に基づき、グラビティーノ問題についての研究を行った。 まず、II型超弦理論の枠組みで超対称性を破らない軽いモジュライ場が存在するモデルの構築が可能なことを示す。 IIB型超弦理論では、軽い複素構造モジュライ場が存在する。初期宇宙において、軽い複素構造モジュライ場の崩壊によって、エントロピーが多く生成され、グラビティーノの量が薄まるため、グラビティーノ問題が回避されることを説明する。上記の内容は、arXiv1603.08399[hep-ph]に基づく。

- Hironori Hattori, Tatsuo Kobayashi, Naoya Omoto, Osamu SetoPhysical Review D - Particles, Fields, Gravitation and Cosmology 92 2 2015年07月13日 [査読有り][通常論文]

Axions in the Peccei-Quinn (PQ) mechanism provide a promising solution to the strong CP problem in the standard model of particle physics. Coherently generated PQ scalar fields could dominate the energy density in the early Universe and decay into relativistic axions, which would conflict with the current dark radiation constraints. We study the possibility that a thermal inflation driven by a U(1) gauged Higgs field dilutes such axions. A well-motivated extra gauged U(1) would be the local B-L symmetry. We also discuss the implication for the case of U(1)B-L and an available baryogenesis mechanism in such cosmology. - Hiroyuki Abe, Tatsuo Kobayashi, Yoshiyuki Tatsuta, Shohei UemuraPhysical Review D - Particles, Fields, Gravitation and Cosmology 92 2 2015年07月06日 [査読有り][通常論文]

We study the μ-term matrix of Higgs pairs induced by the D-brane instanton effects in intersecting D6-brane models compactified on T6. It is found that the μ-term matrix has a certain permutation symmetry and its eigenvalues have large hierarchical structure without fine-tuning. - Hironori Hattori, Tatsuo Kobayashi, Naoya Omoto, Osamu SetoProceedings of Science 22-29-July-2015 2015年The Peccei-Quinn (PQ) mechanism provide a promising solution to the strong CP problem in the standard model of particle physics. Axion appears as its pseudo-Nambu-Goldstone boson of the PQ symetry breaking. Coherently generated PQ scalar fields could dominate the energy density in the early Universe and decay into relativistic axions, which would conflict with the current dark radiation constraints. We show that a thermal inflation driven by a U(1) gauged Higgs field dilutes such axions. We discuss an available baryogenesis mechanism for the U(1)B-L gauge symmetry.
- Hironori Hattori, Tatsuo Kobayashi, Naoya Omoto, Osamu SetoProceedings of the 11th Patras Workshop on Axions, WIMPs and WISPs, PATRAS 2015 228 - 231 2015年Axions in the Peccei-Quinn (PQ) mechanism provide a promising solution to the strong CP problem in the standard model of particle physics. Coherently generated PQ scalar fields could dominate the energy density in the early Universe and decay into relativistic axions, which would conflict with the current dark radiation constraints. We show that a thermal inflation driven by a U(1) gauged Higgs field dilutes such axions. We discuss an available baryogenesis mechanism for the U(1)B-L gauge symmetry.
- 上村 尚平, 小林 達夫, 龍田 佳幸日本物理学会講演概要集 70 37 - 37 一般社団法人日本物理学会 2015年
- 上村 尚平, 安倍 博之, 小林 達夫, 龍田 佳幸日本物理学会講演概要集 70 9 - 9 一般社団法人日本物理学会 2015年
- Hiroyuki Abe, Tatsuo Kobayashi, Hiroshi Ohki, Keigo Sumita, Yoshiyuki TatsutaJOURNAL OF HIGH ENERGY PHYSICS 1406 6 2014年06月 [査読有り][通常論文]

We study discrete flavor symmetries of the models based on a ten-dimensional supersymmetric Yang-Mills (10D SYM) theory compactified on magnetized tori. We assume non-vanishing non-factorizable fluxes as well as the orbifold projections. These setups allow model-building with more various flavor structures. Indeed, we show that there exist various classes of non-Abelian discrete flavor symmetries. In particular, we find that S-3 flavor symmetries can be realized in the framework of the magnetized 10D SYM theory for the first time. - Tatsuo Kobayashi, Osamu SetoPhysical Review D - Particles, Fields, Gravitation and Cosmology 89 10 2014年05月21日 [査読有り][通常論文]

Large field inflation models are favored by the recent BICEP2 that has detected gravitational wave modes generated during inflation. We study general large field inflation models for which the potential contains (constant) quadratic and quartic terms of inflaton field. We show, in this framework, those inflation models can generate the fluctuation with the tensor-to-scalar ratio of 0.2 as well as the scalar spectral index of 0.96 those are very close to the center value of the tensor-to-scalar ratio reported by BICEP2 as well as Planck. Finally, we briefly discuss the particle physics model building of inflation. © 2014 American Physical Society. - Tatsuo Kobayashi, Osamu SetoPHYSICAL REVIEW D 89 10 2014年05月 [査読有り][通常論文]

Large field inflation models are favored by the recent BICEP2 that has detected gravitational wave modes generated during inflation. We study general large field inflation models for which the potential contains (constant) quadratic and quartic terms of inflaton field. We show, in this framework, those inflation models can generate the fluctuation with the tensor-to-scalar ratio of 0.2 as well as the scalar spectral index of 0.96; those are very close to the center value of the tensor-to- scalar ratio reported by BICEP2 as well as Planck. Finally, we briefly discuss the particle physics model building of inflation. - Yuta Hamada, Tatsuo Kobayashi, Shohei UemuraJOURNAL OF HIGH ENERGY PHYSICS 1405 5 2014年05月 [査読有り][通常論文]

We study the flavor structure in intersecting D-brane models. We study anomalies of the discrete flavor symmetries. We analyze the Majorana neutrino masses, which can be generated by D-brane instanton effects. It is found that a certain pattern of mass matrix is obtained and the cyclic permutation symmetry remains unbroken. As a result, trimaximal mixing matrix can be realized if Dirac neutrino mass and charged lepton mass matrices are diagonal. - Hiroyuki Abe, Tatsuo Kobayashi, Hiroshi Ohki, Keigo Sumita, Yoshiyuki TatsutaJOURNAL OF HIGH ENERGY PHYSICS 1404 4 2014年04月 [査読有り][通常論文]

We study the flavor landscape of the visible sector of particle physics models based on a ten-dimensional super Yang-Mills theory compactified on magnetized tori preserving four-dimensional Nu = 1 supersymmetry. Recently, we constructed a semirealistic model which contains the minimal supersymmetric standard model (MSSM) using an ansatz of magnetic fluxes and orbifolding projections. However, we can consider more various configurations of magnetic fluxes and orbifolding projections preserving fourdimensional N = 1 supersymmetry. We research systematically such possibilities for leading to MSSM-like models and study their phenomenological aspects. - Nana G. Cabo Bizet, Tatsuo Kobayashi, Damian K. Mayorga Pena, Susha L. Parameswaran, Matthias Schmitz, Ivonne ZavalaJOURNAL OF HIGH ENERGY PHYSICS 1402 2 2014年02月 [査読有り][通常論文]

We study discrete R-symmetries, which appear in the 4D low energy effective field theory derived from heterotic orbifold models. We derive the R-symmetries directly from the geometrical symmetries of the orbifolds. In particular, we obtain the corresponding R-charges by requiring that the couplings be invariant under these symmetries. This allows for a more general treatment than the explicit computations of correlation functions made previously by the authors, including models with discrete Wilson lines, and orbifold symmetries beyond plane-by-plane rotational invariance. The R-charges obtained in this manner differ from those derived in earlier explicit computations. We study the anomalies associated with these R-symmetries, and comment on the results. - Takashi Hiramatsu, Minoru Eto, Kohei Kamada, Tatsuo Kobayashi, Yutaka OokouchiJOURNAL OF HIGH ENERGY PHYSICS 1401 1 2014年01月 [査読有り][通常論文]

The breaking of U(1) R symmetry plays a crucial role in modeling the breaking of supersymmetry (SUSY). In the models that possess both SUSY preserving and SUSY breaking vacua, tube-like cosmic strings called R-tubes, whose surfaces are constituted by domain walls interpolating a false and a true vacuum with some winding numbers, can exist. Their (in) stability can strongly constrain SUSY breaking models theirselves. In the present study, we investigate the dynamical (in) stability of two colliding metastable tube-like strings by field-theoretic simulations. From them, we find that the strings become unstable, depending on the relative collision angle and speed of two strings, and the false vacuum is eventually filled out by the true vacuum owing to rapid expansion of the strings or unstable bubbles created as remnants of the collision. - Tomo-hiro Abe, Yukihiro Fujimoto, Tatsuo Kobayashi, Takashi Miura, Kenji Nishiwaki, Makoto SakamotoJOURNAL OF HIGH ENERGY PHYSICS 1 2014年01月 [査読有り][通常論文]

We propose new backgrounds of extra dimensions to lead to four-dimensional chiral models with three generations of matter fermions, that is T (2) /Z (N) twisted orbifolds with magnetic fluxes. We consider gauge theory on six-dimensional space-time, which contains the T (2) /Z (N) orbifold with magnetic flux, Scherk-Schwarz phases and Wilson line phases. We classify all the possible Scherk-Schwarz and Wilson line phases on T (2) /Z (N) orbifolds with magnetic fluxes. The behavior of zero modes is studied. We derive the number of zero modes for each eigenvalue of the Z (N) twist, showing explicitly examples of wave functions. We also investigate Kaluza-Klein mode functions and mass spectra. - Florian Beye, Tatsuo Kobayashi, Shogo KuwakinoJOURNAL OF HIGH ENERGY PHYSICS 1 2014年01月 [査読有り][通常論文]

Using Z(3) asymmetric orbifolds in heterotic string theory, we construct N = 1 SUSY three-generation models with the standard model gauge group SU(3)(C) x SU(2)(L) x U(1)(Y) and the left-right symmetric group SU(3)(C) x SU(2)(L) x SU(2)(R) x U(1)(B-L). One of the models possesses a gauge flavor symmetry for the Z(3) twisted matter. - Yuta Hamada, Kohei Kamada, Tatsuo Kobayashi, Yutaka OokouchiJOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS 1 2014年01月 [査読有り][通常論文]

We study the spontaneous R-symmetry breaking model and investigate the cosmological constraints on this model due to the pseudo Nambu-Goldstone boson, R-axion. We consider the R-axion which has relatively heavy mass in order to complement our previous work. In this regime, model parameters, R-axions mass and R-symmetry breaking scale, are constrained by Big Bang Nucleosynthesis and overproduction of the gravitino produced from R-axion decay and thermal plasma. We find that the allowed parameter space is very small for high reheating temperature. For low reheating temperature, the U(1)(R) breaking scale f(a) is constrained as f(a) < 10(12-14) GeV regardless of the value of R-axion mass. - Yuta Hamada, Tatsuo Kobayashi, Atsushi Ogasahara, Yuji Omura, Fumihiro Takayama, Daiki YasuharaJournal of High Energy Physics 2014 10 2014年 [査読有り][通常論文]

Abstract: We revisit the discrete dark matter model with the A< inf> 4< /inf> flavor symmetry originally introduced by M.Hirsch et.al. We show that radiative corrections can lead to non-zero θ< inf> 13< /inf> and the non-zero mass for the lightest neutrino. We find an interesting relation among neutrino mixing parameters and it indicates the sizable deviation of s< inf> 23< /inf> from the maximal angle s< inf> 23< /inf> < sup> 2< /sup> = 1/2 and the degenerate mass spectrum for neutrinos. Also we study the possibilities that the right-handed neutrino is a dark matter candidate. Assuming that the thermal freeze-out explains observed dark matter abundance, TeV-scale right-handed neutrino and flavored scalar bosons are required. In such a case, the flavor symmetry plays an important role for the suppression of lepton flavor violating processes as well as for the stability of dark matter. We show that this scenario is viable within currently existing constraints from collider, low energy experiments and cosmological observations. - Takeshi Araki, Hiroyuki Ishida, Hajime Ishimori, Tatsuo Kobayashi, Atsushi OgasaharaPHYSICAL REVIEW D 88 9 2013年11月 [査読有り][通常論文]

Following the way proposed recently by Hernandez and Smirnov, we seek possible residual symmetries in the quark sector with a focus on the von Dyck groups. We begin with two extreme cases in which both theta(13) and theta(23) or only theta(13) are set to zero. Then, cases where all the Cabibbo-Kobayashi-Maskawa parameters are allowed to take nonzero values are explored. The Z(7) symmetry is favorable to realize only the Cabibbo angle. On the other hand, larger groups are necessary in order to be consistent with all the mixing parameters. Possibilities of embedding the obtained residual symmetries into the Delta(6N(2)) series are also briefly discussed. - Florian Beye, Tatsuo Kobayashi, Shogo KuwakinoNUCLEAR PHYSICS B 875 3 599 - 620 2013年10月 [査読有り][通常論文]

We study heterotic asymmetric orbifold models. By utilizing the lattice engineering technique, we classify (22,6)-dimensional Narain lattices with right-moving non-Abelian group factors which can be starting points for Z(3) asymmetric orbifold construction. We also calculate gauge symmetry breaking patterns. (C) 2013 Elsevier B.V. All rights reserved. - Nana G. Cabo Bizet, Tatsuo Kobayashi, Damian K. Mayorga Pena, Susha L. Parameswaran, Matthias Schmitz, Ivonne ZavalaJOURNAL OF HIGH ENERGY PHYSICS 5 2013年05月 [査読有り][通常論文]

We consider the string theory origin of R-charge conservation laws in heterotic orbifold compactifications, deriving the corresponding string coupling selection rule for factorizable and non-factorizable orbifolds, with prime ordered and non-prime ordered point groups. R-charge conservation arises due to symmetries among the worldsheet instantons that can mediate the couplings. Among our results is a previously missed non-trivial contribution to the conserved R-charges from the gamma-phases in non-prime orbifolds, which weakens the R-charge selection rule. Symmetries among the worldsheet instantons can also lead to additional selection rules for some couplings. We make a similar analysis for Rule 4 or the "torus lattice selection rule". Moreover, we identify a new string selection rule, that we call Rule 6 or the "coset vector selection rule". - Hiroyuki Abe, Tatsuo Kobayashi, Hiroshi Ohki, Akane Oikawa, Keigo SumitaNUCLEAR PHYSICS B 870 1 30 - 54 2013年05月 [査読有り][通常論文]

We present a particle physics model based on a ten-dimensional (10D) super Yang-Mills (SYM) theory compactified on magnetized tori preserving four-dimensional N = 1 supersymmetry. The low-energy spectrum contains the minimal supersymmetric standard model with hierarchical Yukawa couplings caused by a wavefunction localization of the chiral matter fields due to the existence of magnetic fluxes, allowing a semi-realistic pattern of the quark and the lepton masses and mixings. We show supersymmetric flavor structures at low energies induced by a moduli-mediated and an anomaly-mediated supersymmetry breaking. (C) 2013 Elsevier B.V. All rights reserved. - Kohei Kamada, Tatsuo Kobayashi, Keisuke Ohashi, Yutaka OokouchiJOURNAL OF HIGH ENERGY PHYSICS 5 2013年05月 [査読有り][通常論文]

We study stabilization of an unstable cosmic string associated with spontaneously broken U(1)(R) symmetry, which otherwise causes a dangerous roll-over process. We demonstrate that in a gauge mediation model, messengers can receive enough corrections from the thermal plasma of the supersymmetric standard model particles to stabilize the unstable modes of the string. - Yukihiro Fujimoto, Tatsuo Kobayashi, Takashi Miura, Kenji Nishiwaki, Makoto SakamotoPhysical Review D - Particles, Fields, Gravitation and Cosmology 87 8 2013年04月08日 [査読有り][通常論文]

We propose a mechanism to obtain the generation of matter in the standard model. We start from the analysis of the T2/ZN shifted orbifold with magnetic flux, which imposes a ZN symmetry on torus. We also consider several orbifolds such as (T2×T2)/ZN, (T2×T2×T2)/ (ZN×ZN') and (T2×T2×T2)/(Z N×ZN'×ZN'). On such orbifolds, we study the behavior of fermions in two different means - the operator formalism and the explicit analysis of wave functions. For an interesting result, it is found that the number of zero-mode fermions is related to N of the ZN symmetry. In other words, the generation of matter relates to the type of orbifolds. Moreover, we find that shifted orbifold models are severely restricted from realizing three generations. For example, the three-generation model on the type of M4×(T2×T2)/ZN is unique. One can also construct other types of three-generation orbifold models with rich flavor structure. Those results may bring us a realistic model with desired Yukawa structure. © 2013 American Physical Society. - Yuta Hamada, Kohei Kamada, Tatsuo Kobayashi, Yutaka OokouchiJOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS 4 2013年04月 [査読有り][通常論文]

We study general constraints on spontaneous R-symmetry breaking models coming from the cosmological effects of the pseudo Nambu-Goldstone bosons, R-axions. They are substantially produced in the early Universe and may cause several cosmological problems. We focus on relatively long-lived R-axions and find that in a wide range of parameter space, models are severely constrained. In particular, R-axions with mass less than 1 MeV are generally ruled out for relatively high reheating temperature, T-R > 10 GeV. - Minoru Eto, Yuta Hamada, Kohei Kamada, Tatsuo Kobayashi, Keisuke Ohashi, Yutaka OokouchiJOURNAL OF HIGH ENERGY PHYSICS 3 2013年03月 [査読有り][通常論文]

We show that a cosmic string associated with spontaneous U(1)(R) symmetry breaking gives a constraint for supersymmetric model building. In some models, the string can be viewed as a tube-like domain wall with a winding number interpolating a false vacuum and a true vacuum. Such string causes inhomogeneous decay of the false vacuum to the true vacuum via rapid expansion of the radius of the tube and hence its formation would be inconsistent with the present Universe. However, we demonstrate that there exist metastable solutions which do not expand rapidly. Furthermore, when the true vacua are degenerate, the structure inside the tube becomes involved. As an example, we show a "bamboo"-like solution, which suggests a possibility observing an information of true vacua from outside of the tube through the shape and the tension of the tube. - Tatsuo Kobayashi, Hiroki Makino, Ken-ichi Okumura, Takashi Shimomura, Tsubasa TakahashiJOURNAL OF HIGH ENERGY PHYSICS 1 2013年01月 [査読有り][通常論文]

We study the next-to-minimal supersymmetric standard model. We consider soft supersymmetry breaking parameters, which are induced by the mirage mediation mechanism of supersymmetry breaking. We concentrate on the mirage mediation, where the so-called mirage scale is the TeV scale. In this scenario, we can realize the up-type Higgs soft mass of O(200) GeV, while other masses such as gaugino masses and stop masses are heavy such as 1 TeV or more. Cancellation between the effective mu-term and the down-type Higgs soft mass ameliorates the fine-tuning in the electroweak symmetry breaking even for mu = O(500) GeV. The mixing between the doublet and singlet Higgs bosons is suppressed by (kappa/lambda) tan(-1) beta. Then the lightest doublet Higgs mass naturally reaches 125 GeV lifted by the new quartic coupling. The higgsino and singlino are light and their linear combination is the lightest superparticle. - Tatsuo Kobayashi, Susha L. Parameswaran, Saul Ramos-Sanchez, Ivonne ZavalaJOURNAL OF HIGH ENERGY PHYSICS 12 2012年12月 [査読有り][通常論文]
- Tatsuo Azeyanagi, Tatsuo Kobayashi, Atsushi Ogasahara, Koichi YoshiokaPHYSICAL REVIEW D 86 9 2012年11月 [査読有り][通常論文]

We study the D-term effect on runaway directions of the F-term scalar potential. A minimal renormalizable model is presented where supersymmetry is broken without any pseudomoduli. The model is applied to the hidden sector of gauge mediation for spontaneously breaking R symmetry and generating nonvanishing gaugino masses at the one-loop order. - Yuta Hamada, Tatsuo KobayashiPROGRESS OF THEORETICAL PHYSICS 128 5 903 - 923 2012年11月 [査読有り][通常論文]

We study higher dimensional models with magnetic fluxes, which can be derived from superstring theory. We study mass spectrum and wavefunctions of massless and massive modes for spinor, scalar and vector fields. We compute the 3-point couplings and higher order couplings among massless modes and massive modes in 4D low-energy effective field theory. These couplings have non-trivial behaviors, because wavefunctions of massless and massive modes are non-trivial. - Hiroyuki Abe, Tatsuo Kobayashi, Hiroshi Ohki, Keigo SumitaNUCLEAR PHYSICS B 863 1 1 - 18 2012年10月 [査読有り][通常論文]

We present a four-dimensional (4D) N = 1 superfield description of supersymmetric Yang-Mills (SYM) theory in ten-dimensional (10D) spacetime with certain magnetic fluxes in compactified extra dimensions preserving partial N = 1 supersymmetry out of full N = 4. We derive a 41) effective action in N = 1 superspace directly from the 10D superfield action via dimensional reduction, and identify its dependence on dilaton and geometric moduli superfields. A concrete model for three generations of quark and lepton superfields are also shown. Our formulation would be useful for building various phenomenological models based on magnetized SYM theories or D-branes. (C) 2012 Elsevier B.V. All rights reserved. - Tatsuo Kobayashi, Takashi Shimomura, Tsubasa TakahashiPhysical Review D - Particles, Fields, Gravitation and Cosmology 86 1 2012年07月30日 [査読有り][通常論文]

We study the mass, the mixing, and the coupling with the Z boson of the lightest Higgs boson in the next-to-minimal supersymmetric standard model with the Z 3 symmetry. The vacuum structure of the Higgs potential is analyzed, and the new false vacua are discussed. The significant parameter region can be excluded by requiring the realistic vacuum to be deeper than false vacua. In numerical analysis, we also require the couplings to be perturbative up to the Grand Unified Theory scale. Our analysis results in constraints on the properties of the lightest Higgs boson. © 2012 American Physical Society. - Tatsuo Kobayashi, Takashi Shimomura, Tsubasa TakahashiPHYSICAL REVIEW D 86 1 2012年07月 [査読有り][通常論文]

We study the mass, the mixing, and the coupling with the Z boson of the lightest Higgs boson in the next- to-minimal supersymmetric standard model with the Z(3) symmetry. The vacuum structure of the Higgs potential is analyzed, and the new false vacua are discussed. The significant parameter region can be excluded by requiring the realistic vacuum to be deeper than false vacua. In numerical analysis, we also require the couplings to be perturbative up to the Grand Unified Theory scale. Our analysis results in constraints on the properties of the lightest Higgs boson. - Hajime Ishimori, Tatsuo KobayashiPHYSICAL REVIEW D 85 12 2012年06月 [査読有り][通常論文]

We study the lepton flavor models with the flavor symmetry (Z(N) x Z(N) x Z(N))xZ(3). Our models lead nonvanishing discrete values of theta(13) as well as (12) and theta(23) depending on N. For certain values, our models realize the tribimaximal mixing angles with theta(13) = 0. For other values, our models provide discrete deviation from the tribimaximal mixing angles. - Tatsuo Kobayashi, Susha L. Parameswaran, Saúl Ramos-Sánchez, Ivonne ZavalaJournal of High Energy Physics 2012 12 2012年 [査読有り][通常論文]
- Tatsuo KobayashiAIP Conference Proceedings 1467 194 - 199 2012年 [査読有り][通常論文]

We study 4D low-energy effective field theory, derived from heterotic string theory on the orbifolds. In particular, we study Abelian and non-Abelian discrete symmetries and their anomalies. Furthermore, stringy computations also provide with stringy coupling selection rules. © 2012 American Institute of Physics. - Yoshimi Kanehata, Tatsuo Kobayashi, Yasufumi Konishi, Osamu Seto, Takashi ShimomuraPROGRESS OF THEORETICAL PHYSICS 126 6 1051 - 1076 2011年12月 [査読有り][通常論文]

We study constraints to avoid deep unrealistic minima in the next-to-minimal supersymmetric standard model. We analyze the scalar potential along directions where all of and one of the three Higgs fields develop their vacuum expectation values, and find unrealistic minima deeper than the electroweak symmetry breaking (EWSB) vacuum. These unrealistic minima threaten the realization of successful EWSB and therefore should be avoided. Necessary conditions for avoiding these minima result in constraints on parameters. We show that a wide and significant region of the parameter space, especially a large A, is ruled out by our constraints. - Hajime Ishimori, Tatsuo KobayashiJOURNAL OF HIGH ENERGY PHYSICS 10 2011年10月 [査読有り][通常論文]

We study lepton flavor models with the S-4 flavor symmetry. We construct simple models with smaller numbers of flavon fields and free parameters, such that we have predictions among lepton masses and mixing angles. The model with a S-4 triplet flavon is not realistic, but we can construct realistic models with two triplet flavons, or one triplet and one doublet flavons. - Hiroyuki Abe, Kang-Sin Choi, Tatsuo Kobayashi, Hiroshi Ohki, Manabu SakaiINTERNATIONAL JOURNAL OF MODERN PHYSICS A 26 23 4067 - 4082 2011年09月 [査読有り][通常論文]

We study non-Abelian flavor symmetries on orbifolds, S-1/Z(2) and T-2/Z(3). Our extra dimensional models realize D-N, Sigma(2N(2)), Delta(3N(2)) and Delta(6N(2)) including A(4) and S-4. In addition, one can also realize their subgroups such as Q(N), T-7, etc. The S-3 flavor symmetry can be realized on both S-1/Z(2) and T-2/Z(3) orbifolds. - Tatsuo Azeyanagi, Tatsuo Kobayashi, Atsushi Ogasahara, Koichi YoshiokaJOURNAL OF HIGH ENERGY PHYSICS 9 2011年09月 [査読有り][通常論文]

We systematically study supersymmetry breaking with non-vanishing F and D terms. We classify the models into two categories and find that a certain class of models necessarily has runaway behavior of scalar potential, while the other needs the Fayet-Iliopoulos term to break supersymmetry. The latter class is useful to have a simple model of gauge mediation where the vacuum is stable everywhere and the gaugino mass is generated at the one-loop order. - Tetsutaro Higaki, Tatsuo KobayashiPHYSICAL REVIEW D 84 4 2011年08月 [査読有り][通常論文]

We study properties of moduli stabilization in the four dimensional N = 1 supergravity theory with heavy moduli and would-be saxion-axion multiplets, including light string-theoretic axions. We give general formulation for the scenario that heavy moduli and saxions are stabilized while axions remain light, assuming that moduli are stabilized near the supersymmetric solution. One can find stable vacuum, i.e. nontachyonic saxions, in the nonsupersymmetric Minkowski vacua. We also discuss the cases where the moduli are coupled to the supersymmetry breaking sector and/or moduli have contributions to supersymmetry breaking. Futhermore, we study the models with axions originating from matterlike fields. Our analysis on moduli stabilization is applicable even if there are not light axion multiplets. - Tatsuo Kobayashi, Yuichiro Nakai, Manabu SakaiJOURNAL OF HIGH ENERGY PHYSICS 6 2011年06月 [査読有り][通常論文]

We study anomaly mediation models with gauge mediation effects from messengers which have a general renormalizable mass matrix with a supersymmetry-breaking spurion. Our models lead to a rich structure of supersymmetry breaking terms in the visible sector. We derive sum rules among the soft scalar masses for each generation. Our sum rules for the first and second generations are the same as those in general gauge mediation, but the sum rule for the third generation is different because of the top Yukawa coupling. We find the parameter space where the tachyonic slepton problem is solved. We also explore the case in which gauge mediation causes the anomalously small gaugino masses. Since anomaly mediation effects on the gaugino masses exist, we can obtain viable mass spectrum of the visible sector fields. - Kang-Sin Choi, Tatsuo KobayashiPHYSICS LETTERS B 693 3 330 - 333 2010年10月 [査読有り][通常論文]

We study the MSSM in F-theory. Its group is the commutant to a structure group SU(5) x U(1)gamma of a gauge bundle in E(8). The spectrum contains three generations of quarks and leptons plus vectorlike electroweak and colored Higgses. The minimal MSSM Yukawa couplings with matter parity is obtained at the renormalizable level. (c) 2010 Elsevier B.V. All rights reserved. - Constraints from unrealistic vacua in the supersymmetric standard model with neutrino mass operatorsYoshimi Kanehata, Tatsuo Kobayashi, Yasufumi Konishi, Takashi ShimomuraPHYSICAL REVIEW D 82 7 2010年10月 [査読有り][通常論文]

We analyze a scalar potential of the minimal supersymmetric standard model (MSSM) with neutrino mass operators along unbounded-from-below and color and/or charged breaking directions. We show necessary conditions to avoid the potential minima which can be deeper than the realistic vacuum. These conditions would constrain more strongly than conditions in the MSSM without taking into account neutrino mass operators and can improve the predictive power of supersymmetric models with neutrino mass operators. - Tatsuo Kobayashi, Yuichiro Nakai, Ryo TakahashiJOURNAL OF HIGH ENERGY PHYSICS 9 2010年09月 [査読有り][通常論文]

We study superparticle spectra in the superconformal flavor scenario with non-universal gaugino masses. The non-universality of gaugino masses can lead to the wino-like or higgsino-like neutralino LSP. Furthermore, it is shown that the parameter space for the higgsino-like LSP includes the region where the fine-tuning problem can be improved. The degeneracy of soft scalar masses does not drastically change when ratios of gaugino masses are of O(1). The degeneracy of scalar masses for squarks and left-handed sleptons would be good enough to avoid the FCNC problem but that of right-handed slepton masses is weak. However, the overall size of right-handed slepton masses becomes larger when the bino becomes heavier. Thus, that would be favorable to avoid the FCNC problem due to soft scalar masses and A-terms. - Tatsuo Kobayashi, Takashi ShimomuraPHYSICAL REVIEW D 82 3 2010年08月 [査読有り][通常論文]

We consider a model where right-handed neutrinos and sneutrinos are introduced to the minimal supersymmetric standard model. In the scalar potential of this model, there exist trilinear and quartic terms in scalar potential that are proportional to Yukawa couplings of neutrinos. Because of these trilinear and quartic terms, color and/or charge breaking (CCB) and unbounded-from-below (UFB) directions appear along which sneutrinos have a vacuum expectation value, making the vacuum of the electroweak symmetry breaking unstable. We analyze the scalar potential of this model and derive necessary conditions for color and/or charge breaking and unbounded-from-below directions to vanish. - Hiroyuki Abe, Kang-Sin Choi, Tatsuo Kobayashi, Hiroshi OhkiPHYSICAL REVIEW D 81 12 2010年06月 [査読有り][通常論文]

We study the flavor structure of four-dimensional (4D) effective theories, which are derived from extra dimensional theories with magnetic fluxes and non-Abelian Wilson lines. We study zero-mode wave functions in generic case and compute Yukawa couplings as well as four-point couplings. In our models, we also discuss non-Abelian discrete flavor symmetries such as D-4, Delta(27), and Delta(54). The orbifold background is also studied. - Tatsuo Kobayashi, Ryosuke Maruyama, Masaki Murata, Hiroshi Ohki, Manabu SakaiJOURNAL OF HIGH ENERGY PHYSICS 5 2010年05月 [査読有り][通常論文]

We study 10D super Yang-Mills E-8 theory on the 6D torus compactification with magnetic fluxes. We study systematically the possibilities for realizing 4D supersymmetric standard models with three generations of quarks and leptons. We also study quark mass matrices. - Kang-Sin Choi, Tatsuo Kobayashi, Ryosuke Maruyama, Masaki Murata, Yuichiro Nakai, Hiroshi Ohki, Manabu SakaiEUROPEAN PHYSICAL JOURNAL C 67 1-2 273 - 282 2010年05月 [査読有り][通常論文]

We study 10D super Yang-Mills theory with the gauge groups E (6), E (7) and E (8). We consider the torus/orbifold compactification with magnetic fluxes and Wilson lines. They lead to 4D interesting models with three families of quarks and leptons, whose profiles in extra dimensions are quasi-localized because of magnetic fluxes. - Hajime Ishimori, Tatsuo Kobayashi, Hiroshi Ohki, Yusuke Shimizu, Hiroshi Okada, Morimitsu TanimotoPROGRESS OF THEORETICAL PHYSICS SUPPLEMENT 183 1 - 163 2010年 [査読有り][通常論文]

We review pedagogically non-Abelian discrete groups, which play an important role in particle physics. We show group-theoretical aspects for many concrete groups, such as representations and their tensor products. We explain how to derive, conjugacy classes, characters, representations, and tensor products for these groups (with a finite number). We discuss them explicitly for S-N, A(N), T', D-N, Q(N), E(2N(2)), Delta(3N(2)), T-7, Sigma(3N(3)), and Delta(6N(2)), which have been applied for model building in particle physics. We also present typical flavor models by using A(4), S-4, and Delta(54) groups. Breaking patterns of discrete groups and decompositions of multiplets are important for applications of the non-Abelian discrete symmetry. We discuss these breaking patterns of the non-Abelian discrete group, which are a powerful tool for model buildings. We also review briefly anomalies of non-Abelian discrete symmetries by using the path integral approach. - Hiroyuki Abe, Kang-Sin Choi, Tatsuo Kobayashi, Hiroshi OhkiPhysical Review D - Particles, Fields, Gravitation and Cosmology 80 12 2009年12月15日 [査読有り][通常論文]

We study torus/orbifold models with magnetic flux and Wilson line backgrounds. The number of zero modes and their profiles depend on those backgrounds. That has interesting implications from the viewpoint of particle phenomenology. © 2009 The American Physical Society. - Hiroyuki Abe, Kang-Sin Choi, Tatsuo Kobayashi, Hiroshi OhkiPHYSICAL REVIEW D 80 12 2009年12月 [査読有り][通常論文]

We study torus/orbifold models with magnetic flux and Wilson line backgrounds. The number of zero modes and their profiles depend on those backgrounds. That has interesting implications from the viewpoint of particle phenomenology. - Hajime Ishimori, Tatsuo Kobayashi, Hiroshi Okada, Yusuke Shimizu, Morimitsu TanimotoJOURNAL OF HIGH ENERGY PHYSICS 12 2009年12月 [査読有り][通常論文]

We study a Delta(54) x Z(2) flavor model for leptons and sleptons. The tri-bimaximal mixing can be reproduced for arbitrary neutrino masses if certain vacuum alignments of scalar fields are realized. The deviation from the tri-bimaximal mixing of leptons is predicted. The predicted upper bound for sin theta(13) is 0.07. The value of sin theta(23) could be deviated from the maximal mixing considerably while sin theta(12) is hardly deviated from 1/root 3. We also study SUSY breaking terms in the slepton sector. Three families of left-handed and right-handed slepton masses are almost degenerate. Our model leads to smaller values of flavor changing neutral currents than the present experimental bounds. - Hiroyuki Abe, Kang-Sin Choi, Tatsuo Kobayashi, Hiroshi OhkiNUCLEAR PHYSICS B 820 1-2 317 - 333 2009年10月 [査読有り][通常論文]

We study non-Abelian discrete flavor symmetries, which appear in magnetized brane models. For example, D-4, Delta(27) and Delta(54) flavor symmetries can appear with matter fields in several representations. We also study the orbifold background, where non-Abelian flavor symmetries are broken in a certain way. (C) 2009 Published by Elsevier B.V. - Hiroyuki Abe, Kang-Sin Choi, Tatsuo Kobayashi, Hiroshi OhkiNUCLEAR PHYSICS B 814 1-2 265 - 292 2009年06月 [査読有り][通常論文]

We study three generation models in the four-dimensional spacetime, which can be derived from the ten-dimensional N = 1 super-Yang-Mills theory on the orbifold background with a non-vanishing magnetic flux. We classify the flavor structures and show possible patterns of Yukawa matrices. Some examples of numerical studies are also shown. (C) 2009 Elsevier B.V. All rights reserved. - Hiroyuki Abe, Kang-Sin Choi, Tatsuo Kobayashi, Hiroshi OhkiJOURNAL OF HIGH ENERGY PHYSICS 6 2009年06月 [査読有り][通常論文]

We compute three-point and higher order couplings in magnetized brane models. We show that higher order couplings are written as products of three-point couplings. This behavior is the same as higher order amplitudes by conformal field theory calculations e. g. in intersecting D-brane models. - Hajime Ishimori, Tatsuo Kobayashi, Hiroshi Okada, Yusuke Shimizu, Morimitsu TanimotoJOURNAL OF HIGH ENERGY PHYSICS 4 2009年04月 [査読有り][通常論文]

We present the lepton flavor model with Delta(54), which appears typically in heterotic string models on the T-2/Z(3) orbifold. Our model reproduces the tri-bimaximal mixing in the parameter region around degenerate neutrino masses or two massless neutrinos. We predict the deviation from the tri-bimaximal mixing by putting the experimental data of neutrino masses in the normal hierarchy of neutrino masses. The upper bound of sin(2)theta(13) is 0.01. There is the strong correlation between theta(23) and theta(13). Unless theta(23) is deviated from the maximal mixing considerably, theta(13) remains to be tiny. - Hiroyuki Abe, Tetsutaro Higaki, Tatsuo Kobayashi, Kazutoshi Ohta, Yuji Omura, Haruhiko TeraoPhysical Review D - Particles, Fields, Gravitation and Cosmology 79 4 2009年02月02日 [査読有り][通常論文]

We study the duality cascade of softly broken supersymmetric theories. We investigate the renormalization group (RG) flow of SUSY breaking terms as well as supersymmetric couplings. It is found that the magnitudes of SUSY breaking terms are suppressed in most regimes of the RG flow through the duality cascade. At one stage of cascading, the gaugino mass of the strongly coupled sector and scalar masses converge to certain values, which are determined by the gauge coupling and the gaugino mass of the weakly coupled sector. At the next stage, the strongly and weakly coupled sectors are interchanged with each other. We also show the possibility that cascading would be terminated by the gauge symmetry breaking, which is induced by the so-called B-term. © 2009 The American Physical Society. - Hiroyuki Abe, Tetsutaro Higaki, Tatsuo Kobayashi, Kazutoshi Ohta, Yuji Omura, Haruhiko TeraoPHYSICAL REVIEW D 79 4 2009年02月 [査読有り][通常論文]

We study the duality cascade of softly broken supersymmetric theories. We investigate the renormalization group (RG) flow of SUSY breaking terms as well as supersymmetric couplings. It is found that the magnitudes of SUSY breaking terms are suppressed in most regimes of the RG flow through the duality cascade. At one stage of cascading, the gaugino mass of the strongly coupled sector and scalar masses converge to certain values, which are determined by the gauge coupling and the gaugino mass of the weakly coupled sector. At the next stage, the strongly and weakly coupled sectors are interchanged with each other. We also show the possibility that cascading would be terminated by the gauge symmetry breaking, which is induced by the so-called B-term. - Hajime Ishimori, Tatsuo Kobayashi, Yuji Omura, Morimitsu TanimotoJOURNAL OF HIGH ENERGY PHYSICS 12 2008年12月 [査読有り][通常論文]

We study the supersymmetric model with the A(4) lepton flavor symmetry, in particular soft supersymmetry breaking terms, which are predicted from the A(4) lepton flavor symmetry. We evaluate soft slepton masses and A-terms within the framework of supergravity theory. Constraints due to experiments of flavor changing neutral current processes are examined. - Hajime Ishimori, Tatsuo Kobayashi, Yuji Omura, Morimitsu TanimotoJOURNAL OF HIGH ENERGY PHYSICS 12 2008年12月 [査読有り][通常論文]

We study the supersymmetric model with the A(4) lepton flavor symmetry, in particular soft supersymmetry breaking terms, which are predicted from the A(4) lepton flavor symmetry. We evaluate soft slepton masses and A-terms within the framework of supergravity theory. Constraints due to experiments of flavor changing neutral current processes are examined. - Tatsuo Kobayashi, Yuji Omura, Koichi YoshiokaPHYSICAL REVIEW D 78 11 2008年12月 [査読有り][通常論文]

Flavor symmetry has been widely studied for figuring out the masses and mixing angles of standard model fermions. In this paper we present a framework for handling flavor symmetry breaking where the symmetry breaking is triggered by boundary conditions of scalar fields in extra-dimensional space. The alignment of scalar expectation values is achieved without referring to any details of scalar potential and its minimization procedure. As applications to non-Abelian discrete flavor symmetries, illustrative lepton mass models are constructed where the S-3 and A(4) flavor symmetries are broken down to the directions leading to the tribimaximal form of lepton mixing and realistic mass patterns. - Takeshi Araki, Tatsuo Kobayashi, Jisuke Kubo, Saul Ramos-Sanchez, Michael Ratz, Patrick K. S. VaudrevangeNUCLEAR PHYSICS B 805 1-2 124 - 147 2008年12月 [査読有り][通常論文]

We derive anomaly constraints for Abelian and non-Abelian discrete symmetries using the path integral approach. We survey anomalies of discrete symmetries in heterotic orbifolds and find a new relation between such anomalies and the so-called 'anomalous' U(1). (C) 2008 Elsevier B.V. All rights reserved. - Hiroyuki Abe, Tatsuo Kobayashi, Hiroshi OhkiJOURNAL OF HIGH ENERGY PHYSICS 9 2008年09月 [査読有り][通常論文]

We study (4+2n)-dimensional N=1 super Yang-Mills theory on the orbifold background with non-vanishing magnetic fluxes. In particular, we study zero- modes of spinor fields. The flavor structure of our models is different from one in magnetized torus models, and would be interesting in realistic model building. - Kang-Sin Choi, Tatsuo KobayashiNUCLEAR PHYSICS B 797 1-2 295 - 321 2008年07月 [査読有り][通常論文]

We calculate couplings of arbitrary order from correlation functions among twisted strings, using conformal field theory. Twisted strings arise in heterotic string compactified on orbifolds yielding matter fields in the low energy limit. We calculate completely the classical and the quantum amplitude including normalization, up to a contribution from Kahler potential. The classical action has saddle points which are interpreted as worldsheet instantons described by metastable untwisted strings, formed by twisted strings distributed at certain fixed points. This understanding generalizes the area rule, in the case that the locations of twisted strings do not form a polygon, and provides a general rule for calculating these kinds of instanton corrections. An interpretation of couplings involving linearly combined states is given, which commonly appear in non-prime order orbifolds. The quantum part of the amplitude is given by ratios of gamma functions with order one arguments. (C) 2008 Elsevier B.V. All rights reserved. - Hiroyuki Abe, Tetsutaro Higaki, Tatsuo Kobayashi, Yuji OmuraJOURNAL OF HIGH ENERGY PHYSICS 4 2008年04月 [査読有り][通常論文]

We study moduli stabilization, the dynamical supersymmetry (SUSY) breaking, the uplifting of SUSY anti-de Sitter (AdS) vacuum and the sequestering of hidden sector in a five-dimensional supergravity model, where all modes of the visible sector and the hidden sector are originated from bulk fields. We clarify couplings between the visible and hidden sectors. The expressions for the visible sector soft SUSY breaking terms as well as the hidden sector potential are shown explicitly in our model. The sequestering is achieved dynamically by a wavefunction localization in extra dimension. We find that the tree-level soft scalar mass and the A-term can be suppressed at a SUSY breaking Minkowski minimum where the radius modulus is stabilized, while gaugino masses would be a mirage type. - Hiroyuki Abe, Tatsuo Kobayashi, Yuji OmuraPHYSICAL REVIEW D 77 6 2008年03月 [査読有り][通常論文]

We study the scenario that conformal dynamics leads to metastable supersymmetry breaking vacua. At a high energy scale, the superpotential is not R-symmetric and has a supersymmetric minimum. However, conformal dynamics suppresses several operators along renormalization group flow toward the infrared fixed point. Then we can find an approximately R-symmetric superpotential, which has a metastable supersymmetry breaking vacuum, and the supersymmetric vacuum moves far away from the metastable supersymmetry breaking vacuum. We show a 4D simple model. Furthermore, we can construct 5D models with the same behavior, because of the AdS/CFT duality. - David G. Cerdeno, Carlos Munoz, Tatsuo KobayashiJOURNAL OF HIGH ENERGY PHYSICS 1 2008年01月 [査読有り][通常論文]

We analyse the phenomenology of orbifold scenarios from the heterotic superstring, and the resulting theoretical predictions for the direct detection of neutralino dark matter. In particular, we study the parameter space of these constructions, computing the low-energy spectrum and taking into account the most recent experimental and astrophysical constraints, as well as imposing the absence of dangerous charge and colour breaking minima. In the remaining allowed regions the spin-independent part of the neutralino-proton cross section is calculated and compared with the sensitivity of dark matter detectors. In addition to the usual non universalities of the soft terms in orbifold scenarios due to the modular weight dependence, we also consider D-term contributions to scalar masses. These are generated by the presence of an anomalous U(1), providing more flexibility in the resulting soft terms, and are crucial in order to avoid charge and colour breaking minima. Thanks to the D-term contribution, large neutralino detection cross sections can be found, within the reach of projected dark matter detectors. - Tatsuo KobayashiSUPERSYMMETRY AND THE UNIFICATION OF FUNDAMENTAL INTERACTIONS 1078 405 - 407 2008年 [査読有り][通常論文]

We study nonperturbative superpotentials with positive exponents, i.e. the form like Ae(aT) with positive constant a and the modulus T. - Hiroyuki Abe, Tetsutaro Higaki, Tatsuo KobayashiPhysical Review D - Particles, Fields, Gravitation and Cosmology 76 10 2007年11月05日 [査読有り][通常論文]

We study moduli stabilization and a realization of de Sitter vacua in generalized F-term uplifting scenarios of the Kachru-Kallosh-Linde-Trivedi-type anti-de Sitter vacuum, where the uplifting sector X directly couples to the light Kähler modulus T in the superpotential through, e.g., stringy instanton effects. F-term uplifting can be achieved by a spontaneous supersymmetry breaking sector, e.g., the Polonyi model, the O'Raifeartaigh model, and the Intriligator-Seiberg-Shih model. Several models with the X-T mixing are examined, and qualitative features in most models even with such mixing are almost the same as those in the Kachru-Kallosh-Linde-Trivedi scenario. One of the quantitative changes, which are relevant to the phenomenology, is a larger hierarchy between the modulus mass mT and the gravitino mass m3/2, i.e., mT/m3/2=O(a2), where a∼4π2. In spite of such a large mass, the modulus F term is suppressed not like FT=O(m3/2/a2), but like FT=O(m3/2/a) for ln (MPl/m3/2)∼a, because of an enhancement factor coming from the X-T mixing. Then we typically find a mirage-mediation pattern of gaugino masses of O(m3/2/a), while the scalar masses would be generically of O(m3/2). © 2007 The American Physical Society. - Hiroyuki Abe, Tatsuo Kobayashi, Yuji OmuraJOURNAL OF HIGH ENERGY PHYSICS 11 2007年11月 [査読有り][通常論文]

We study N=1 global and local supersymmetric theories with a continuous global U(1)(R) symmetry as models of dynamical supersymmetry (SUSY) breaking. We introduce explicit R-symmetry breaking terms into such models, in particular a generalized O'Raifeartaigh model. Such explicit R-symmetry breaking terms can lead to a SUSY preserving minimum. We classify explicit R-symmetry breaking terms by the structure of newly appeared SUSY stationary points as a consequence of the R-breaking effect, which could make the SUSY breaking vacuum metastable. We show that the R-breaking terms are basically divided into two categories. One of them does not generate a SUSY solution, or yields SUSY solutions that disappear in the case of supergravity when we tune a parameter so that the original SUSY breaking minimum becomes a Minkowski vacuum. We also show that the general argument by Nelson and Seiberg for a dynamical SUSY breaking still holds with a local SUSY except for a certain nontrivial case, and present concrete examples of the exception. - Hiroyuki Abe, Tetsutaro Higaki, Tatsuo KobayashiPHYSICAL REVIEW D 76 10 2007年11月 [査読有り][通常論文]

We study moduli stabilization and a realization of de Sitter vacua in generalized F-term uplifting scenarios of the Kachru-Kallosh-Linde-Trivedi-type anti-de Sitter vacuum, where the uplifting sector X directly couples to the light Kahler modulus T in the superpotential through, e.g., stringy instanton effects. F-term uplifting can be achieved by a spontaneous supersymmetry breaking sector, e.g., the Polonyi model, the O'Raifeartaigh model, and the Intriligator-Seiberg-Shih model. Several models with the X-T mixing are examined, and qualitative features in most models even with such mixing are almost the same as those in the Kachru-Kallosh-Linde-Trivedi scenario. One of the quantitative changes, which are relevant to the phenomenology, is a larger hierarchy between the modulus mass m(T) and the gravitino mass m(3/2), i.e., m(T)/m(3/2)=O(a(2)), where a similar to 4 pi(2). In spite of such a large mass, the modulus F term is suppressed not like F-T=O(m(3/2)/a(2)), but like F-T=O(m(3/2)/a) for ln(M-Pl/m(3/2))similar to a, because of an enhancement factor coming from the X-T mixing. Then we typically find a mirage-mediation pattern of gaugino masses of O(m(3/2)/a), while the scalar masses would be generically of O(m(3/2)). - Hiroyuki Abe, Yeong Gyun Kim, Tatsuo Kobayashi, Yasuhiro ShimizuJOURNAL OF HIGH ENERGY PHYSICS 9 2007年09月 [査読有り][通常論文]

We study the TeV scale partial mirage unification scenario, where the gluino and wino masses are degenerate around a TeV scale, but the bino mass is not degenerate. This scenario has phenomenologically interesting aspects. First, because of the degeneracy between the gluino and wino masses, this scenario does not have the little hierarchy problem, that is, the higgisino mass is around 150 GeV. The lightest superparticle is a mixture of the bino and higgsino, and can lead to a right amount of thermal relic density as a dark matter candidate. - Takeshi Araki, Kang-Sin Choi, Tatsuo Kobayashi, Jisuke Kubo, Hiroshi OhkiPHYSICAL REVIEW D 76 6 2007年09月 [査読有り][通常論文]

Anomalies of discrete R symmetries appearing in heterotic orbifold models are studied. We find that the mixed anomalies for different gauge groups satisfy the universal Green-Schwarz condition, indicating that these anomalies are canceled by the Green-Schwarz mechanism. An exact relation between the anomaly coefficients of the discrete R symmetries and one-loop beta-function coefficients is obtained. We also find that the discrete R symmetries have a good chance to be unbroken down to the supersymmetry breaking scale. Even below this scale a Z(2) subgroup is unbroken, and it may be an origin of the R parity of the minimal supersymmetric standard model. Relations between the R-symmetry anomalies and T-duality anomalies are also investigated. - Pyungwon Ko, Tatsuo Kobayashi, Jae-Hyeon Park, Stuart RabyPHYSICAL REVIEW D 76 3 2007年08月 [査読有り][通常論文]

In this paper we show how some flavor symmetries may be derived from the heterotic string, when compactified on a 6D orbifold. In the body of the paper we focus on the D-4 family symmetry, recently obtained in Z(3)xZ(2) orbifold constructions. We show how this flavor symmetry constrains fermion masses, as well as the soft SUSY breaking mass terms. Flavor symmetry breaking can generate the hierarchy of fermion masses and at the same time the flavor symmetry suppresses large flavor changing neutral current processes. - Hiroyuki Abe, Tatsuo Kobayashi, Yuji OmuraPhysical Review D - Particles, Fields, Gravitation and Cosmology 76 1 2007年07月09日 [査読有り][通常論文]

We study, in a bottom-up approach, the fine-tuning problem between soft supersymmetry (SUSY) breaking parameters and the μ term for the successful electroweak symmetry breaking in the minimal supersymmetric standard model. It is shown that certain ratios among nonuniversal gaugino masses at the grand unified theory (GUT) scale are necessary to reduce fine-tuning. In addition, when all the gaugino masses are regarded as independent ones in their origins, a small gluino mass M3 120GeV and a nonvanishing A term At∼O(M3) associated with top squarks are also required at the GUT scale as well as the nonuniversality. On the other hand, when we consider some UV theory, which fixes ratios of soft SUSY breaking parameters as certain values aside from the overall magnitude, heavier spectra are allowed. It is favored that the gluino and W-ino masses are almost degenerate at the weak scale, while a wider region of b-ino mass is favorable. © 2007 The American Physical Society. - Kiwoon Choi, Kwang Sik Jeong, Tatsuo Kobayashi, Ken-Ichi OkumuraPhysical Review D - Particles, Fields, Gravitation and Cosmology 75 9 2007年05月31日 [査読有り][通常論文]

TeV scale mirage mediation has been proposed as a supersymmetry-breaking scheme reducing the fine-tuning for electroweak symmetry breaking in the minimal supersymmetric extension of the standard model. We discuss a moduli stabilization setup for TeV scale mirage mediation which allows an extradimensional interpretation for the origin of supersymmetry breaking and naturally gives a weak-scale size of the Higgs B parameter. The setup utilizes the holomorphic gauge kinetic functions depending on both the heavy dilaton and the light volume modulus whose axion partners are assumed to be periodic fields. We also examine the low-energy phenomenology of TeV scale mirage mediation, particularly the constraints from electroweak symmetry breaking and flavor changing neutral current processes. © 2007 The American Physical Society. - Kiwoon Choi, Kwang Sik Jeong, Tatsuo Kobayashi, Ken-ichi OkumuraPHYSICAL REVIEW D 75 9 2007年05月 [査読有り][通常論文]

TeV scale mirage mediation has been proposed as a supersymmetry-breaking scheme reducing the fine-tuning for electroweak symmetry breaking in the minimal supersymmetric extension of the standard model. We discuss a moduli stabilization setup for TeV scale mirage mediation which allows an extradimensional interpretation for the origin of supersymmetry breaking and naturally gives a weak-scale size of the Higgs B parameter. The setup utilizes the holomorphic gauge kinetic functions depending on both the heavy dilaton and the light volume modulus whose axion partners are assumed to be periodic fields. We also examine the low-energy phenomenology of TeV scale mirage mediation, particularly the constraints from electroweak symmetry breaking and flavor changing neutral current processes. - Tatsuo Kobayashi, Hans Peter Nilles, Felix Ploeger, Stuart Raby, Michael RatzNUCLEAR PHYSICS B 768 1-2 135 - 156 2007年04月 [査読有り][通常論文]

We study the origin of non-Abelian discrete flavor symmetries in superstring theory. We classify all possible non-Abelian discrete flavor symmetries which can appear in heterotic orbifold models. These symmetries include D-4 and Delta(54). We find that the symmetries of the couplings are always larger than the symmetries of the compact space. This is because they are a consequence of the geometry of the orbifold combined with the space group selection rules of the string. We also study possible breaking patterns. Our analysis yields a simple geometric understanding of the realization of non-Abelian flavor symmetries. (c) 2007 Elsevier B.V. All rights reserved. - Stefan Forste, Tatsuo Kobayashi, Hiroshi Ohki, Kei-jiro TakahashiJOURNAL OF HIGH ENERGY PHYSICS 3 2007年03月 [査読有り][通常論文]

We classify compactification lattices for supersymmetric Z(2) x Z(2) orbifolds. These lattices include factorisable as well as non-factorisable six-tori. Different models lead to different numbers of fixed points/tori. A lower bound on the number of fixed tori per twisted sector is given by four, whereas an upper bound consists of 16 fixed tori per twisted sector. Thus, these models have a variety of generation numbers. For example, in the standard embedding, the smallest number of net generations among these classes of models is equal to six, while the largest number is 48. Conditions for allowed Wilson lines and Yukawa couplings are derived. - Takeshi Chiba, Tatsuo Kobayashi, Masahide Yamaguchi, Jun'ichi YokoyamaPhysical Review D - Particles, Fields, Gravitation and Cosmology 75 4 2007年02月20日 [査読有り][通常論文]

Recent astrophysical observations indicate that the proton-electron mass ratio and the fine structure constant have gone through nontrivial time evolution. We discuss their time variation in the context of a dilaton runaway scenario with gauge coupling unification at the string scale Ms. We show that the choice of adjustable parameters allows them to fit the same order magnitude of both variations and their (opposite) signs in such a scenario. © 2007 The American Physical Society. - Hiroyuki Abe, Tetsutaro Higaki, Tatsuo Kobayashi, Yuji OmuraPHYSICAL REVIEW D 75 2 2007年01月 [査読有り][通常論文]

We study moduli stabilization with F-term uplifting. As a source of uplifting F-term, we consider spontaneous supersymmetry breaking models, e.g. the Polonyi model and the Intriligator-Seiberg-Shih model. We analyze potential minima by requiring almost vanishing vacuum energy and evaluate the size of modulus F-term. We also study soft SUSY-breaking terms. In our scenario, the mirage mediation is dominant in gaugino masses. Scalar masses can be comparable with gaugino masses or much heavier, depending on couplings with spontaneous supersymmetry breaking sector. - Tatsuo Kobayashi, Yuji Omura, Haruhiko TeraoPHYSICAL REVIEW D 74 5 2006年09月 [査読有り][通常論文]

We study realization of the democratic form of Yukawa matrices by infrared fixed points. We investigate renormalization-group flows of Yukawa couplings in models with a single Yukawa matrix for three families, and up and down-sector Yukawa matrices. It is found that each model has its certain pattern of renormalization-group flows of Yukawa matrices. We apply them to the charged lepton sector and quark sector, and examine in which situation our class of models can lead to realistic results for the mass ratios and mixing angles between the second and third families. We also study corresponding A-terms. The A-terms approach toward the universal form with no physical CP-violating phase. Thus, constraints due to various neutral flavor changing processes except for mu -> e gamma are found to be satisfied by this dynamics. In order to suppress the electric dipole moments as well as mu -> e gamma sufficiently, more alignment of the A-terms with some reason is required. - Hiroyuki Abe, Tetsutaro Higaki, Tatsuo KobayashiPHYSICAL REVIEW D 74 4 2006年08月 [査読有り][通常論文]

We study two steps of moduli stabilization in type IIB flux compactification with gaugino condensations. We consider the condition that one can integrate out heavy moduli first with light moduli remaining. We give appendix, where detail study is carried out for potential minima of the model with a six dimensional compact space with h(1,1)=h(2,1)=1, including the model, whose respective moduli with h(1,1),h(2,1)not equal 1 are identified. - Tatsuo Kobayashi, Haruhiko Terao, Akito TsuchiyaPHYSICAL REVIEW D 74 1 2006年07月 [査読有り][通常論文]

It is shown that fine-tuning of the Higgs parameters stronger than a few percent is required at best in the models with gauge mediated supersymmetry breaking. With the aim of solving this problem, we consider a new type of models in which the top Yukawa coupling is induced at TeV scale through mass mixing with unknown matter fields. Then it is found that the fine-tuning problem can be eliminated essentially. We discuss some phenomenological features of this model and also consider the extension to the next-to-minimal models. - H Abe, T Higaki, T KobayashiNUCLEAR PHYSICS B 742 187 - 207 2006年05月 [査読有り][通常論文]

We study supersymmetric models with double gaugino condensations in the hidden sector, where the gauge couplings depend on two light moduli of superstring theory. We perform a detailed analysis of this class of model and show that there is no stable supersymmetric minimum with finite vacuum values of moduli fields. Instead, we find that the supersymmetry breaking occurs with moduli stabilized and negative vacuum energy. That yields moduli-dominated soft supersymmetry breaking terms. To realize slightly positive (or vanishing) vacuum energy, we add uplifting potential. We discuss uplifting does not change qualitatively the vacuum expectation values of moduli and the above feature of supersymmetry breaking. (c) 2006 Elsevier B.V. All rights reserved. - K Choi, KS Jeong, T Kobayashi, K OkumuraPHYSICS LETTERS B 633 2-3 355 - 361 2006年02月 [査読有り][通常論文]

Motivated by the KKLT string compactification involving a supersymmetry-breaking uplifting potential, we examine 4D effective supergravity with a generic form of uplifting potential, focusing on the possibility that the resulting mixed modulus-anomaly mediated soft terms realize the little hierarchy between the Higgs boson masses m(H) and the sparticle masses m(SUSY). It is noted that for some type of uplifting potential, the anomaly-mediated contribution to m(H)(2) at M-GUT can cancel the subsequent renormalization group evolution of m(H)(2) down to TeV scale, thereby the model can naturally realize the little hierarchy m(H)(2) similar to m(SUSY)(2)/8 pi(2) which is desirable for the lightest Higgs boson mass to satisfy the experimental bound. In such models, the other Higgs mass parameters mu and B can have the desirable size mu similar to B similar to mH without severe fine-tuning of parameters, although the gravitino is much heavier than the Higgs boson. Those models for the little hierarchy avoid naturally the dangerous SUSY flavor and CP violations, and predict nearly degenerate low energy gaugino masses, pure Higgsino LSP and also a specific relation between the stop and gaugino masses. (c) 2005 Elsevier B.V. All rights reserved. - T Kobayashi, C MunozJOURNAL OF HIGH ENERGY PHYSICS 1 2006年01月 [査読有り][通常論文]

In realistic four-dimensional string constructions the presence of anomalous U(1)'s is generic. In addition, the associated Fayet-Iliopoulos contribution to the D-term can break the extra gauge symmetries. As a consequence, physical particles can appear combined with other states. We show that even if a three-generation standard-like model has originally flavour-independent soft scalar masses, the particle mixing contribution may generate non-universality among them. Thus FCNC effects which were apparently absent reappear. We also discuss the size of these contributions in an explicit model, and how they can be suppressed. - Hiroyuki Abe, Tetsutaro Higaki, Tatsuo KobayashiPhysical Review D - Particles, Fields, Gravitation and Cosmology 73 4 2006年 [査読有り][通常論文]

We study KKLT type models with moduli-mixing superpotential. In several string models, gauge kinetic functions are written as linear combinations of two or more moduli fields. Their gluino condensation generates moduli-mixing superpotential. We assume one of moduli fields is frozen already around the string scale. It is found that Kähler modulus can be stabilized at a realistic value without tuning 3-form fluxes because of gluino condensation on (non-)magnetized D-brane. Furthermore, we do not need to highly tune parameters in order to realize a weak gauge coupling and a large hierarchy between the gravitino mass and the Planck scale, when there exists nonperturbative effects on D3-brane. SUSY breaking patterns in our models have a rich structure. Also, some of our models have cosmologically important implications, e.g., on the overshooting problem and the destabilization problem due to finite temperature effects as well as the gravitino problem and the moduli problem. © 2006 The American Physical Society. - Tatsuo Kobayashi, Haruhiko Terao, Akito TsuchiyaPhysical Review D - Particles, Fields, Gravitation and Cosmology 74 1 2006年 [査読有り][通常論文]

It is shown that fine-tuning of the Higgs parameters stronger than a few percent is required at best in the models with gauge mediated supersymmetry breaking. With the aim of solving this problem, we consider a new type of models in which the top Yukawa coupling is induced at TeV scale through mass mixing with unknown matter fields. Then it is found that the fine-tuning problem can be eliminated essentially. We discuss some phenomenological features of this model and also consider the extension to the next-to-minimal models. © 2006 The American Physical Society. - T Higaki, N Kitazawa, T Kobayashi, K TakahashiPHYSICAL REVIEW D 72 8 2005年10月 [査読有り][通常論文]

We study flavor structure and the coupling selection rule in intersecting D-brane configurations on T(2)xT(2)xT(2) as well as its orbifold/orientifold compactifications. We formulate the selection rule for Yukawa couplings and its extensions to generic n-point couplings. We investigate the possible flavor structure, which can appear from intersecting D-brane configuration. Then, we show there is a certain rule among intersecting numbers for states corresponding to allowed Yukawa couplings, and also it is found that their couplings are determined by discrete Abelian symmetries. Our studies on the flavor structure and the coupling selection rule in this class of models show that there is no solution to derive realistic Yukawa matrices only from stringy 3-point couplings at the tree-level within the framework of the minimal matter content of the supersymmetric standard model. - T Kobayashi, H Nakano, H TeraoPHYSICAL REVIEW D 71 11 2005年06月 [査読有り][通常論文]

In the scenarios with heavy top squarks, mass parameters of the Higgs field must be fine-tuned due to a large logarithmic correction to the soft scalar mass. We consider a new possibility that the top-Yukawa coupling is small above TeV scale. The large top mass is induced from strong Yukawa interaction of the Higgs with another gauge sector, in which supersymmetry breaking parameters are given to be small. Then it is found that the logarithmic correction to the Higgs soft scalar mass is suppressed in spite of the strong coupling, and the fine-tuning is ameliorated. We propose an explicit model coupled to a superconformal gauge theory which realizes the above situation. - Tatsuo Kobayashi, Hiroaki Nakano, Haruhiko TeraoPhysical Review D - Particles, Fields, Gravitation and Cosmology 71 11 2005年06月01日 [査読有り][通常論文]

In the scenarios with heavy top squarks, mass parameters of the Higgs field must be fine-tuned due to a large logarithmic correction to the soft scalar mass. We consider a new possibility that the top-Yukawa coupling is small above TeV scale. The large top mass is induced from strong Yukawa interaction of the Higgs with another gauge sector, in which supersymmetry breaking parameters are given to be small. Then it is found that the logarithmic correction to the Higgs soft scalar mass is suppressed in spite of the strong coupling, and the fine-tuning is ameliorated. We propose an explicit model coupled to a superconformal gauge theory which realizes the above situation. © 2005 The American Physical Society. - P Ko, T Kobayashi, JH ParkPHYSICAL REVIEW D 71 9 2005年05月 [査読有り][通常論文]

We systematically study the possibility for realizing realistic values of lepton mass ratios and mixing angles by using only renormalizable Yukawa couplings derived from the heterotic Z(6)-I orbifold. We assume one pair of up and down sector Higgs fields. We consider both the Dirac neutrino mass scenario and the seesaw scenario with degenerate right-handed majorana neutrino masses. It is found that realistic values of the charged lepton mass ratios, m(e)/m(tau) and m(mu)/m(tau), the neutrino mass squared difference ratio, Delta m(31)(2)/Delta m(21)(2), and the lepton mixing angles can be obtained in certain cases. - T Kobayashi, H Shirano, H TeraoPROGRESS OF THEORETICAL PHYSICS 113 5 1077 - 1099 2005年05月 [査読有り][通常論文]

We consider the dynamical realization of the democratic type of Yukawa coupling matrices as the Pendelton-Ross infrared fixed points. Such fixed points of the Yukawa couplings become possible through the introduction of many Higgs fields, which are made superheavy, but one massless mode. Explicitly, we consider a strongly coupled supersymmetric GUT based on SU(5) x SU(5), in which rapid convergence to the infrared fixed point generates sufficiently large mass hierarchy for quarks and leptons. In particular, it is found that the remarkable difference between mixing angles in the quark and lepton sectors can be explained as a simple dynamical consequence. We also discuss a possible scenario leading to a realistic mass spectra and mixing angles for quarks and leptons. In this scheme, the Yukawa couplings not only for top but also for bottom appear close to their quasi-fixed points at low energy and, therefore, tan beta should be large. - Pyungwon Ko, Tatsuo Kobayashi, Jae-Hyeon ParkPhysical Review D - Particles, Fields, Gravitation and Cosmology 71 9 1 - 9 2005年05月01日 [査読有り][通常論文]

We systematically study the possibility for realizing realistic values of lepton mass ratios and mixing angles by using only renormalizable Yukawa couplings derived from the heterotic Z6-I orbifold. We assume one pair of up and down sector Higgs fields. We consider both the Dirac neutrino mass scenario and the seesaw scenario with degenerate right-handed majorana neutrino masses. It is found that realistic values of the charged lepton mass ratios, me/mτ and mμ/mτ, the neutrino mass squared difference ratio, Δm312/ Δm212, and the lepton mixing angles can be obtained in certain cases. © 2005 The American Physical Society. - N Kitazawa, T Kobayashi, N Maru, N OkadaEUROPEAN PHYSICAL JOURNAL C 40 4 579 - 587 2005年04月 [査読有り][通常論文]

The structure of Yukawa coupling matrices is investigated in type IIA T-6/(Z(2) x Z(2)) orientifold models with intersecting D-branes. Yukawa coupling matrices are difficult to be made realistic in conventional models in which the generation structure emerges by the multiple intersection of D-branes in the factorized T-6 = T-2 x T-2 x T-2. We study the new type of favor structure, where Yukawa couplings are dynamically generated, and show this type of models lead to non-trivial structures of Yukawa coupling matrices, which can be realistic. - T Kobayashi, H Nakano, H Terao, Y YamadaPROGRESS OF THEORETICAL PHYSICS 113 2 413 - 427 2005年02月 [査読有り][通常論文]

We consider a mechanism for realizing the desired decoupling of the strongly-coupled sector which is supposed to generate hierarchical structure of the Yukawa couplings. In our mechanism, the same strongly-coupled sector is responsible for generating a sufficiently flat potential and a large vacuum expectation value (VEV) of a gauge-singlet scalar field by suppressing its soft scalar mass and self-coupling. A vacuum instability is caused by a supersymmetry-breaking A-term of order 10 TeV. We explicitly demonstrate the infrared convergence of soft scalar masses due to strongly-coupled dynamics and show that the soft mass of the singlet is at most of the same order as the soft masses of squarks and sleptons, Which are much more strongly suppressed than the A-term. The physical mass scale of the decoupling is calculated in a self-consistent way. We also reinterpret the result in terms of an RG-improved effective potential. - Orbifold grand uni#12;ed theories from heterotic string compacti#12;cation on a Z6 orbifold 2005年 [査読有り][通常論文]
- T Kobayashi, K YoshiokaJOURNAL OF HIGH ENERGY PHYSICS 11 2004年11月 [査読有り][通常論文]

Higher-dimensional field theory has been applied to explore various issues in recent particle physics such as the gauge hierarchy problem. In order for such approaches to be viable, a crucial ingredient is to fix the sizes of extra dimensions at some finite values, which sizes are generically free parameters in the theory. In this paper, we present several schemes to determine the radius of extra dimension in warped five-dimensional theory. In every case, a non-vanishing Fayet-Iliopoulos term for abelian gauge factor plays a crucial role for the radius stabilization. It is radiatively generated in the presence of charged matter fields and the compactification is therefore spontaneous, not forced by selected operators. The low-energy supersymmetry is broken or unbroken, and the radius can be fixed to give a small or large scale hierarchy without any fine tuning of parameters. We also discuss a model of the radius stabilization correlated with Yukawa hierarchy and supersymmetry breaking. - P Ko, T Kobayashi, JH ParkPHYSICS LETTERS B 598 3-4 263 - 272 2004年09月 [査読有り][通常論文]

We study systematically the possibility for realizing realistic values of quark mass ratios m(c)/m(t) and m(s)/m(b) and the mixing angle V-cb by using only renormalizable Yukawa couplings derived from heterotic orbifold models. We assume one pair of up and down sector Higgs fields. We find many realistic Yukawa matrices. (C) 2004 Published by Elsevier B.V. - T Kobayashi, H TeraoJOURNAL OF HIGH ENERGY PHYSICS 7 2004年07月 [査読有り][通常論文]

We study the little hierarchy between mass parameters in the Higgs sector and other SUSY breaking masses. This type of spectrum can relieve the fine-tuning problem in the MSSM Higgs sector. Our scenario can be realized by superconformal dynamics. The spectrum in our scenario has significant implications in other phenomenological aspects like the relic abundance of the lightest neutralino and relaxation of the unbounded-from-below constraints. - T Higaki, T Kobayashi, O SetoJOURNAL OF HIGH ENERGY PHYSICS 7 2004年07月 [査読有り][通常論文]

We study the D-term inflation scenario with a nonperturbative Kahler potential of the dilaton field. Although the FI term which leads an inflationary expansion is given by the derivative of the Kahler potential with respect to the dilaton in heterotic string models with anomalous U(1), the too large magnitude is problematic for a viable D-term inflation. In this paper, we point out that the Kahler potential with a nonperturbative term can reduce the magnitude of FI term to desired values while both the dilaton stabilization and D-term domination in the potential are realized by nonperturbative superpotential. - T Kobayashi, S Raby, RJ ZhangPHYSICS LETTERS B 593 262 - 270 2004年07月 [査読有り][通常論文]

A three-generation Pati-Salam model is constructed by compactifying the heterotic string on a particular T-6/Z(6) Abelian symmetric orbifold with two discrete Wilson lines. The compactified space is taken to be the Lie algebra lattice G(2) circle plus SU(3) circle plus SO(4). When one dimension of the SO(4) lattice is large compared to the string scale, this model reproduces many features of a 5d SO(10) grand unified theory compactified on an S-1/Z(2) orbifold. (Of course, with two large extra dimensions we can obtain a 6d SO(10) grand unified theory.) We identify the orbifold parities and other ingredients of the orbifold grand unified theories in the string model. Our construction provides a UV completion of orbifold grand unified theories, and gives new insights into both field theoretical and string theoretical constructions. (C) 2004 Elsevier B.V. All rights reserved. - K Choi, DY Kim, IW Kim, T KobayashiEUROPEAN PHYSICAL JOURNAL C 35 2 267 - 275 2004年06月 [査読有り][通常論文]

We examine the soft supersymmetry breaking parameters in supersymmetric theories on a slice of AdS(5) which generate the hierarchical Yukawa couplings by dynamically quasi-localizing the bulk matter fields in an extra dimension. Such models can be regarded as the AdS dual of the recently studied 4-dimensional models which contain a supersymmetric CFT to generate the hierarchical Yukawa couplings. In such models, if supersymmetry breaking is mediated by the bulk radion superfield and/or some brane chiral superfields, potentially dangerous flavor-violating soft parameters are suppressed with an appropriate correlation with the Yukawa coupling suppression, thereby avoiding the SUSY flavor problem in a natural manner. We present some models of radion-dominated supersymmetry breaking which yield a highly predictive form of soft parameters in this framework, and discuss the constraints from flavor-changing rare processes. Most of the discussions in this paper can be applied also to models with a flat extra dimension in which the Yukawa hierarchy is generated by quasi-localizing the bulk matter fields in the extra dimension. - T Higaki, Y Kawamura, T Kobayashi, H NakanoPHYSICAL REVIEW D 69 8 2004年04月 [査読有り][通常論文]

We study the D-term contribution for anomalous U(1) symmetries in type I string models and derive a general formula for the D-term contribution, assuming that the dominant source of supersymmetry breaking is given by the F terms of the dilaton, (overall) moduli, or twisted moduli fields. On the basis of the formula, we also point out that there are several features different from the case of heterotic string models. The differences originate from the different forms of the Kahler potential between twisted moduli fields in type I string models and the dilaton field in heterotic string models. - T Higaki, Y Kawamura, T Kobayashi, H NakanoPHYSICS LETTERS B 582 3-4 257 - 262 2004年03月 [査読有り][通常論文]

We study the dilaton stabilization in models with anomalous U(l) symmetry by adding specific string-motivated, non-perturbative corrections to the tree-level dilaton Kahler potential. We find that the non-perturbative effects can stabilize the dilaton at a desirably large value. We also observe that the size of Fayet-Iliopoulos term is reduced at the stabilized point. (C) 2004 Published by Elsevier B.V. - Stefan Groot Nibbelink, Mark Hillenbach, Tatsuo Kobayashi, Martin G. A. WalterPhysical Review D - Particles, Fields, Gravitation and Cosmology 69 4 2004年02月13日 [査読有り][通常論文]

We investigate the structure of local anomalies of heterotic [Formula Presented] theory on [Formula Presented] We show that the untwisted states lead to anomalies in 10, four, and in particular six dimensions. At each of the six dimensional fixed spaces of this orbifold the twisted states ensure, that the anomalies factorize separately. As some of these twisted states live on [Formula Presented] they give rise to four dimensional anomalies as well. At all four dimensional fixed points at worst a single Abelian anomaly can arise. Additionally we show that the anomalies in all these dimensions factorize in a universal way and can be canceled simultaneously. © 2004 The American Physical Society. - SG Nibbelink, M Hillenbach, T Kobayashi, MGA WalterPHYSICAL REVIEW D 69 4 2004年02月 [査読有り][通常論文]

We investigate the structure of local anomalies of heterotic E(8)xE'(8) theory on T-6/Z(4). We show that the untwisted states lead to anomalies in 10, four, and in particular six dimensions. At each of the six dimensional fixed spaces of this orbifold the twisted states ensure, that the anomalies factorize separately. As some of these twisted states live on T-2/Z(2), they give rise to four dimensional anomalies as well. At all four dimensional fixed points at worst a single Abelian anomaly can arise. Additionally we show that the anomalies in all these dimensions factorize in a universal way and can be canceled simultaneously. - T Kobayashi, O SetoPHYSICAL REVIEW D 69 2 2004年01月 [査読有り][通常論文]

We investigate the possibility of D-term inflation within the framework of type-I string-inspired models. Although the D-term inflation model has the excellent property that it is free of the so-called eta problem, two serious problems appear when we embed D-term inflation in string theory: the magnitude of the FI term and the rolling motion of the dilaton. In the present paper, we analyze the potential of D-term inflation in type-I-inspired models and study the behavior of dilaton and twisted moduli fields. Adopting the nonperturbative superpotential induced by gaugino condensation, the twisted moduli can be stabilized. If the dilaton is in a certain range, it evolves very slowly and does not run away to infinity. Thus D-term-dominated vacuum energy becomes available for driving inflation. By studying the density perturbation generated by the inflation model, we derive the constraints on model parameters and give some implications on D-term inflation in type-I-inspired models. - Tetsutaro Higaki, Yoshiharu Kawamura, Tatsuo Kobayashi, Hiroaki NakanoPhysical Review D - Particles, Fields, Gravitation and Cosmology 69 8 9 2004年 [査読有り][通常論文]

We study the D-term contribution for anomalous [Formula Presented] symmetries in type I string models and derive a general formula for the D-term contribution, assuming that the dominant source of supersymmetry breaking is given by the F terms of the dilaton, (overall) moduli, or twisted moduli fields. On the basis of the formula, we also point out that there are several features different from the case of heterotic string models. The differences originate from the different forms of the Kähler potential between twisted moduli fields in type I string models and the dilaton field in heterotic string models. © 2004 The American Physical Society. - Tetsutaro Higaki, Yoshiharu Kawamura, Tatsuo Kobayashi, Hiroaki NakanoPhysical Review D 69 8 2004年 [査読有り][通常論文]

We study the D-term contribution for anomalous U(1) symmetries in type I string models and derive a general formula for the D-term contribution, assuming that the dominant source of supersymmetry breaking is given by the F terms of the dilaton, (overall) moduli, or twisted moduli fields. On the basis of the formula, we also point out that there are several features different from the case of heterotic string models. The differences originate from the different forms of the Kahler potential between twisted moduli fields in type I string models and the dilaton field in heterotic string models. © 2004 The American Physical Society. - Tatsuo Kobayashi, Jisuke Kubo, Haruhiko TeraoPhysics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics 568 1-2 83 - 91 2003年08月21日 [査読有り][通常論文]

The exact discrete flavor symmetry, S3 in the hadronic sector and S3 x Z2 in the leptonic sector, which has been recently found, is introduced in a supersymmetric extension of the standard model. We investigate the supersymmetric flavor problem, and explicitly find that thanks to the flavor symmetry the dangerous FCNC processes and CP-violating phases are sufficiently suppressed. © 2003 Elsevier B.V. All rights reserved. - T Higaki, T KobayashiPHYSICAL REVIEW D 68 4 2003年08月 [査読有り][通常論文]

We consider a model with dilaton and twisted moduli fields, which is inspired by type I string models. The stabilization of their vacuum expectation values is studied. We find that the stabilization of the twisted moduli field has different aspects from dilaton stabilization. - T Kobayashi, H Nakano, H Terao, K YoshiokaPROGRESS OF THEORETICAL PHYSICS 110 2 247 - 267 2003年08月 [査読有り][通常論文]

In this paper, we study flavor violation in supersymmetric models with gauged flavor symmetries. There are several sources of flavor violation in these theories. The dominant flavor violation is the tree-level D-term contribution to scalar masses generated by flavor symmetry breaking. We present a new approach for suppressing these phenomenologically dangerous effects by separating the flavor-breaking sector from the supersymmetry-breaking sector. This separation can be achieved either in geometrical setups or in a dynamical way. We also point out that radiative corrections from the gauginos of gauged flavor symmetries can yield sizable generation-dependent masses of scalars. Generically the gaugino mass effects are not suppressed, even if the dominant D-term contribution is suppressed. We also analyze the constraints on the flavor symmetry sector resulting from these flavor-violating corrections. - T Kobayashi, O LebedevPHYSICS LETTERS B 566 1-2 164 - 170 2003年07月 [査読有り][通常論文]

Realistic heterotic string models require the presence of the Wilson lines. We study the effect of continuous Wilson lines on the heterotic Yukawa couplings for Abelian orbifold compactifications and find that the presence of continuous Wilson lines affects the magnitude of the twisted Yukawa couplings resulting in their stronger hierarchy. (C) 2003 Published by Elsevier B.V. - T Kobayashi, O LebedevPHYSICS LETTERS B 565 1-4 193 - 200 2003年07月 [査読有り][通常論文]

In the framework of Z(N) orbifolds, we discuss effects of heterotic string backgrounds including discrete Wilson lines on the Yukawa matrices and their connection to CP violation. (C) 2003 Published by Elsevier B.V. - T Kobayashi, N Maru, K YoshiokaEUROPEAN PHYSICAL JOURNAL C 29 2 277 - 284 2003年07月 [査読有り][通常論文]

In this letter, we discuss a four-dimensional model with modulus fields which are responsible for supersymmetry breaking. Given a non-trivial moduli dependence of the action, the model is found to give a proper description of higher-dimensional supersymmetry breaking. We explicitly calculate the gaugino and scalar mass spectrum and show that several classes of scenarios proposed in the literature are described in certain regions of the parameter space of the moduli vacuum expectation values. The model in other generic regions of the moduli space gives unexplored scenarios (mass spectra) of supersymmetry breaking in four dimensions. - H Abe, T Higaki, T KobayashiPROGRESS OF THEORETICAL PHYSICS 109 5 809 - 830 2003年05月 [査読有り][通常論文]

We systematically study VEVs of a gauge scalar field Sigma in a bulk U(1) vector multiplet and scalar fields in brane/bulk hypermultiplets charged under U(1) in the 5D S-1/Z(2) orbifold model with generic FI terms. A non-trivial VEV of Sigma generates bulk mass terms for U(1) charged fields, and their zero modes have non-trivial profiles. In particular, in the SUSY-breaking case, bosonic and fermionic zero modes have Gaussian profiles. Such non-trivial profiles are useful to explain hierarchical couplings. A toy model for SUSY breaking is studied, and it yields sizable D-term contributions to scalar masses. Because the overall magnitude of D-term contributions is the same everywhere in the bulk and also on both branes, we have to take into account these contributions and other SUSY-breaking terms to obtain a realistic description. We also give profiles and mass eigenvalues of higher modes. - T Kobayashi, H Nakano, T Noguchi, H TeraoJOURNAL OF HIGH ENERGY PHYSICS 2 2003年02月 [査読有り][通常論文]

We propose a scenario that leads to hierarchical Yukawa couplings and degenerate sfermion masses at the same time, in the context of extra-dimensional models, which can be naturally embedded in a wide class of string models. The hierarchy of Yukawa couplings and degeneracy of sfermion masses can be realized thanks to superconformal gauge dynamics. The sfermion mass degeneracy is guaranteed by taking the superconformal fixed point to be family independent. In our scenario, the origin of Yukawa hierarchy is attributed to geometry of compactified dimensions and the consequent volume dependence of gauge couplings in the superconformal sectors. The difference in these gauge couplings is dynamically transferred to the hierarchy of the Yukawa couplings. Thus, our scenario combines a new dynamical approach and the conventional geometrical approach of the supersymmetric flavor problem. - H Abe, T Kobayashi, N Maru, K YoshiokaPHYSICAL REVIEW D 67 4 2003年02月 [査読有り][通常論文]

We present four-dimensional gauge theories that describe physics on five-dimensional curved (warped) backgrounds, which includes bulk fields with various spins (vectors, spinors, and scalars). Field theory on the AdS(5) geometry is examined as a simple example of our formulation. Various properties of bulk fields on this background, e.g., the mass spectrum and field localization behavior, can be achieved within a fully four-dimensional framework. Moreover, that gives a localization mechanism for massless vector fields. We also consider supersymmetric cases, and show in particular that the conditions on bulk masses imposed by supersymmetry on warped backgrounds are derived from a four-dimensional supersymmetric theory on the flat background. As a phenomenological application, models are shown to generate hierarchical Yukawa couplings. Finally, we discuss possible underlying mechanisms which dynamically realize the required couplings to generate curved geometries. - K. S. Babu, Tatsuo Kobayashi, Jisuke KuboPhysical Review D - Particles, Fields, Gravitation and Cosmology 67 7 2003年 [査読有り][通常論文]

We study a finite [Formula Presented] grand unified model based on the non-Abelian discrete symmetry [Formula Presented] This model leads to the democratic structure of the mass matrices for the quarks and leptons. In the soft supersymmetry breaking sector, the scalar trilinear couplings are aligned and the soft scalar masses are degenerate, thus solving the SUSY flavor problem. © 2003 The American Physical Society. - Hiroyuki Abe, Tatsuo Kobayashi, Nobuhito Maru, Koichi YoshiokaPhysical Review D - Particles, Fields, Gravitation and Cosmology 67 4 2003年 [査読有り][通常論文]

We present four-dimensional gauge theories that describe physics on five-dimensional curved (warped) backgrounds, which includes bulk fields with various spins (vectors, spinors, and scalars). Field theory on the (Formula presented) geometry is examined as a simple example of our formulation. Various properties of bulk fields on this background, e.g., the mass spectrum and field localization behavior, can be achieved within a fully four-dimensional framework. Moreover, that gives a localization mechanism for massless vector fields. We also consider supersymmetric cases, and show in particular that the conditions on bulk masses imposed by supersymmetry on warped backgrounds are derived from a four-dimensional supersymmetric theory on the flat background. As a phenomenological application, models are shown to generate hierarchical Yukawa couplings. Finally, we discuss possible underlying mechanisms which dynamically realize the required couplings to generate curved geometries. © 2003 The American Physical Society. - Tatsuo KobayashiPhysical Review D - Particles, Fields, Gravitation and Cosmology 65 1 2002年12月11日 [査読有り][通常論文]

We propose a new type of supersymmetric model coupled to superconformal (SC) field theories, leading simultaneously to hierarchical Yukawa couplings and completely degenerate sfermion masses. We consider models with an extra Abelian gauge symmetry to generate hierarchical structure for couplings between the standard model (SM) sector and the SC sector. Interestingly, this hierarchy is inversely transferred to the Yukawa couplings in the SM sector. In this type of model, the flavor-independent structure of the superconformal fixed point guarantees that the sfermion masses of the first and the second generations are completely degenerate at low energy. © 2001 The American Physical Society. - Tatsuo Kobayashi, Hiroaki Nakano, Tatsuya Noguchi, Haruhiko TeraoPhysical Review D - Particles, Fields, Gravitation and Cosmology 66 9 2002年11月25日 [査読有り][通常論文]

We discuss issues in a scenario where hierarchical Yukawa couplings are generated through the strong dynamics of superconformal field theories (SCFTs). Independently of the mediation mechanism of supersymmetry breaking, the infrared convergence property of SCFTs can provide an interesting solution to the supersymmetric flavor problem sfermion masses are suppressed around the decoupling scale of SCFTs and eventually become degenerate to some degree, thanks to family-independent radiative corrections governed by the gaugino masses of the minimal supersymmetric standard model (MSSM). We discuss under what conditions the degeneracy of the sfermion mass can be estimated in a simple manner. We also discuss the constraints from lepton flavor violations. We then explicitly study sfermion mass degeneracy within the framework of grand unified theories coupled to SCFTs. It is found that the degeneracy for right-handed sleptons becomes worse in the conventional SU(5) model than in the MSSM. On the other hand, in the flipped (Formula presented) model, each right-handed lepton is still an SU(5) singlet, whereas the B-ino mass (Formula presented) is determined by two independent gaugino masses of (Formula presented) These two properties enable us to have an improved degeneracy for the right-handed sleptons. We also speculate on how further improvement can be obtained in the SCFT approach. © 2002 The American Physical Society. - T Kobayashi, H Nakano, T Noguchi, H TeraoPHYSICAL REVIEW D 66 9 2002年11月 [査読有り][通常論文]

We discuss issues in a scenario where hierarchical Yukawa couplings are generated through the strong dynamics of superconformal field theories (SCFTs). Independently of the mediation mechanism of supersymmetry breaking, the infrared convergence property of SCFTs can provide an interesting solution to the supersymmetric flavor problem; sfermion masses are suppressed around the decoupling scale of SCFTs and eventually become degenerate to some degree, thanks to family-independent radiative corrections governed by the gaugino masses of the minimal supersymmetric standard model (MSSM). We discuss under what conditions the degeneracy of the sfermion mass can be estimated in a simple manner. We also discuss the constraints from lepton flavor violations. We then explicitly study sfermion mass degeneracy within the framework of grand unified theories coupled to SCFTs. It is found that the degeneracy for right-handed sleptons becomes worse in the conventional SU(5) model than in the MSSM. On the other hand, in the flipped SU(5)xU(1) model, each right-handed lepton is still an SU(5) singlet, whereas the B-ino mass M-1 is determined by two independent gaugino masses of SU(5)xU(1). These two properties enable us to have an improved degeneracy for the right-handed sleptons. We also speculate on how further improvement can be obtained in the SCFT approach. - T Kobayashi, H TeraoPROGRESS OF THEORETICAL PHYSICS 107 4 785 - 792 2002年04月 [査読有り][通常論文]

We study the so-called "'Kaluza-Klein regularization". We calculate one-loop corrections to the Higgs mass due to Kaluza-Klein modes explicitly in a model with SUSY breaking mass splitting between bosonic and fermionic modes. We perform the proper time cutoff at 1/Lambda(2) and the KK level truncation at l. It is shown that a finite result is obtained as long as l >> RLambda for the compactification radius R. - T Kobayashi, H Nakano, H TeraoPHYSICAL REVIEW D 65 1 2002年01月 [査読有り][通常論文]

We propose a new type of supersymmetric model coupled to superconformal (SC) field theories, leading simultaneously to hierarchical Yukawa couplings and completely degenerate sfermion masses. We consider models with an extra Abelian gauge symmetry to generate hierarchical structure for couplings between the standard model (SM) sector and the SC sector. Interestingly, this hierarchy is inversely transferred to the Yukawa couplings in the SM sector. In this type of model, the flavor-independent structure of the superconformal fixed point guarantees that the sfermion masses of the first and the second generations are completely degenerate at low energy. - Tatsuo Kobayashi, Hiroaki Nakano, Haruhiko TeraoPhysical Review D 65 1 2002年 [査読有り][通常論文]

We propose a new type of supersymmetric model coupled to superconformal (SC) field theories, leading simultaneously to hierarchical Yukawa couplings and completely degenerate sfermion masses. We consider models with an extra Abelian gauge symmetry to generate hierarchical structure for couplings between the standard model (SM) sector and the SC sector. Interestingly, this hierarchy is inversely transferred to the Yukawa couplings in the SM sector. In this type of model, the flavor-independent structure of the superconformal fixed point guarantees that the sfermion masses of the first and the second generations are completely degenerate at low energy. © 2001 The American Physical Society. - T Kobayashi, H TeraoPHYSICAL REVIEW D 64 7 2001年10月 [査読有り][通常論文]

We study soft supersymmetric (SUSY) breaking parameters in the Nelson-Strassler-type models: SUSY standard models coupled with superconformal field theories (SCFT's). In this type of model, soft SUSY breaking parameters including sfermion masses can be suppressed around the decoupling scale of SCFT's. We clarify the condition to derive exponential suppression of sfermion masses within the framework of pure SCFT's. Such behavior is favorable for degeneracy of sfermion masses. However, the realistic sfermion masses are not quite degenerate due to the gauge couplings and the gaugino masses in the standard model sector. We show the sfermion mass spectrum obtained in such models. The aspect of suppression for the soft SUSY breaking parameters is also demonstrated in an explicit model. We also give a mechanism generating the mu term of the electroweak scale by a singlet field coupled with the SCFT. - M Bando, T Kobayashi, T Noguchi, K YoshiokaPHYSICAL REVIEW D 63 11 2001年06月 [査読有り][通常論文]

In this paper we study renormalization-group evolutions of Yukawa matrices enhanced by Kaluza-Klein excited modes and analyze their infrared fixed-point structure. We derive the necessary conditions to obtain hierarchies between generations on the fixed point. These conditions restrict how the fields in the models can extend to higher dimensions. Several specific mechanisms to realize the conditions are presented. We also take into account the generation mixing effects and find a scenario where the mixing angles become small at low energy even with large initial values at a high-energy scale. A toy model is shown to lead to realistic quark mass matrices. - M Drees, YG Kim, T Kobayashi, MM NojiriPHYSICAL REVIEW D 63 11 2001年06月 [査読有り][通常論文]

We compare predictions for the spin-independent contribution to the neutralino-proton scattering cross section sigma (xp) and for the anomalous magnetic dipole moment of the muon, a(mu)= (g(mu) - 2)/2, in models with gravity-mediated supersymmetry breaking. We nearly always find a positive correlation between these two measurables; i.e., scenarios with larger a(mu) also tend to have larger sigma (xp), but the detailed prediction differs greatly between models. Tn particular, we find that for the popular MSUGRA scenario with universal soft breaking masses at the scale of grand unification, measurements of a(mu) currently seem more promising. On the other hand, if scalar soft breaking masses at the GUT scale receive sizable contributions from SO(10) D terms, one often finds scenarios with large sigma (xp) but a(mu) below the currently foreseen sensitivity. A string-inspired model with a nonuniversal scalar spectrum at the GUT scale falls between these two cases. - T Kobayashi, O VivesPHYSICS LETTERS B 506 3-4 323 - 330 2001年05月 [査読有り][通常論文]

We analyze the flavor structure of the trilinear couplings in the different theoretical models of SUSY breaking. We generically obtain A(ij) = A(i)(L) + A(j)(R) in all the models examined. In fact, this is the rigorous form when SUSY breaking effects appear through the Kahler metric of chiral fields or through wave-function renormalization. Indeed, the low-energy phenomenological requirements from the absence of charge and color breaking minima and the measurements in flavor changing neutral current (FCNC) observables strongly favor this restricted form of the trilinear matrices. As a straightforward consequence the number of unknown parameters associated with the trilinear couplings is decreased in a factor of 2. (C) 2001 Elsevier Science B.V. All rights reserved. - T KobayashiPHYSICS LETTERS B 506 1-2 187 - 192 2001年05月 [査読有り][通常論文]

We study soft SUSY breaking terms in heterotic M-theory. We show that both weakly and strongly coupled heterotic string models lead to the same relations of soft SUSY breaking terms, A = -M and m(2) = M-2/3, up to O((alphaT/S)(2)). (C) 2001 Published by Elsevier Science B.V. - T Kobayashi, J Kubo, M Mondragon, G ZoupanosINTERNATIONAL JOURNAL OF MODERN PHYSICS A 16 11 2053 - 2057 2001年04月 [査読有り][通常論文]

Finite Unified Theories are N = 1 supersymmetric GUT's that can be made finite beyond the unification point, including the softly broken sector. The new characteristic predictions of FUTs are: 1) The lightest Higgs boson mass is predicted to be in the window 120-130 GeV, in case the LSP is neutralino, while in case the LSP is the <(<tau>)over bar> (which can be consistently accommodated in presence of bilinear R-parity violating terms) it can be as light as 111 GeV. 2) The s-spectrum starts above several hundreds of GeV. - SUSY standard models coupled with SCFTs 2001年 [査読有り][通常論文]
- Tatsuo KobayashiPhysical Review D - Particles, Fields, Gravitation and Cosmology 64 7 15 2001年 [査読有り][通常論文]

We study soft supersymmetric (SUSY) breaking parameters in the Nelson-Strassler-type models: SUSY standard models coupled with superconformal field theories (SCFT’s). In this type of model, soft SUSY breaking parameters including sfermion masses can be suppressed around the decoupling scale of SCFT’s. We clarify the condition to derive exponential suppression of sfermion masses within the framework of pure SCFT’s. Such behavior is favorable for degeneracy of sfermion masses. However, the realistic sfermion masses are not quite degenerate due to the gauge couplings and the gaugino masses in the standard model sector. We show the sfermion mass spectrum obtained in such models. The aspect of suppression for the soft SUSY breaking parameters is also demonstrated in an explicit model. We also give a mechanism generating the μ term of the electroweak scale by a singlet field coupled with the SCFT. © 2001 The American Physical Society. - Masako Bando, Tatsuo Kobayashi, Tatsuya Noguchi, Koichi YoshiokaPhysical Review D - Particles, Fields, Gravitation and Cosmology 63 11 9 2001年 [査読有り][通常論文]

In this paper we study renormalization-group evolutions of Yukawa matrices enhanced by Kaluza-Klein excited modes and analyze their infrared fixed-point structure. We derive the necessary conditions to obtain hierarchies between generations on the fixed point. These conditions restrict how the fields in the models can extend to higher dimensions. Several specific mechanisms to realize the conditions are presented. We also take into account the generation mixing effects and find a scenario where the mixing angles become small at low energy even with large initial values at a high-energy scale. A toy model is shown to lead to realistic quark mass matrices. © 2001 The American Physical Society. - T Kobayashi, K YoshiokaPHYSICAL REVIEW LETTERS 85 26 5527 - 5530 2000年12月 [査読有り][通常論文]

The four-dimensional effective theories with compactified extra dimensions are considered. We then discuss a new framework for communicating supersymmetry breaking to the visible sector where the radius modulus, which determines the size of extra dimensions, has an auxiliary vacuum expectation value. The modulus couplings generate mass splitting in Kaluza-Klein supermultiplets and they act as messengers of supersymmetry breaking. The soft mas ses are expressed in terms of renormalization-group functions and the sparticle spectrum is determined by what kind of field propagates in the bulk. - T Kobayashi, H TeraoPHYSICS LETTERS B 489 1-2 233 - 242 2000年09月 [査読有り][通常論文]

We examine the (softly broken) supersymmetric gauge-Higgs-Yukawa theories satisfying the compositeness conditions at a certain scale. In these theories the Higgs superfields can be regarded as the chiral composite fields. It is found that there are the fundamental theories, which contain the dimension 5 interactions and the hard SUSY breaking in perturbation, turn to be renormalizable and also softly broken theories in the nonperturbative framework. The soft SUSY breaking parameters as well as the Yukawa coupling in the corresponding gauge-Higgs-Yukawa theories are restricted by the renormalization group invariant relations. (C) 2000 Published by Elsevier Science B.V. - T Kobayashi, K YoshiokaPHYSICS LETTERS B 486 1-2 223 - 227 2000年07月 [査読有り][通常論文]

We study new renormalization-group invariant quantities of soft supersymmetry breaking parameters other than the ratio of gaugino mass to gauge coupling squared by using the spurion method. The obtained invariants are useful to probe supersymmetry breaking and mu-term generation mechanisms at high-energy scale. We also discuss the convergence behavior of fixed points of supersymmetry breaking parameters. (C) 2000 Elsevier Science B.V. All rights reserved. - T Kobayashi, J Kubo, H ShimabukuroNUCLEAR PHYSICS B 580 1-2 3 - 28 2000年07月 [査読有り][通常論文]

We study soft SUSY breaking parameters in M-theory with and without 5-brane moduli fields. We investigate successful electroweak symmetry breaking and the positivity of stau mass squared. A value of alpha(T + (T) over bar) is phenomenologically constrained. Also we have phenomenological constraints on magnitudes of derivatives of the Kahler potential for the 5-brane moduli field. We study mass spectra in the allowed regions. (C) 2000 Elsevier Science B.V. All rights reserved. - S Khalil, T Kobayashi, O VivesNUCLEAR PHYSICS B 580 1-2 275 - 288 2000年07月 [査読有り][通常論文]

Non-universality in the soft breaking terms is a common feature in most superstring inspired SUSY models. This property is required to obtain sizeable CP violation effects from SUSY and, on the other hand, can be used to avoid the electric dipole moment (EDM) constraints. We take advantage of these qualities and explore a class of SUSY models based on type I string theory where scalar masses, gaugino masses and trilinear couplings are non-universal. In this framework, we show that, in the presence of large SUSY phases, the bounds on the electric dipole moments can be controlled without fine-tuning. At the same time, we find that these phases, free from EDM constraints, lead to large contributions to the observed CP phenomena in Kaon system and, in particular, to direct CP violation in epsilon'/epsilon. (C) 2000 Elsevier Science B.V. All rights reserved. - M Bando, T Kobayashi, T Noguchi, K YoshiokaPHYSICS LETTERS B 480 1-2 187 - 192 2000年05月 [査読有り][通常論文]

We discuss the existence of hierarchy of Yukawa couplings in the models with extra spatial dimensions. The hierarchical structure is induced by the power behavior of the cutoff dependence of the evolution equations which yield large suppressions of couplings at the compactification scale. The values of coupling constants at this scale can be made stable almost independently of the initial input parameters by utilizing the infrared fixed point. We find that the Yukawa couplings converge to the fixed points very quickly because of the enhanced energy dependence of the suppression factor from extra dimensions as well as in the case of large gauge couplings at high-energy scale. (C) 2000 Published by Elsevier Science B.V. All rights reserved. - M Chaichian, T KobayashiPHYSICS LETTERS B 481 1 26 - 32 2000年05月 [査読有り][通常論文]

We compare two approaches in supersymmetric confinement, the Seiberg formulation and the superconvergence rule. For the latter, the critical point is gamma(00) = 0 in the Landau gauge We find 4 gamma(00) = beta(0) is the critical point for most of confining theories without a tree-level superpotential in the Seiberg formulation, in particular, in the large N-c and N-f limit. We show how confining theories with a discrete symmetry and a tree-level superpotential connect these two critical points: the large and small discrete symmetry limits correspond to the critical points 4 gamma(00) = beta(0) and gamma(00) = 0 respectively. (C) 2000 Elsevier Science B.V. All rights reserved. - K Huitu, Y Kawamura, T Kobayashi, K PuolamakiHIGH ENERGY PHYSICS 99, PROCEEDINGS 786 - 788 2000年 [査読有り][通常論文]

We study phenomenological aspects of supersymmetric SU(5) grand unified theories with non-universal gaugino masses. For large tan beta, we investigate constraints from the requirement of successful electroweak symmetry breaking, the positivity of stau mass squared and the b --> s gamma decay rate. We also calculate loop corrections to the bottom mass due to superpartners. - Y Kawamura, T Kobayashi, M WatanabeINTERNATIONAL JOURNAL OF MODERN PHYSICS A 15 1 81 - 103 2000年01月 [査読有り][通常論文]

We study mu and B-parameters in the minimal supersymmetric standard model (MSSM) based on the radiative electroweak symmetry breaking scenario using "bottom-up" approach and show how useful our approach is to select a phenomenologically viable model beyond the MSSM under the assumption that the underlying theory is a string model or a gauge-Yukawa unified gauge model. - T Kobayashi, J Kubo, M Mondragon, G ZoupanosHIGH ENERGY PHYSICS 99, PROCEEDINGS 804 - 806 2000年 [査読有り][通常論文]

The search for renormalization group invariant (RGI) relations among the couplings of GUTs holding beyond the unification point leads to the predictive scheme of Gauge-Yukawa Unified Theories. In this class of theories, of particular interest are those GUTs which are finite beyond the unification paint even to all orders in perturbation theory, i.e. exactly finite. Extending the search to the soft supersymmetry-breaking (SSB) sector of these theories we obtain exact RGI relations and exact finite theories also in this sector. Taking into account several phenomenological constraints we obtain an accurate prediction of the standard model (SM) Higgs mass in addition to the previous successful prediction of the top quark mass. Another interesting and distinct feature of these models is that their predictions for the spectrum beyond the SM starts above several hundreds of GeV. - K Huitu, T KobayashiPHYSICS LETTERS B 470 1-4 90 - 94 1999年12月 [査読有り][通常論文]

We study the gauge coupling unification with extra dimensions. We Lake into account corrections due to the higher dimensional operators. We show the prediction of alpha(3)(M-Z) is sensitive to such corrections, even if c(Phi)/M = O(0.01). We also discuss the b - tau Yukawa unification. (C) 1999 Published by Elsevier Science B.V. All rights reserved. - K Huitu, Y Kawamura, T Kobayashi, K PuolamakiPHYSICS LETTERS B 468 1-2 111 - 117 1999年11月 [査読有り][通常論文]

We study non-universal corrections to the gauge couplings due to higher dimensional operators in supersymmetric SU(5) grand unified theories. The corrections are, in general, parametrized by three components originating from 24, 75 and 200 representations. We consider the prediction of alpha(3)(M-z) along each 24, 75 and 200 direction, and their linear combinations. The magnitude of GUT scale and its effects on proton decay are discussed. Non-SUSY case is also examined. (C) 1999 Published by Elsevier Science B.V. All rights reserved. - SM Harun-or-Rashid, T Kobayashi, H ShimabukuroPHYSICS LETTERS B 466 2-4 95 - 99 1999年11月 [査読有り][通常論文]

We study F-term inflation in M-theory with and without five-brane moduli fields. We show the slow rolling condition is not satisfied in M-theory without five-brane moduli fields, but it can be satisfied in the case with non-vanishing F-terms of five-brane moduli fields. (C) 1999 Elsevier Science B.V. All rights reserved. - S Khalil, T Kobayashi, A MasieroPHYSICAL REVIEW D 60 7 1999年10月 [査読有り][通常論文]

We study the CP phases of the soft supersymmetry breaking terms in string-inspired models with nonuniversal trilinear couplings. We show that such nonuniversality plays an important role on all CP violating processes. In particular these new supersymmetric sources of CP violation may significantly contribute to the observed CP phenomena in kaon physics while respecting the severe bound on the electric dipole moment of the neutron. [S0556-2821(99)03217-8]. - S Khalil, T KobayashiPHYSICS LETTERS B 460 3-4 341 - 347 1999年08月 [査読有り][通常論文]

We study contributions of supersymmetric CP phases to the CP violation epsilon'/epsilon in models with asymmetric A-matrices. We consider asymmetric A-matrices, which are obtained from string-inspired supergravity. We show that a certain type of asymmetry of A-matrices enhances supersymmetric contributions to the CP violation epsilon'/epsilon and the supersymmetric contribution to epsilon'/epsilon can be of order of the KTeV result, epsilon'/epsilon similar to 10(-3). (C) 1999 Elsevier Science B.V. All rights reserved. - T Kobayashi, J Kubo, M Mondragon, G ZoupanosNUCLEAR PHYSICS B 550 1-2 99 - 122 1999年06月 [査読有り][通常論文]

The evolution of the parameters including those in the soft supersymmetry-breaking (SSB) sector is studied in the minimal supersymmetric standard model (MSSM) with a certain set of Kaluza-Klein towers which has been recently considered by Dienes et al. We use the continuous Wilson renormalization group technique to derive the one-loop matching condition between the effective, renormalizable and original, unrenormalizable theories. We investigate whether the assumption of a large compactification radius in the model is consistent with the gauge coupling unification, the b-tau unification and the radiative breaking of the electroweak gauge symmetry with the universal SSB terms. We calculate the superpartner spectrum under the assumption of the universal SSB parameters to find differences between the model and the MSSM. (C) 1999 Elsevier Science B.V. All rights reserved. - T Kobayashi, J Kubo, M Mondragon, G ZoupanosACTA PHYSICA POLONICA B 30 6 2013 - 2027 1999年06月 [査読有り][通常論文]

The recent developments in the soft supersymmetry breaking (SSB) sector of Gauge-Yukawa and Finite Unified Theories permit the derivation of exact renormalization group invariant results also in this sector of the theory. Of particular interest is a RGI sum rule for the soft scalar masses holding to all-orders in perturbation theory. In the case of Finite Unified Theories the sum rule ensures the all-loop finiteness also in their SSB sector and in this way are promoted to completely finite ones. Using the sum rule we investigate the minimal supersymmetric Gauge-Yukawa and two Finite-Gauge-Yukawa SU(5) models. The characteristic features of these models are: a) the old agreement of the top quark mass prediction remains unchanged, b) the lightest Higgs boson is predicted to be around 120 GeV; c) the s-spectrum starts above several hundreds of GeV. - T Kobayashi, J Kubo, M Mondragon, G ZoupanosPARTICLES AND FIELDS 490 279 - 309 1999年 [査読有り][通常論文]

All - loop Finite Unified Theories (FUTs) are very interesting N = 1 GUTs in which a complete reduction of couplings has been achieved. FUTs realise an old field theoretical dream and have remarkable predictive power. Reduction of dimensionless couplings in N = 1 GUTs is achieved by searching for renormalization group invariant (RGI) relations among them holding beyond the unification scale. Finiteness results from the fact that there exists RGI relations among dimensionless couplings that guarantee the vanishing of the beta - functions in certain N = 1 supersymmetric GUTS even to all orders. Recent developments in the soft supersymmetry breaking (SSB) sector of N = 1 GUTs and FUTs lead to exact RGI relations also in this sector of the theories. Of particular interest is a RGI sum rule for the sok scaler masses holding to all orders. The characteristic features of SU(5) models that have been constructed based on the above tools are: a) the old agreement of the top quark prediction with the measured value remains unchanged, b) the lightest Higgs boson is predicted to be around 120 GeV, c) the s - spectrum starts above several hundreds of GeV. - M Chaichian, T KobayashiPHYSICS LETTERS B 442 1-4 192 - 198 1998年12月 [査読有り][通常論文]

We study softly broken N = 1 supersymmetric QCD with the gauge group SU(N-c) and N-f flavors of quark pairs. We investigate vacuum structure of the theory with generic soft supersymmetry breaking terms. Trilinear soft breaking terms play an essential role in determining vacua. For N-f = N-c + 1, chiral symmetry is broken for a sufficiently large magnitude of trilinear couplings, while it is unbroken in the case with only soft masses. In the case where appearance of trilinear coupling terms is allowed, i.e. for N-f greater than or equal to N-c + 1, we have two possible vacua, the trivial and non-trivial ones. Otherwise, we only have the non-trivial vacuum. which corresponds to the non-trivial vacuum in the N-f greater than or equal to N-c + 1 theory. (C) 1998 Elsevier Science B.V. All rights reserved. - M Chaichian, T KobayashiPHYSICS LETTERS B 442 1-4 192 - 198 1998年12月 [査読有り][通常論文]

We study softly broken N = 1 supersymmetric QCD with the gauge group SU(N-c) and N-f flavors of quark pairs. We investigate vacuum structure of the theory with generic soft supersymmetry breaking terms. Trilinear soft breaking terms play an essential role in determining vacua. For N-f = N-c + 1, chiral symmetry is broken for a sufficiently large magnitude of trilinear couplings, while it is unbroken in the case with only soft masses. In the case where appearance of trilinear coupling terms is allowed, i.e. for N-f greater than or equal to N-c + 1, we have two possible vacua, the trivial and non-trivial ones. Otherwise, we only have the non-trivial vacuum. which corresponds to the non-trivial vacuum in the N-f greater than or equal to N-c + 1 theory. (C) 1998 Elsevier Science B.V. All rights reserved. - A Karch, T Kobayashi, J Kubo, G ZoupanosPHYSICS LETTERS B 441 1-4 235 - 242 1998年11月 [査読有り][通常論文]

Applying the recently obtained results on the renormalization of soft supersymmetry-breaking parameters, we investigate the infrared behaviour of the softly broken supersymmetric QCD as well as its dual theory in the conformal window. Under general assumptions on beta-functions, it is shown that the soft supersymmetry-breaking parameters asymptotically vanish in the infrared limit so that superconformal symmetry in softly broken supersymmetric QCD and in its dual theory revives at the infrared fixed point, provided the soft scalar masses satisfy certain renormalization group invariant relations. If these relations are not satisfied, there exist marginal operators in both theories that lead to the breaking of supersymmetry and also colour symmetry. (C) 1998 Elsevier Science B.V. All rights reserved. - Y Kawamura, T Kobayashi, H ShimabukuroPHYSICS LETTERS B 436 1-2 108 - 116 1998年09月 [査読有り][通常論文]

We study phenomenological aspects of supersymmetric SO(10) GUTs with sum rules among soft SUSY breaking parameters, In particular, the sum rule related to the stau mass leads to the constraints from the requirements of successful electroweak breaking and the positivity of stau mass squared. The bottom quark mass is also estimated. (C) 1998 Elsevier Science B.V. All rights reserved. - S Khalil, T KobayashiNUCLEAR PHYSICS B 526 1-3 99 - 114 1998年08月 [査読有り][通常論文]

We study Yukawa unification in string models with moduli-dominant SUSY breaking. This type of SUSY breaking in general leads to non-universal soft masses, i.e. soft scalar masses and gaugino masses. Such non-universality is important for phenomenological aspects of Yukawa unification, i.e., successful electroweak breaking, SUSY corrections to the bottom mass and the branching ratio of b --> s gamma, We show three regions in the whole parameter space that lead to successful electroweak breaking and allow small SUSY corrections to the bottom mass. For these three regions we investigate the b --> s gamma decay and mass spectra, (C) 1998 Elsevier Science B.V. - Y Kawamura, T Kobayashi, J KuboPHYSICS LETTERS B 432 1-2 108 - 113 1998年07月 [査読有り][通常論文]

Assuming that the universal sum rules for soft supersymmetry-breaking terms, which have been recently found both in four-dimensional superstring models and in supersymmertic gauge-Yukawa unified gauge models, are satisfied above and at the grand unification scale, we investigate their low energy consequences and derive sum rules in the superpartner spectrum of the minimal supersymmetric standard model (MSSM). These sum rules contain only observable parameters of the MSSM, and could be experimentally verified in future. (C) 1998 Elsevier Science B.V. All rights reserved. - T Kobayashi, ZZ XingINTERNATIONAL JOURNAL OF MODERN PHYSICS A 13 13 2201 - 2215 1998年05月 [査読有り][通常論文]

Four simple but realistic patterns of quark mass matrices are derived from orbifold models of superstring theory in the absence of gauge symmetries. Two of them correspond to the Ramond-Roberts-Ross types, which have five texture zeros in up and down quark sectors. The other two, with four texture zeros, preserve the structural parallelism between up and down sectors. The phenomenological consequences of these mass matrices on flavor mixings and CP violation are analyzed at the weak scale. With the same input values of quark mass ratios, we find that only one or two of the four patterns can be in good agreement with current experimental data. - T Kobayashi, J Kubo, G ZoupanosPHYSICS LETTERS B 427 3-4 291 - 299 1998年05月 [査読有り][通常論文]

It is proven that the recently found, renormalization-group invariant sum rule for the soft scalar masses in softly-broken N=l supersymmetric gauge-Yukawa unified theories can be extended to all orders in perturbation theory. Ln the case of finite unified theories, the sum rule ensures the all-loop finiteness in the soft supersymmetry breaking sector. As a byproduct the exact beta function for the soft scalar masses in the Novikov-Shifman-Vainstein-Zakharov (NSVZ) scheme for softly-broken supersymmetric QCD is obtained. It is also found that the singularity appearing in the sum rule in the NSVZ scheme exactly coincides with that which has been previously found in a certain class of superstring models in which the massive string states are organized into N=4 supermultiplets. (C) 1998 Elsevier Science B.V. All rights reserved. - Y Kawamura, T Kobayashi, M WatanabePHYSICS LETTERS B 419 1-4 157 - 166 1998年02月 [査読有り][通常論文]

We give a generic method to select a realistic mu-term generation mechanism based on the radiative electroweak symmetry breaking scenario and study which type is hopeful within the framework of string theory. We discuss effects of the moduli F-term condensation and D-term contribution to soft scalar masses. (C) 1998 Elsevier Science B.V. - T Kobayashi, J Kubo, M Mondragon, G ZoupanosNUCLEAR PHYSICS B 511 1-2 45 - 68 1998年02月 [査読有り][通常論文]

Requiring the soft supersymmetry-breaking (SSB) parameters in finite gauge-Yukawa unified models to be finite up to and including two-loop order, we derive a two-loop sum rule for the soft scalar masses, It is shown that this sum rule coincides with that of a certain class of string models in which the massive string states are organized into N = 4 supermultiplets. We investigate the SSB sector of two finite SU(5) models. Using the sum rule which allows non-universality of the SSB terms and requiring that the lightest superparticle is neutral, we constrain the parameter space of the SSB sector in each model. (C) 1998 Elsevier Science B.V. - T Kobayashi, J Kubo, M Mondragon, G ZoupanosBEYOND THE STANDARD MODEL 464 - 471 1998年 [査読有り][通常論文]

Gauge-Yukawa unified theories, and in particular those which are finite beyond the unification scale, have been extended to include a soft supersymmetry breaking (SSB) sector. In the case of the Finite Unified Theories a new solution to the condition of two-loop finiteness of the SSB parameters is found, which requires a sum rule for the relevant scalar masses. The sum rule permits violations of the universality of the scalar masses which is found to lead to phenomenological problems. The minimal supersymmetric SU(5) Gauge-Yukawa model and two Finite-Gauge-Yukawa models have been examined using the sum rule. The characteristic features of these models are: a) the old agreement of the top quark mass prediction persists using the new data, b) the lightest Higgs boson is predicted to be around 120 GeV, c) the s-spectrum starts above 200 GeV. - N Aizawa, T Kobayashi, HT SatoINTERNATIONAL JOURNAL OF MODERN PHYSICS A 12 32 5867 - 5883 1997年12月 [査読有り][通常論文]

Based on the quantum superspace construction of q-deformed algebra, we discuss a supersymmetric extension of the deformed Virasoro algebra, which is a subset of the q-W-infinity algebra which recently appeared in the context of two-dimensional string theory. We analyze two types of deformed super Virasoro algebra as well as their osp(1,2) subalgebras. Applying our quantum superspace structure to conformal field theory, we find the same type of deformation of affine sl(2) algebra. - Y Kawamura, T KobayashiPHYSICAL REVIEW D 56 7 3844 - 3859 1997年10月 [査読有り][通常論文]

We derive a formula of soft supersymmetry-breaking scalar masses from four-dimensional string models within a more generic framework. We consider the effects of extra gauge symmetry breaking including an anomalous U(1) breaking through flat directions, that is, D-term and F-term contributions, particle mixing effects, and heavy-light mass mixing effects. Some phenomenological implications are discussed based on our mass formula. - Y Kawamura, S Khalil, T KobayashiNUCLEAR PHYSICS B 502 1-2 37 - 58 1997年09月 [査読有り][通常論文]

We study moduli-dominated SUSY breaking within the framework of string models. This type of SUSY breaking in general leads to non-universal soft masses, i.e. soft scalar masses and gaugino masses. Further gauginos are Lighter than sfermions. This non-universality has phenomenologically important implications. We investigate radiative electroweak symmetry breaking in the mass spectrum derived from moduli-dominated SUSY breaking, where the lightest chargino and neutralino are almost gauginos. Moreover, constraints from the branching ratio of b --> s gamma and the relic abundance of the LSP are also considered. The mass spectrum of moduli-dominated SUSY breaking is favorable to the experimental bound of the b --> s gamma decay decreasing its branching ratio. We obtain an upper bound for the gravitino mass from the cosmological constraint. (C) 1997 Elsevier Science B.V. - Y Kawamura, T Kobayashi, J KuboPHYSICS LETTERS B 405 1-2 64 - 70 1997年07月 [査読有り][通常論文]

It is shown that a certain sum rule for soft supersymmetry-breaking scalar masses, which has been recently found in a certain class of superstring models, is universal for gauge-Yukawa unified models. To explain this coincidence, we argue that the low-energy remnant of the target-space duality invariance in the effective supergravity of compactified superstrings can be identified with the (broken) scale invariance in gauge-Yukawa unified models, and that gauge-Yukawa unification which is indispensable for the sum rule to be satisfied follows from the matching of anomalies. (C) 1997 Published by Elsevier Science B.V. - T Kobayashi, H NakanoNUCLEAR PHYSICS B 496 1-2 103 - 131 1997年07月 [査読有り][通常論文]

''Anomalous'' U(1) gauge symmetry with a Green-Schwarz anomaly cancellation mechanism is discussed in the orbifold construction of four-dimensional heterotic string models. Some conditions are given as criteria to have ''anomalous'' U(1) in orbifold string models. In particular, ''anomalous'' U(1) is absent if the massless twisted matter has no mixing between visible and hidden sectors or if a certain type of discrete symmetries are found. We then give a general procedure for classifying orbifold models with ''anomalous'' U(1) and for identifying the ''anomalous'' U(1) basis. We illustrate our procedure in Z(3) and Z(4) orbifold models. According to our procedure, the classification of ''anomalous'' U(1) can be reduced to classification in the absence of a Wilson line. We also discuss discrete symmetries left unbroken after the ''anomalous'' U(1) breaking. This includes a possible relation between ''anomalous'' U(1) and discrete R symmetries. (C) 1997 Elsevier Science B.V. - Y Kawamura, T Kobayashi, T KomatsuPHYSICS LETTERS B 400 3-4 284 - 294 1997年05月 [査読有り][通常論文]

We study flat directions and soft scalar masses using a Z(3) orbifold model with SU(3) x SU(2) x U(1) gauge group and extra gauge symmetries including an anomalous U(1) symmetry. Soft scalar masses contain D-term contributions and particle mixing effects after symmetry breaking and they are parametrized by a few parameters. Some specific relations among scalar masses are obtained. (C) 1997 Elsevier Science B.V. - T Kobayashi, ZZ XingMODERN PHYSICS LETTERS A 12 8 561 - 571 1997年03月 [査読有り][通常論文]

We propose a simple but realistic pattern of quark mass matrices at the string scale, which can be derived from orbifold models of superstring theory with no use of gauge symmetries. This pattern is left-right symmetric and preserves the structural parallelism between up and down quark sectors. Its phenomenological consequences on flavor mixing and CP-violation are confronted with current experiments at the weak scale by using the renormalization group equations in the framework of minimal supersymmetric standard model. We find that good agreement is achievable without fine-tuning. - Yoshiharu Kawamura, Tatsuo KobayashiPhysical Review D - Particles, Fields, Gravitation and Cosmology 56 7 3844 - 3859 1997年 [査読有り][通常論文]

We derive a formula of soft supersymmetry-breaking scalar masses from four-dimensional string models within a more generic framework. We consider the effects of extra gauge symmetry breaking including an anomalous U(1) breaking through flat directions, that is, [Formula presented]-term and [Formula presented]-term contributions, particle mixing effects, and heavy-light mass mixing effects. Some phenomenological implications are discussed based on our mass formula. © 1997 The American Physical Society. - Y Kawamura, T KobayashiNUCLEAR PHYSICS B 481 3 539 - 576 1996年12月 [査読有り][通常論文]

We study generic features related to matter contents and flat directions in Z(2n) orbifold models. It is shown that Z(2n) orbifold models have massless conjugate pairs, R and (R) over bar, in certain twisted sectors as well as one of untwisted subsectors. Using these twisted sectors, Z(2n) orbifold models are classified into two types. Conjugate pairs, R and (R) over bar,lead to D-flatness as [R] = [(R) over bar] not equal 0. We investigate generic superpotentials derived from orbifold models so as to show that this direction is indeed a flat direction. - Y Kawamura, T KobayashiPHYSICS LETTERS B 388 4 867 - 868 1996年11月 [査読有り][通常論文]
- T Kobayashi, ZZ XingMODERN PHYSICS LETTERS A 11 31 2531 - 2537 1996年10月 [査読有り][通常論文]

We study the Kielanowski parametrization of the Kobayashi-Maskawa (KM) matrix V. A new two-angle parametrization is investigated explicitly and compared with the Kielanowski ansatz. Both of them are symmetric matrices and lead to \V-13/V-23\ = 0.129. Necessary corrections to the off-diagonal symmetry of Si are also discussed. - T Kobayashi, Y YamagishiPHYSICS LETTERS B 381 1-3 169 - 176 1996年07月 [査読有り][通常論文]

We study effects of SUSY particle decouplings on a quasi fixed point (QFP) of Yukawa coupling. From renormalization group analysis it is shown that if the SUSY breaking scale M(S) is large (greater than or similar to 1 TeV), effects of decoupling of Higgsinos and squarks raise the top Yukawa QFP. Tnis tendency is enhanced in most cases of non-universal SUSY breaking. For the case of M(S) less than or similar to 1 TeV, the decoupling of gluinos lowers m(t)(QFP). We checked some parameter dependencies for the top Yukawa QFP. The bottom-top Yukawa unified case is also studied. When top quark mass is measured more precisely, some patterns of soft mass spectra could be excluded if a rather large initial top Yukawa coupling is realized by the underlying theory. - T Kobayashi, HT SatoMODERN PHYSICS LETTERS A 11 7 535 - 543 1996年03月 [査読有り][通常論文]

We study quantum deformed gl(n) and igl(n) algebras on a quantum space discussing multi-parametric extension. We realize elements of deformed gl(n) and igl(n) algebras by a quantum fermionic space. We investigate a map between deformed igl(2) algebras of our basis and other basis. - R Altendorfer, T KobayashiINTERNATIONAL JOURNAL OF MODERN PHYSICS A 11 5 903 - 920 1996年02月 [査読有り][通常論文]

We study the gauge coupling unification of the minimal supersymmetric standard model with nonuniversal soft scalar and gaugino masses. The unification scale of the gauge couplings is estimated for nonuniversal cases. It is sensitive to the nonuniversality. It turns out that these cases can be combined with the assumption of string unification, which leads to a prediction of sin(2) theta(W)(M(Z)) and k(1), the normalization of the U(1)(Y) generator. String unification predicts that k(1) = 1.3-1.4. These values have nontrivial implications for string model building. Two-loop corrections are also calculated. Some of these cases exhibit a large discrepancy between experiment and string unification. We calculate string threshold corrections to explain the discrepancy. - T KOBAYASHIPHYSICS LETTERS B 358 3-4 253 - 258 1995年09月 [査読有り][通常論文]

We study left-right symmetric quark mass matrices whose up- and down-sectors have the same structure. This type of realistic mass matrices are derived from orbifold models. - T KOBAYASHI, M KONMURA, D SUEMATSU, K YAMADA, Y YAMAGISHIPROGRESS OF THEORETICAL PHYSICS 94 3 417 - 433 1995年09月 [査読有り][通常論文]

The electric dipole moment of the neutron (EDMN) is re-examined in the framework of non-universal soft supersymmetry breaking. We review some features of the relation between the EDMN and non-universal soft supersymmetry breaking terms. It is shown that the constraints on the soft scalar masses and soft CP phases have a rather strong dependence on the non-universality of soft breaking terms. We also show that the soft CP phase phi(B) which has no natural suppression mechanism may not contribute significantly to the EDMN in a certain parameter region where the radiative symmetry breaking occurs successfully. If this is the case, phi(B) may not need to be so small. - T KOBAYASHIPHYSICS LETTERS B 354 3-4 264 - 270 1995年07月 [査読有り][通常論文]

We study nonrenormalizable coupling terms in Z(N) orbifold models. Nontrivial selection rules of couplings are provided and cannot be understood in terms of a simple symmetry of effective field theories. We also discuss phenomenological implications of theses selection rules for the quark mass matrices. - T KOBAYASHI, D SUEMATSU, K YAMADA, Y YAMAGISHIPHYSICS LETTERS B 348 3-4 402 - 410 1995年04月 [査読有り][通常論文]

We study soft scalar masses in comparison with gaugino masses in 4-dimensional string models. In general non-universal soft masses are derived in orbifold models. We give conditions on modular weights which lead to a large non-degeneracy in the soft scalar mass spectrum. This non-universality is applied to the unification of gauge coupling constants in the minimal string model. - T KOBAYASHIINTERNATIONAL JOURNAL OF MODERN PHYSICS A 10 10 1393 - 1411 1995年04月 [査読有り][通常論文]

We study the minimal supersymmetric standard model derived from Z(N) x Z(M) orbifold models. Moduli-dependent threshold corrections of gauge couplings are investigated to explain the measured values of the coupling constants. We also study the Yukawa couplings of the models. We find that the Z(2) x Z'(6), Z(2) x Z(6), Z(3) x Z(6) and Z(6) x Z(6) orbifold models have the possibility of deriving Yukawa couplings for the second and third generations as well as the measured gauge coupling constants. Allowed models are shown explicitly by combinations of modular weights for the matter fields. - H KAWABE, T KOBAYASHI, N OHTSUBONUCLEAR PHYSICS B 434 1-2 210 - 230 1995年01月 [査読有り][通常論文]

We examine whether a minimal string model possessing the same massless spectra as the MSSM can be obtained from Z(4), Z(6) and Z(8) orbifold constructions. Using an anomaly cancellation condition of the target space duality symmetry, we derive allowable values of a level k(1) of U(1)(Y) for the minimal string model on the orbifolds through computer analyses. We investigate threshold corrections of the gauge coupling constants of SU(3), SU(2) and U(1)Y and examine consistencies of the model with the LEP experiments. It is found that Z(4) and Z(8)-II cannot derive the minimal string model but Z(6)-I, Z(6)-II and Z(8)-I are possible to derive it with k(1) = 29/21, 1 less than or equal to k(1) less than or equal to 32/21 and 1 less than or equal to k(1) less than or equal to 41/21, respectively, We obtain explicitly allowed combinations of modular weights. The minimum values of the moduli on unrotated planes are estimated within the ranges of the levels. Further we investigate what kinds of hidden sectors are consistent with the minimal string models. Also their gauge coupling constants of the hidden groups are estimated, We discuss Yukawa couplings of the models. - T KOBAYASHI, CS LIMPHYSICS LETTERS B 343 1-4 122 - 127 1995年01月 [査読有り][通常論文]

We study CP in orbifold models. It is found that the orbifolds always have some automorphisms as CP symmetry. The symmetries are restricted non-trivially due to geometrical structure of the orbifolds. Explicit analysis on Yukawa couplings also shows that CP is not violated in orbifold models. - T KOBAYASHI, D SUEMATSU, Y YAMAGISHIPHYSICS LETTERS B 329 1 27 - 32 1994年06月 [査読有り][通常論文]

Gauge coupling unification is studied in the MSSM with non-universal soft supersymmetry breaking terms. If gaugino masses are sufficiently smaller than scalar soft masses and the scalar soft masses have also certain types of non-universality, the gauge coupling unification scale can be larger than 3 x 10(16) GeV even within the MSSM contents. String unification may not need a large threshold correction or a large modulus value. We also discuss the relation to the string model building. - H KAWABE, T KOBAYASHI, N OHTSUBOPHYSICS LETTERS B 325 1-2 77 - 84 1994年04月 [査読有り][通常論文]

We study a minimal string model possessing the same massless spectra as the MSSM on Z(N) x Z(M) orbifolds. Threshold corrections of the gauge coupling constants of SU(3), SU(2) and U(1)Y are investigated in a case of an overall modulus. Using computer analyses, we search ranges of levels of U(1)Y allowed by the LEP experiments. It is found that Z3 x Z3 can not derive the minimal string model for a M(Z) SUSY breaking scale. The minimum values of the overall moduli are estimated within the ranges of the levels. - H KAWABE, T KOBAYASHI, N OHTSUBOPHYSICS LETTERS B 322 4 331 - 339 1994年02月 [査読有り][通常論文]

We study the construction of the minimal supersymmetric standard model from the Z8 orbifold models. We use a target-space duality anomaly cancellation and a unification of gauge couplings as constraints. It is shown that some models obtained through a systematical search realize the unification of SU(3) and SU(2) coupling constants. - H KAWABE, T KOBAYASHI, N OHTSUBOPHYSICS LETTERS B 322 4 331 - 339 1994年02月 [査読有り][通常論文]

We study the construction of the minimal supersymmetric standard model from the Z8 orbifold models. We use a target-space duality anomaly cancellation and a unification of gauge couplings as constraints. It is shown that some models obtained through a systematical search realize the unification of SU(3) and SU(2) coupling constants. - T KOBAYASHI, N OHTSUBOINTERNATIONAL JOURNAL OF MODERN PHYSICS A 9 1 87 - 125 1994年01月 [査読有り][通常論文]

We study the geometrical and phenomenological aspects of Z(N) orbifold models. Conditions on-the background moduli and Wilson lines are clarified. We investigate the explicit Z(N)-invariant states through a modified GSO projection and calculate Yukawa coupling conditions due to space-group and SO(10) invariance. We estimate a contribution of a holomorphic instanton to the couplings. - T KOBAYASHI, N OHTSUBO, K TANIOKAINTERNATIONAL JOURNAL OF MODERN PHYSICS A 8 20 3553 - 3563 1993年08月 [査読有り][通常論文]

We construct a ''shifted'' Z4 x Z4 orbifold model equivalent to a twisted Z4 X Z4 orbifold model at an enhancement point. It is proved that the N = 2 algebra of the 2(6) model is coincident with that of the ''shifted'' orbifold model. The correspondence between the Z4 x Z4 orbifold model and the 2(6) model is investigated with respect to massless spectra and Yukawa couplings. Also studied are Yukawa couplings and some other aspects of the models away from the enhancement point. - T KOBAYASHI, T UEMATSUZEITSCHRIFT FUR PHYSIK C-PARTICLES AND FIELDS 58 4 559 - 565 1993年06月 [査読有り][通常論文]

We investigate quantum deformation of conformal algebras by constructing the quantum space for sl(q)(4). The differential calculus on the quantum space and the action of the quantum generators are studied. We derive deformed su(2, 2) algebra from the deformed sl(4) algebra using the quantum 4-spinor and its conjugate spinor. The quantum 6-vector in so(q)(4, 2) is constructed as a tensor product of two sets of 4-spinors. We obtain the q-deformed conformal algebra with the suitable assignment of the generators which satisfy the reality condition. The deformed Poincare algebra is derived through a contraction procedure. - T KOBAYASHI, T UEMATSUPHYSICS LETTERS B 306 1-2 27 - 33 1993年05月 [査読有り][通常論文]

A quantum deformation of four-dimensional superconformal algebra realized on quantum superspace is investigated. We study the differential calculus and the action of the quantum generators corresponding to sl(q)(1\4) which act on the quantum superspace. We derive deformed su(1\2, 2) algebras from the deformed sl(1\4) algebra. Through a contraction procedure we obtain a deformed super-Poincare algebra. - T KOBAYASHI, T UEMATSUPHYSICS LETTERS B 306 1-2 27 - 33 1993年05月 [査読有り][通常論文]

A quantum deformation of four-dimensional superconformal algebra realized on quantum superspace is investigated. We study the differential calculus and the action of the quantum generators corresponding to sl(q)(1\4) which act on the quantum superspace. We derive deformed su(1\2, 2) algebras from the deformed sl(1\4) algebra. Through a contraction procedure we obtain a deformed super-Poincare algebra. - H KAWABE, T KOBAYASHI, N OHTSUBOPROGRESS OF THEORETICAL PHYSICS 88 2 431 - 442 1992年08月 [査読有り][通常論文]

We explicitly construct a two-dimensional 'shifted' Z6 orbifold model equivalent to a two-dimensional twisted Z6 orbifold model at an enhancement point. Then we study equivalence of the shifted Z6 super-orbifold model and the 1.4 model. As applications to the (2,2) heterotic string theories, we investigate correspondence of the Z6 X Z6 (Z3 X Z6) orbifold model and (1.4)3 (1(3)(1.4)2) model. - T KOBAYASHIPROGRESS OF THEORETICAL PHYSICS SUPPLEMENT 110 277 - 290 1992年 [査読有り][通常論文]

We construct a 'shifted' Z4 x Z4 orbifold model equivalent to a twisted Z4 x Z4 orbifold model at an enhancement point. It is proved that an N=2 superconformal algebra of a 2(2) theory coincides with that of the 2-dim 'shifted' Z4 orbifold model. We relate chiral primary states of the 2(6) model to 'shifted' ground states of the Z4 x Z4 orbifold model. Yukawa couplings are also studied. - T KOBAYASHI, N OHTSUBOPHYSICS LETTERS B 262 4 425 - 431 1991年06月 [査読有り][通常論文]

SO(10) momenta and fixed points are studied for obtaining non-vanishing Yukawa couplings of Z(N) x Z(M) orbifold models. As an example, we investigate explicity three-point couplings of Z4 x Z4 and Z3 x Z3 orbifolds with the standard embedding and no discrete torsion. Comparing with the 2(6) and 1(9) model by Gepner's coset construction, respectively, we find discrepancies with respect to the numbers of (27) (27) (27) couplings, while the numbers of 27 are identical. - T KOBAYASHI, N OHTSUBOPHYSICS LETTERS B 245 3-4 441 - 446 1990年08月 [査読有り][通常論文]
- T KOBAYASHI, N OHTSUBOPHYSICS LETTERS B 245 3-4 441 - 446 1990年08月 [査読有り][通常論文]
- Y KATSUKI, Y KAWAMURA, T KOBAYASHI, N OHTSUBO, Y ONO, K TANIOKAPHYSICS LETTERS B 227 3-4 381 - 386 1989年08月 [査読有り][通常論文]
- Y KATSUKI, Y KAWAMURA, T KOBAYASHI, N OHTSUBO, Y ONO, K TANIOKAPHYSICS LETTERS B 227 3-4 381 - 386 1989年08月 [査読有り][通常論文]
- Y KATSUKI, Y KAWAMURA, T KOBAYASHI, N OHTSUBO, K TANIOKAPROGRESS OF THEORETICAL PHYSICS 82 1 171 - 182 1989年07月 [査読有り][通常論文]
- Y KATSUKI, Y KAWAMURA, T KOBAYASHI, N OHTSUBOPHYSICS LETTERS B 212 3 339 - 342 1988年09月 [査読有り][通常論文]

- Ishimori Hajime, Kobayashi Tatsuo, Ohki Hiroshi, Okada Hiroshi, Shimizu Yusuke, Tanimoto Morimitsu

Springer-Verlag 2012年 (ISBN: 9783642308048) - Yukawa International, Symposium, 小林 達夫, Kugo Taichi

Progress of Theoretical Physics 2009年 - 小林 達夫, 京都大学大学院理学研究科

[小林達夫] 2008年 - 小林 達夫, 京都大学大学院理学研究科

[小林達夫] 2004年 - 小林 達夫

小林達夫 1991年

- Shota Kikuchi, Tatsuo Kobayashi, Kaito Nasu, Hajime Otsuka, Shohei Takada, Hikaru Uchida Journal of High Energy Physics 2023 (4) 2023年01月24日We revisit the modular weights in type IIB magnetized D-brane models. The simple analysis of wave function shows that the four-dimensional matter fields have the modular weight -1/2, but it may shift as one in type IIA intersecting D-brane models. For example, the localized gauge flux as well as the localized curvature can shift the modular weight in the magnetized D-brane models. Such corrections do not affect physical couplings such as physical Yukawa couplings. However, it leads to differences in supersymmetry breaking sfermion masses, which depend on the modular weights, although the $A$-term coefficients and the sum of sfermion masses squared seem to be the same between two models.
- Shota Kikuchi, Tatsuo Kobayashi, Kaito Nasu, Shohei Takada, Hikaru Uchida 2023年01月10日We study modular symmetric quark flavor models without fine-tuning. Mass matrices are written in terms of modular forms, and modular forms in the vicinity of the modular fixed points become hierarchical depending on their residual charges. Thus modular symmetric flavor models in the vicinity of the modular fixed points have a possibility to describe mass hierarchies without fine-tuning. Since describing quark hierarchies without fine-tuning requires $Z_n$ residual symmetry with $n\geq 6$, we focus on $\Gamma_6$ modular symmetry in the vicinity of the cusp $\tau=i\infty$ where $Z_6$ residual symmetry remains. We use only modular forms belonging to singlet representations of $\Gamma_6$ to make our analysis simple. Consequently, viable quark flavor models are obtained without fine-tuning.
- Quark and lepton flavor structure in magnetized orbifold models at residual modular symmetric pointsKouki Hoshiya, Shota Kikuchi, Tatsuo Kobayashi, Hikaru Uchida 2022年09月15日We study quark and lepton mass matrices derived from magnetized $T^2/\mathbb{Z}_2$ orbifold models. Quark and lepton masses have a large hierarchy. In addition, mixing angles are large in the lepton sector, while those are small in the quark sector. We find that this structure can be realized in certain flavor models, which are identified by the zero points of the zero-mode wave functions of fermions and Higgs modes. We classify such realistic flavor models. Fixed points $\tau=i$, $e^{2\pi i/3}$ and $i\infty$ of the modulus $\tau$ play a role in realizing a large mass hierarchy through our scenario, where residual $S$, $ST$, and $T$ symmetries remain and the lightest Higgs modes can correspond to eigenstates of residual symmetries at the leading order. As a result, we find that there are 24 flavor models in total which can be realistic in a vicinity of $S$-symmetric vacuum but no flavor models for $ST$ and $T$-symmetric vacua.
- Shota Kikuchi, Tatsuo Kobayashi, Morimitsu Tanimoto, Hikaru Uchida 2022年06月17日We study the CP violation and the CP phase of quark mass matrices in modular flavor symmetric models. The CP symmetry remains at $\tau = e^{2 \pi i/3}$ by a combination of the $T$-symmetry of the modular symmetry. However, the $T$-symmetry breaking may lead to the CP violation at the fixed point $\tau = e^{2 \pi i/3}$. We study such a possibility in magnetized orbifold models as examples of modular flavor symmetric models. These models, in general, have more than one candidates for Higgs modes, while generic string compactifications also lead to several Higgs modes. These Higgs modes have different behaviors under the $T$-transformation. The light Higgs mode can be a linear combination of those modes so as to lead to realistic quark mass matrices. The CP phase of mass matrix does not appear in a certain case, which is determined by the $T$-transformation behavior. Deviation from it is important to realize the physical CP phase. We discuss an example leading to non-vanishing CP phase at the fixed point $\tau = e^{2 \pi i/3}$.
- Shota Kikuchi, Tatsuo Kobayashi, Kaito Nasu, Hajime Otsuka, Shohei Takada, Hikaru Uchida 2022年03月28日We study the modular symmetry of soft supersymmetry breaking terms. Soft scalar masses and $A$-term coefficients are invariant under the modular symmetry when we regard $F$-term as a spurion with the modular weight $-2$. Their flavor structure is determined by the same symmetry as Yukawa couplings, i.e., fermion masses. The modular symmetric behavior of $\mu$-term and $B$-term depends on how the $\mu$-term is generated.
- Shota Kikuchi, Tatsuo Kobayashi, Kaito Nasu, Hikaru Uchida, Shohei Uemura 2022年02月11日We study the modular symmetry anomaly in magnetized orbifold models. The non-perturbative effects such as D-brane instanton effects can break tree-level symmetry. We study which part of the modular symmetry is broken explicitly by Majorana mass terms with three generations of neutrinos. The modular weight of neutrino mass terms does not match with other coupling terms in tree-level Lagrangian. In addition, the $Z_N$ symmetry of the modular flavor symmetry is broken and a certain normal subgroup of the modular flavor symmetry remains in neutrino mass terms.
- Shota Kikuchi, Tatsuo Kobayashi, Hajime Otsuka, Morimitsu Tanimoto, Hikaru Uchida, Kei Yamamoto 2022年01月12日We study a scenario to derive four-dimensional modular flavor symmetric models from higher dimensional theory by assuming the compactification consistent with the modular symmetry. In our scenario, wavefunctions in extra dimensional compact space are modular forms. That leads to constraints on combinations between modular weights and $\Gamma_N$ ($\Gamma_N')$ representations of matter fields. We also present illustrating examples.
- Tatsuo Kobayashi, Hajime Otsuka European Physical Journal C 82 (1) 2022年01月We study the minimal flavor violation in the context of string effective field theory. Stringy selection rules indicate that n-point couplings among fermionic zero-modes and lightest scalar modes in the string effective action are given by a product of Yukawa couplings which are regarded as spurion fields of stringy and geometrical symmetries. Hence, Yukawa couplings determine the dynamics of flavor and CP violations. This observation strongly supports the hypothesis of minimal flavor violation in the Standard Model effective field theory.
- Shota Kikuchi, Tatsuo Kobayashi, Yuya Ogawa, Hikaru Uchida 2021年12月03日We study quark mass matrices derived from magnetized $T^2/\mathbb{Z}_2$ orbifold models. Yukawa matrices at three modular fixed points, $\tau=i, e^{2\pi i/3}$ and $i\infty$ are invariant under $S$, $ST$ and $T$-transformations. We study these invariances on $T^2/\mathbb{Z}_2$ twisted orbifold. We find that Yukawa matrices have a kind of texture structures although ones at $\tau=i\infty$ are not realistic. We classify Yukawa textures at $\tau=i$ and $e^{2\pi i/3}$. Moreover we investigate the conditions such that quark mass matrix constructed by Yukawa textures becomes approximately rank one matrix, which is favorable to lead to hierarchical masses between the third generation and the others. It is found that realistic quark mass matrices can be obtained around the $S$-invariant vacuum and $ST$-invariant vacuum. As an illustrating example, we show the realization of the quark mass ratios and mixing based on Fritzch and Fritzch-Xing mass matrices.
- Tatsuo Kobayashi, Hikaru Uchida 2021年11月21日We study anomalies of non-Abelian discrete symmetries; which part of non-Abelian group is anomaly free and which part can be anomalous. It is found that the anomaly-free elements of the group $G$ generate a normal subgroup $G_0$ of $G$ and the residue class group $G/G_0$, which becomes the anomalous part of $G$, is isomorphic to a single cyclic group. The derived subgroup $D(G)$ of $G$ is useful to study the anomaly structure. This structure also constrains the structure of the anomaly-free subgroup; the derived subgroup $D(G)$ should be included in the anomaly-free subgroup. We study the detail structure of the anomaly-free subgroup from the structure of the derived subgroup in various discrete groups. For example, when $G=S_n \simeq A_n \rtimes Z_2$ and $G=\Delta(6n^2) \simeq \Delta(3n^2) \rtimes Z_2$, in particular, $A_n$ and $\Delta(3n^2)$ are at least included in the anomaly-free subgroup, respectively. This result holds in any arbitrary representations.
- Tatsuo Kobayashi, Hiroshi Okada, Yuta Orikasa 2021年11月10日We propose a new mechanism of stabilization for dark matter under fixed points of modular $A_4$ symmetry, thanks to recovering a residual symmetry for each of fixed points at $\tau=\omega,\ i, i\times\infty$. We find that a model at $\tau =i$ successfully reproduces the neutrino oscillation data without conflict of the DM stability.
- Kouki Hoshiya, Shota Kikuchi, Tatsuo Kobayashi, Kaito Nasu, Hikaru Uchida, Shohei Uemura 2021年03月12日We study Majorana neutrino masses induced by D-brane instanton effects in magnetized orbifold models. We classify possible cases, where neutrino masses can be induced. Three and four generations are favored in order to generate neutrino masses by D-brane instantons. Explicit mass matrices have specific features. Their diagonalizing matrices correspond to the bimaximal mixing matrix in the case with even magnetic fluxes, independently of the modulus value $\tau$. On the other hand, for odd magnetic fluxes, diagonalizing matrices correspond nearly to the tri-bimaximal mixing matrix near $\tau =i$, while they become the bimaximal mixing matrix for larger ${\rm Im}\tau$. For even fluxes, neutrino masses are modular forms of the weight 1 on $T^2/\mathbb{Z}_2$, and they have symmetries such as $S_4'$ and ${\Delta'}(96)\times \mathbb{Z}_3$.
- Tatsuo Kobayashi, Takashi Shimomura, Morimitsu Tanimoto 2021年02月20日We study the soft supersymmetry (SUSY) breaking terms due to the modulus F-term in the modular flavor models of leptons. It is found that the soft SUSY breaking terms are constrained by the modular forms, and specific patterns are derived. Those phenomenological implications are discussed in such as the lepton flavor violation, $\mu \rightarrow e + \gamma$ decay. In order to examine numerically, two modular flavor $A_4$ models are taken. The SUSY breaking scale is significantly constrained by inputting the observed upper bound of the $\mu \rightarrow e + \gamma$ decay. The SUSY mass scale is larger than around $8$ TeV and $5$ TeV for the two $A_4$ models, respectively. Therefore, the current experimental upper bound for the $\mu \to e + \gamma$ decay corresponds to the new physics of the SUSY particle at the $5$ -- $10$ TeV scale in the modular flavor models. The predicted branching ratio depends on a modulus $\tau$ significantly. It decreases of one order at the large ${\rm Im}\,\tau$. We also calculate the branching ratios of tauon decays to $e + \gamma$ and $\mu + \gamma$. Predicted ones are at most ${\cal O}(10^{-15})$, which are much below the current experimental bounds.
- Shota Kikuchi, Tatsuo Kobayashi, Hikaru Uchida 2021年01月04日We study the modular symmetry on magnetized toroidal orbifolds with Scherk-Schwarz phases. In particular, we investigate finite modular flavor groups for three-generation modes on magnetized orbifolds. The three-generation modes can be the three-dimensional irreducible representations of covering groups and central extended groups of $\Gamma_N$ for $N=3,4,5,7,8,16$, that is, covering groups of $\Delta(6(N/2)^2)$ for $N=$ even and central extensions of $PSL(2,\mathbb{Z}_{N})$ for $N=$odd with Scherk-Schwarz phases. We also study anomaly behaviors.
- Kouki Hoshiya, Shota Kikuchi, Tatsuo Kobayashi, Yuya Ogawa, Hikaru Uchida 2020年12月01日We study orbifolding by the $\mathbb{Z}_2^{\rm (per)}$ permutaion of $T^2_1 \times T^2_2$ with magnetic fluxes and its twisted orbifolds. We classify the possible three generation models which lead to non-vanishing Yukawa couplings on the magnetized $T^2_1 \times T^2_2$ and orbifolds including the $\mathbb{Z}_2^{\rm (per)}$ permutation and $\mathbb{Z}_2^{\rm (t)}$ twist. We also study the modular symmetry on such orbifold models. As an illustrating model, we examine the realization of quark masses and mixing angles.
- Shota Kikuchi, Tatsuo Kobayashi, Shintaro Takada, Takuya H. Tatsuishi, Hikaru Uchida 2020年05月26日We study the modular symmetry in $T^2$ and orbifold comfactifications with magnetic fluxes. There are $|M|$ zero-modes on $T^2$ with the magnetic flux $M$. Their wavefunctions as well as massive modes behave as modular forms of weight $1/2$ and represent the double covering group of $\Gamma \equiv SL(2,\mathbb{Z})$, $\widetilde{\Gamma} \equiv \widetilde{SL}(2,\mathbb{Z})$. Each wavefunction on $T^2$ with the magnetic flux $M$ transforms under $\widetilde{\Gamma}(2|M|)$, which is the normal subgroup of $\widetilde{SL}(2,\mathbb{Z})$. Then, $|M|$ zero-modes are representations of the quotient group $\widetilde{\Gamma}'_{2|M|} \equiv \widetilde{\Gamma}/\widetilde{\Gamma}(2|M|)$. We also study the modular symmetry on twisted and shifted orbifolds $T^2/\mathbb{Z}_N$. Wavefunctions are decomposed into smaller representations by eigenvalues of twist and shift. They provide us with reduction of reducible representations on $T^2$.
- Tatsuo Kobayashi, Takaaki Nomura, Takashi Shimomura 2019年12月02日We discuss type-II seesaw models adopting modular $A_4$ symmetry in supersymmetric framework. In our approach, the models are classified by the assignment of $A_4$ representations and modular weights for leptons and triplet Higgs fields. Then neutrino mass matrix is characterized by modulus $\tau$ and two free parameters. Carrying out numerical analysis, we find allowed parameter sets which can fit the neutrino oscillation data. For the allowed parameter sets, we obtain the predictions in neutrino sector such as CP violating phases and the lightest neutrino mass. Finally we also show the predictions for the branching ratios of doubly charged scalar boson focusing on the case where the doubly charged scalar boson dominantly decays into charged leptons.
- Tatsuo Kobayashi, Yusuke Shimizu, Kenta Takagi, Morimitsu Tanimoto, Takuya H. Tatsuishi, Hikaru Uchida 2019年10月25日We study the spontaneous $CP$ violation through the stabilization of the modulus $\tau$ in modular invariant flavor models. The $CP$-invaraiant potentential has the minimum only at ${\rm Re}[\tau] = 0$ or 1/2. From this prediction, we study $CP$ violation in modular invariant flavor models. The physical $CP$ phase is vanishing. The important point for the $CP$ conservation is the $T$ transformation in the modular symmetry. One needs the violation of $T$ symmetry to realize the spontaneous $CP$ violation.
- Tatsuo Kobayashi, Yusuke Shimizu, Kenta Takagi, Morimitsu Tanimoto, Takuya H. Tatsuishi 2019年09月09日We study the modulus stabilization in an $A_4$ model whose $A_4$ flavor symmetry is originated from the $S_4$ modular symmetry. We can stabilize the modulus so that the $A_4$ invariant superpotential leads to the realistic lepton masses and mixing angles. We also discuss the phenomenological aspect of the present model as a consequence of the modulus stabilization.
- Tatsuo Kobayashi, Yusuke Shimizu, Kenta Takagi, Morimitsu Tanimoto, Takuya H. Tatsuishi 2019年07月22日We study a flavor model with $A_4$ symmetry which originates from $S_4$ modular group. In $S_4$ symmetry, $Z_2$ subgroup can be anomalous, and then $S_4$ can be violated to $A_4$. Starting with a $S_4$ symmetric Lagrangian at the tree level, the Lagrangian at the quantum level has only $A_4$ symmetry when $Z_2$ in $S_4$ is anomalous. We obtain modular forms of two singlets and a triplet representations of $A_4$ by decomposing $S_4$ modular forms into $A_4$ representations. We propose a new $A_4$ flavor model of leptons by using those $A_4$ modular forms. We succeed in constructing a viable neutrino mass matrix through the Weinberg operator for both normal hierarchy (NH) and inverted hierarchy (IH) of neutrino masses. Our predictions of the CP violating Dirac phase $\delta_{CP}$ and the mixing $\sin^2\theta_{23}$ depend on the sum of neutrino masses for NH.
- Tatsuo Kobayashi, Yusuke Shimizu, Kenta Takagi, Morimitsu Tanimoto, Takuya H. Tatsuishi 2019年06月25日We present a flavor model with the $S_3$ modular invariance in the framework of SU(5) GUT. The $S_3$ modular forms of weights $2$ and $4$ give the quark and lepton mass matrices with a common complex parameter, the modulus $\tau$. The GUT relation of down-type quarks and charged leptons is imposed by the VEV of adjoint 24-dimensional Higgs multiplet in addition to the VEVs of $5$ and $\bar 5$ Higgs multiples of SU(5). The observed CKM and PMNS mixing parameters as well as the mass eigenvalues are reproduced properly. We discuss the leptonic CP phase and the effective mass of the neutrinoless double beta decay with the sum of neutrino masses.
- Yuki Kariyazono, Tatsuo Kobayashi, Shintaro Takada, Shio Tamba, Hikaru Uchida 2019年04月16日We study modular symmetry anomalies in four-dimensional low-energy effective field theory, which is derived from six-dimensional supersymmetric $U(N)$ Yang-Mills theory by magnetic flux compactification. The gauge symmetry $U(N)$ is broken to $U(N_a) \times U(N_b)$ by magnetic fluxes. It is found that Abelian subgroup of the modular symmetry corresponding to discrete part of $U(1)$ can be anomalous, but other elements independent of $U(1)$ in the modular symmetry are always anomaly-free.
- Tatsuo Kobayashi, Yusuke Shimizu, Kenta Takagi, Morimitsu Tanimoto, Takuya H. Tatsuishi, Hikaru Uchida 2018年12月23日We study a flavor model that the quark sector has the $S_3$ modular symmetry,while the lepton sector has the $A_4$ modular symmetry. Our model leads to characteristic quark mass matrices which are consistent with experimental data of quark masses, mixing angles and the CP violating phase. The lepton sector is also consistent with the experimental data of neutrino oscillations. We also study baryon and lepton number violations in our flavor model.
- Tatsuo Kobayashi, Shio Tamba 2018年11月28日We study modular transformation of holomorphic Yukawa couplings in magnetized D-brane models. It is found that their products are modular forms, which are non-trivial representations of finite modular subgroups, e.g. $S_3$, $S_4$, $\Delta(96)$ and $\Delta(384)$.
- Junichiro Kawamura, Tatsuo Kobayashi, Natsumi Nagata 2018年08月24日 [査読無し][通常論文]

The Next-to-Minimal Supersymmetric Standard Model (NMSSM) provides a natural

framework to realize a low-scale supersymmetric (SUSY) model, where a singlet

superfield is added to the minimal model to generate a SUSY-scale higgsino mass

term with its vacuum expectation value. Due to the presence of the extra

singlet field, the vacuum conditions to realize the correct electroweak

symmetry-breaking become fairly restrictive especially if we impose

universality conditions at the unification scale. In this paper, we show that a

non-universal gaugino mass spectrum can significantly relax this restriction

even though the scalar masses and trilinear couplings are subject to

universality conditions. With the gaugino non-universality, we find that

higgsino can be the lightest SUSY particle and its thermal relic abundance can

reproduce the observed dark matter density in a wide range of parameter space

in which the 125~GeV Higgs-boson mass is obtained. This higgsino-like dark

matter may be probed in direct detection experiments. We also find that there

is an upper bound on the masses of supersymmetric particles in this scenario,

and many model points predict colored particles such as gluino to be within the

reach of a future 100~TeV collider. Implications for no-scale/gaugino-mediation

models are also discussed. - Tatsuo Kobayashi, Naoya Omoto, Yusuke Shimizu, Kenta Takagi, Morimitsu Tanimoto, Takuya H. Tatsuishi 2018年08月09日 [査読無し][通常論文]

We study the phenomenological implications of the modular symmetry $\Gamma(3)

\simeq A_4$ of lepton flavors facing recent experimental data of neutrino

oscillations. The mass matrices of neutrinos and charged leptons are

essentially given by fixing the expectation value of modulus $\tau$, which is

the only source of modular invariance breaking. We introduce no flavons in

contrast with the conventional flavor models with $A_4$ symmetry. We classify

our neutrino models along with the type I seesaw model, the Weinberg operator

model and the Dirac neutrino model. In the normal hierarchy of neutrino masses,

the seesaw model is available by taking account of recent experimental data of

neutrino oscillations and the cosmological bound of sum of neutrino masses. The

predicted $\sin^2\theta_{23}$ is restricted to be larger than $0.54$ and

$\delta_{CP}=\pm (50^{\circ}\mbox{--}180^{\circ})$. Since the correlation of

$\sin^2\theta_{23}$ and $\delta_{CP}$ is sharp, the prediction is testable in

the future. It is remarkable that the effective mass $m_{ee}$ of the

neutrinoless double beta decay is around $22$\,meV while the sum of neutrino

masses is predicted to be $145$\,meV. On the other hand, for the inverted

hierarchy of neutrino masses, only the Dirac neutrino model is consistent with

the experimental data. - Tatsuo Kobayashi, Takaaki Nomura, Hiroshi Okada Phys. Rev. D 98, 055025 (2018) 2018年05月18日 [査読無し][通常論文]

We investigate origins of predictive one-zero neutrino mass textures in a

systematic way. Here we search Abelian continuous(discrete) global symmetries,

and non-Abelian discrete symmetries, and show how to realize these neutrino

masses. Then we propose a concrete model involving a dark matter candidate and

an extra gauge boson, and show their phenomenologies. - Tatsuo Kobayashi, Naoya Omoto, Hajime Otsuka, Takuya H. Tatsuishi Phys. Rev. D 97, 106006 (2018) 97 (10) 2018年05月15日 [査読無し][通常論文]

We propose a new type of K\"ahler moduli stabilization mechanisms in type IIB

superstring theory on Calabi-Yau manifolds with the positive Euler number. The

overall K\"ahler modulus can be perturbatively stabilized by radiative

corrections due to sparticles. Its minimum is the anti-de Sitter vacuum, where

supersymmetry is broken. We can uplift it to the de Sitter vacuum by

introducing anti-D-branes, keeping the modulus stabilized. Although our

numerical results depend on the choice of the cutoff scale and degeneracies of

sparticles, at any rate there exist the parameter spaces where the masses of

Kaluza-Klein and stringy modes are larger than the cutoff scale. Furthermore,

this stabilization scenario predicts an ultralight axion. - Tatsuo Kobayashi, Satoshi Nagamoto, Shintaro Takada, Shio Tamba, Takuya H. Tatsuishi Phys. Rev. D 97, 116002 (2018) 2018年04月18日 [査読無し][通常論文]

We study the modular symmetry in magnetized D-brane models on $T^2$.

Non-Abelian flavor symmetry $D_4$ in the model with magnetic flux $M=2$ (in a

certain unit) is a subgroup of the modular symmetry. We also study the modular

symmetry in heterotic orbifold models. The $T^2/Z_4$ orbifold model has the

same modular symmetry as the magnetized brane model with $M=2$, and its flavor

symmetry $D_4$ is a subgroup of the modular symmetry. - Kenji Kadota, Tatsuo Kobayashi, Ikumi Saga, Keigo Sumita Journal of High Energy Physics 2018 (4) 2018年04月01日 [査読無し][通常論文]

We propose a new model of single-field D-term inflation in supergravity, where the inflation is driven by a single modulus field which transforms non-linearly under the U(1) gauge symmetry. One of the notable features of our modulus D-term inflation scenario is that the global U(1) remains unbroken in the vacuum and hence our model is not plagued by the cosmic string problem which can exclude most of the conventional D-term inflation models proposed so far due to the CMB observations. - Tatsuo Kobayashi, Kentaro Tanaka, Takuya H. Tatsuishi Phys. Rev. D 98, 016004 (2018) 2018年03月28日 [査読無し][通常論文]

We study the lepton flavor models, whose flavor symmetries are finite

subgroups of the modular group such as $S_3$ and $A_4$. In our models,

couplings are also nontrivial representations of these groups and modular

functions of the modulus. We study the possibilities that these models realize

realistic values of neutrino masses and lepton mixing angles. - Kenji Kadota, Tatsuo Kobayashi, Keigo Sumita Journal of Cosmology and Astroparticle Physics 2017 (11) 2017年11月21日 [査読無し][通常論文]

We propose a new model of the D-term hybrid inflation in the framework of supergravity. Although our model introduces, analogously to the conventional D-term inflation, the inflaton and a pair of scalar fields charged under a U(1) gauge symmetry, we study the logarithmic and exponential dependence on the inflaton field, respectively, for the Kähler and superpotential. This results in a characteristic one-loop scalar potential consisting of linear and exponential terms, which realizes the small-field inflation dominated by the Fayet-Iliopoulos term. With the reasonable values for the coupling coefficients and, in particular, with the U(1) gauge coupling constant comparable to that of the Standard Model, our D-term inflation model can solve the notorious problems in the conventional D-term inflation, namely, the CMB constraints on the spectral index and the generation of cosmic strings. - Tatsuo Kobayashi, Yuji Omura, Osamu Seto, Kazuki Ueda JOURNAL OF HIGH ENERGY PHYSICS (11) 2017年11月 [査読無し][通常論文]

It is one of the major issues to realize a vacuum which breaks supersymmetry (SUSY) and R-symmetry, in a supersymmetric model. We study the model, where the same sector breaks the gauge symmetry and SUSY. In general, the SUSY breaking model without gauge symmetry has a flat direction at the minimum of F-term scalar potential. When we introduce U(1) gauge symmetry to such a SUSY breaking model, there can appear a runaway direction. Such a runway direction can be lifted by loop effects, and the gauge symmetry breaking and SUSY breaking are realized. The R-symmetry, that is assigned to break SUSY, is also spontaneously broken at the vacuum. This scenario can be extended to non-Abelian gauge theories. We also discuss application to the Pati-Salam model and the SU(5) grand unified theory. We see that non-vanishing gaugino masses are radiatively generated by the R-symmetry breaking and the gauge messenger contribution. - Tatsuo Kobayashi, Satoshi Nagamoto PHYSICAL REVIEW D 96 (9) 2017年11月 [査読無し][通常論文]

We study T-2/Z(N) orbifold models with magnetic fluxes. We propose a systematic way to analyze the number of zero-modes and their wave functions by use of modular transformation. Our results are consistent with the previous results, and our approach is more direct and analytical than the previous ones. The index theorem implies that the zero-mode number of the Dirac operator on T-2 is equal to the index M, which corresponds to the magnetic flux in a certain unit. Our results show that the zero-mode number of the Dirac operator on T-2/Z(N) is equal to left perpendicularM/Nright perpendicular + 1 except one case on the T-2/Z(3) orbifold. - Tatsuo Kobayashi, Osamu Seto, Takashi Shimomura, Yuko Urakawa MODERN PHYSICS LETTERS A 32 (27) 2017年09月 [査読無し][通常論文]

We investigate cosmological constraints on the original relaxion scenario proposed by Graham, Kaplan and Rajendran. We first discuss the appropriate sign choice of the terms in the scalar potential, when the QCD axion is the relaxion with a relaxion-inflaton coupling proposed in the original paper. We next derive the cosmologically consistent ranges of the mass and a coupling of the relaxion for both the QCD relaxion and non-QCD relaxion. The mass range is obtained by 10(-5) eV << m(phi) less than or similar to 10(4) eV. We also find that a strong correlation between the Hubble parameter at the relaxion stabilization and the scale A of non-QCD strong dynamics, which generates the non-perturbative relaxion cosine potential. For a higher relaxion mass, a large scale A becomes available. However, for its lower mass, A should be small and constructing such a particle physics model is challenging. - Tatsuo Kobayashi, Naoya Omoto, Hajime Otsuka, Takuya H. Tatsuishi PHYSICAL REVIEW D 96 (4) 2017年08月 [査読無し][通常論文]

We propose a new type of moduli stabilization scenario where the supersymmetric and supersymmetry-breaking minima are degenerate at the leading level. The inclusion of the loop corrections originating from the matter fields resolves this degeneracy of vacua. Light axions are predicted in one of our models. - Hiroyuki Abe, Tatsuo Kobayashi, Keigo Sumita, Shohei Uemura Phys. Rev. D 96, 026019 (2017) 96 (2) 2017年07月 [査読無し][通常論文]

We study K\"ahler moduli stabilizations in semi-realistic magnetized D-brane

models based on $ Z_2\times Z_2'$ toroidal orbifolds. In type IIB

compactifications, 3-form fluxes can stabilize the dilaton and complex

structure moduli fields, but there remain some massless closed string moduli

fields, K\"ahler moduli. The magnetic fluxes generate Fayet-Iliopoulos terms,

which can fix ratios of K\"ahler moduli. On top of that, we consider D-brane

instanton effects to stabilize them in concrete D-brane models and investigate

the brane configurations to confirm that the moduli fields can be stabilized

successfully. In this paper, we treat two types of D-brane models. One is based

on D9-brane systems respecting the Pati-Salam model. The other is realized in a

D7-brane system breaking the Pati-Salam gauge group. We find suitable

configurations where the D-brane instantons can stabilize the moduli fields

within both types of D-brane models, explaining an origin of a small constant

term of the superpotential which is a key ingredient for successful moduli

stabilizations. - Tatsuo Kobayashi, Shohei Uemura, Junji Yamamoto PHYSICAL REVIEW D 96 (2) 2017年07月 [査読無し][通常論文]

We investigate the axion inflation model derived by polyinstanton effects in type II superstring theories. Polyinstanton effects are instanton effects corrected by another instanton and they can generate the modulus-axion potential with the double exponential function. Although the axion has a period of small value, this potential can have a flat region because its derivatives are exponentially suppressed by nonperturbative effects. From the view point of the cosmic inflation, such a potential is interesting. In this paper, we numerically study the possibilities of realizing the cosmic inflation. We also study their spectral index and other cosmological observables, numerically. - Kensuke Akita, Tatsuo Kobayashi, Hajime Otsuka JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS (4) 2017年04月 [査読無し][通常論文]

String theory generically predicts the coupling between the Affleck-Dine field and axion field through higher-dimensional operators. We thus explore the Affleck-Dine baryogenesis on an axion background. It turns out that the axion oscillation produces an enough amount of baryon asymmetry of the Universe just after the inflation, even without a soft supersymmetry-breaking A-term. This baryogenesis scenario is applicable to the string axion inflation. - Tatsuo Kobayashi, Kenji Nishiwaki, Yoshiyuki Tatsuta JOURNAL OF HIGH ENERGY PHYSICS (4) 2017年04月 [査読無し][通常論文]

We study the CP-violating phase of the quark sector on T-2/Z(N) (N = 2, 3, 4, 6) with non-vanishing magnetic fluxes, where properties of possible origins of the CP violation are investigated minutely. In this system, a non-vanishing value is mandatory in the real part of the complex modulus parameter tau of the two-dimensional torus in order to explain the CP violation in the quark sector. On T-2 without orbifolding, underlying discrete flavor symmetries severely restrict the form of Yukawa couplings and it is very difficult to reproduce the observed pattern in the quark sector including the CP-violating phase delta(CP). When multiple Higgs doublets emerge on T-2/Z(2), the mass matrices of the zero -mode fermions can be written in the Gaussian textures by choosing appropriate configurations of vacuum expectation values of the Higgs fields. When such Gaussian textures of mass matrices are realized, we show that all of the quark profiles, which are mass hierarchies among the quarks, quark mixing angles, and delta(CP) can be simultaneously realized. - Tatsuo Kobayashi, Osamu Seto, Takuya H. Tatsuishi 2017年03月29日 [査読無し][通常論文]

In string-inspired supergravity theory, K\"{a}hler metric of chiral matter

fields often has a pole. Such K\"{a}hler metric is interesting from the

viewpoint of the framework of the pole inflation, where the scalar potential

can be stretched out to be flat around the pole for a canonically normalized

field and inflation can be realized. However, when K\"{a}hler metric has a

pole, the scalar potential can also have a pole at the same point in

supergravity theory. We study such supergravity models with the pole, and

provide numerical analysis of inflationary dynamics and resultant density

perturbation. In contrast with usual pole inflation models, inflation in this

supergravity based model occurs not on the pole but region apart from the pole.

We show that the existence of the pole in the scalar potential is crucial

nevertheless. We also examine attractor behavior of our model. - Tatsuo Kobayashi, Akane Oikawa, Naoya Omoto, Hajime Otsuka, Ikumi Saga PHYSICAL REVIEW D 95 (6) 2017年03月 [査読無し][通常論文]

We study general class of small-field axion inflations, which are the mixture of polynomial and sinusoidal functions suggested by the natural and axion monodromy inflations. The axion decay constants leading to the successful axion inflations are severely constrained in order not to spoil the big bang nucleosynthesis and overproduce the isocurvature perturbation originating from the QCD axion. We in turn find that the cosmologically favorable axion decay constants are typically of order the grand unification scale or the string scale, which is consistent with the prediction of closed-string axions. - Hiroyuki Abe, Tatsuo Kobayashi, Keigo Sumita, Yoshiyuki Tatsuta PHYSICAL REVIEW D 95 (1) 2017年01月 [査読無し][通常論文]

We study supersymmetric (SUSY) models derived from the ten-dimensional SUSY Yang-Mills theory compactified on magnetized orbifolds, with nonvanishing Fayet-Iliopoulos (FI) terms induced by magnetic fluxes in extra dimensions. Allowing the presence of FI- terms relaxes a constraint on flux configurations in SUSY model building based on magnetized backgrounds. In this case, charged fields develop their vacuum expectation values to cancel the FI- terms in the D-flat directions of fluxed gauge symmetries, which break the gauge symmetries and lead to a SUSY vacuum. Based on this idea, we propose a new class of SUSY magnetized orbifold models with three generations of quarks and leptons. Especially, we construct a model where the right-handed sneutrinos develop their vacuum expectation values which restore the supersymmetry but yield lepton number violating terms below the compactification scale, and show their phenomenological consequences. - Hiroyuki Abe, Tatsuo Kobayashi, Hajime Otsuka, Yasufumi Takano, Takuya H. Tatsuishi PHYSICAL REVIEW D 94 (12) 2016年12月 [査読無し][通常論文]

We study the flavor structure in SO(32) heterotic string theory on six-dimensional tori with magnetic fluxes. Specifically, we focus on models with the flavor symmetries SU(3)(f) and Delta(27). In both models, we can realize the realistic quark masses and mixing angles. - Kenji Kadota, Tatsuo Kobayashi, Akane Oikawa, Naoya Omoto, Hajime Otsuka, Takuya H. Tatsuishi JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS (10) 2016年10月 [査読無し][通常論文]

We study an axion inflation model recently proposed within the framework of type IIB superstring theory, where we pay a particular attention to a sub-Planckian axion decay constant. Our axion potential can lead to the small field inflation with a small tensor-to-scalar ratio, and a typical reheating temperature can be as low as GeV. - Hiroyuki Abe, Tatsuo Kobayashi, Keigo Sumita NUCLEAR PHYSICS B 911 606 -622 2016年10月 [査読無し][通常論文]

We construct several dynamical supersymmetry breaking (DSB) models within a single ten-dimensional supersymmetric Yang-Mills (SYM) theory, compactified on magnetized tori with or without orbifolding. We study the case that the supersymmetry breaking is triggered by a strong dynamics of SU(NC) SYM theory with N-F flavors contained in the four-dimensional effective theory. We show several configurations of magnetic fluxes and orbifolds, those potentially yield, below the compactification scale, the field contents and couplings required for triggering DSB. We especially find a class of self-complete DSB models on orbifolds, where all the extra fields are eliminated by the orbifold projection and DSB successfully occurs within the given framework. Comments on some perspectives for associating the obtained DSB models with the other sectors, such as the visible sector and another hidden sector for, e.g., stabilizing moduli, are also given. (C) 2016 The Authors. Published by Elsevier B.V. - Florian Beye, Tatsuo Kobayashi, Shogo Kuwakino PHYSICS LETTERS B 760 63 -68 2016年09月 [査読無し][通常論文]

We study dilaton stabilization in heterotic string models. By utilizing the asymmetric orbifold construction, we construct an explicit three-generation model whose matter content in the visible sector is the supersymmetric standard model with additional vectorlike matter. This model does not contain any geometric moduli fields except the dilaton field. Model building at a symmetry enhancement point in moduli space enlarges the rank of the hidden gauge group. By analyzing multiple hidden gauge sectors, the dilaton field is stabilized by the racetrack mechanism. We also discuss a supersymmetry breaking scenario and F-term uplifting. (C) 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license. - Tatsuo Kobayashi, Satoshi Nagamoto, Shohei Uemura 2016年08月22日 [査読無し][通常論文]

We study the modular symmetry in four-dimensional low-energy effective field

theory, which is derived from type IIB magnetized D-brane models and type IIA

intersecting D-brane models. We analyze modular symmetric behaviors of

perturbative terms and non-perturbative terms induced by D-brane instanton

effects. Anomalies are also investigated and such an analysis on anomalies

suggests corrections in effective field theory. - Yukihiro Fujimoto, Tatsuo Kobayashi, Kenji Nishiwaki, Makoto Sakamoto, Yoshiyuki Tatsuta PHYSICAL REVIEW D 94 (3) 2016年08月 [査読無し][通常論文]

Based on the results of the classification by T.-h. Abe et al., Nucl. Phys. B894, 374 (2015)., we exhaustively investigate Yukawa sector of U(8) model on magnetized orbifolds T-2 / Z(2), T-2 / Z(3), T-2 / Z(4) and T-2 / Z(6) by evaluating ratios of the mass eigenvalues of the three states in all the possible configurations with one and two Higgs pairs where three generations are realized in fermions. Because of the smearing effect via kinetic mixing, one can realize a hierarchy such as 10(-2)-10(-3), but it is very difficult to achieve the mass ratio between the up and top quarks (m(up) / m(top) similar to 10(-5)) on the complicated magnetized orbifolds T-2 / Z(N) (N similar to 3,4,6). - Tatsuo Kobayashi, Daisuke Nitta, Yuko Urakawa JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS (8) 2016年08月 [査読無し][通常論文]

Modular invariance is a striking symmetry in string theory, which may keep stringy corrections under control. In this paper, we investigate a phenomenological consequence of the modular invariance, assuming that this symmetry is preserved as well as in a four dimensional (4D) low energy effective field theory. As a concrete setup, we consider a modulus field T whose contribution in the 4D effective field theory remains invariant under the modular transformation and study inflation drived by T. The modular invariance restricts a possible form of the scalar potenntial. As a result, large field models of inflation are hardly realized. Meanwhile, a small field model of inflation can be still accomodated in this restricted setup. The scalar potential traced during the slow-roll inflation mimics the hilltop potential V-ht, but it also has a non-negligible deviation from V-ht. Detecting the primordial gravitational waves predicted in this model is rather challenging. Yet, we argue that it may be still possible to falsify this model by combining the information in the reheating process which can be determined self-completely in this setup. - Hiroyuki Abe, Tatsuo Kobayashi, Hajime Otsuka, Yasufumi Takano, Takuya H. Tatsuishi PROGRESS OF THEORETICAL AND EXPERIMENTAL PHYSICS (5) 2016年05月 [査読無し][通常論文]

We study SO(32) heterotic string theory on a torus with magnetic fluxes. Non-vanishing fluxes can lead to non-universal gauge kinetic functions for SU(3) x SU(2) x U(1)gamma, which is an important feature of SO(32) heterotic string theory in contrast to the E-8 x E-8 theory. It is found that the experimental values of gauge couplings are realized with O(1) values of moduli fields based on realistic models with the SU(3) x SU(2) x U(1)gamma gauge symmetry and three chiral generations of quarks and leptons without chiral exotics. - Kensuke Akita, Tatsuo Kobayashi, Akane Oikawa, Hajime Otsuka JOURNAL OF HIGH ENERGY PHYSICS (5) 2016年05月 [査読無し][通常論文]

We study the moduli-induced gravitino problem within the framework of the phenomenologically attractive mirage mediations. The huge amount of gravitino generated by the moduli decay can be successfully diluted by introducing an extra light modulus field which does not induce the supersymmetry breaking. Since the lifetime of extra modulus field becomes longer than usually considered modulus field, our proposed mechanism is applied to both the low- and high-scale supersymmetry breaking scenarios. We also point out that such an extra modulus field appears in the flux compactification of type II string theory. - Tatsuo Kobayashi, Akane Oikawa, Hajime Otsuka PHYSICAL REVIEW D 93 (8) 2016年04月 [査読無し][通常論文]

We propose a new type of axion inflation with complex structure moduli in the framework of type IIB superstring theory compactified on the Calabi-Yau manifold. The inflaton is identified as the axion for the complex structure moduli whose potential is originating from instantonic corrections appearing through the period vector of the mirror Calabi-Yau manifold. The axionic shift symmetry is broken down to the discrete one by the inclusion of the instantonic correction and certain three-form fluxes. Our proposed inflation scenario is compatible with Kahler moduli stabilization. We also study a typical reheating temperature in the case of complex structure moduli inflation. - Tatsuo Kobayashi, Yoshiyuki Tatsuta, Shohei Uemura PHYSICAL REVIEW D 93 (6) 2016年03月 [査読無し][通常論文]

We study the effects of D-brane instantons wrapping rigid cycles on the Z(2) x Z(2)' toroidal orbifold. We compute Majorana masses induced by rigid D-brane instantons and realize bimaximal mixing matrices in certain models. We can also derive more generic mass matrices in other models. The bimaximal mixing Majorana mass matrix provides a possibility for explaining observed mixing angles. We also compute the mu-term matrix among more than one pair of Higgs fields induced by rigid D-brane instantons. - Kei Hagimoto, Tatsuo Kobayashi, Hiroki Makino, Ken-ichi Okumura, Takashi Shimomura JOURNAL OF HIGH ENERGY PHYSICS (2) 2016年02月 [査読無し][通常論文]

We study the next-to-minimal supersymmetric standard model (NMSSM) with the TeV scale mirage mediation, which is known as a solution for the little hierarchy problem in supersymmetry. Our previous study showed that 125 GeV Higgs boson is realized with O (10)% fine-tuning for 1.5 TeV gluino (1TeV stop) mass. The mu term could be as large as 500 GeV without sacrificing the fine-tuning thanks to a cancellation mechanism. The singlet-doublet mixing is suppressed by tan beta. In this paper, we further extend this analysis. We argue that approximate scale symmetries play a role behind the suppression of the singlet-doublet mixing. They reduce the mixing matrix to a simple form that is useful to understand the results of the numerical analysis. We perform a comprehensive analysis of the fine-tuning including the singlet sector by introducing a simple formula for the fine-tuning measure. This shows that the singlet mass of the least fine-tuning is favored by the LEP anomaly for moderate tan beta. We also discuss prospects for the precision measurements of the Higgs couplings at LHC and ILC and direct/indirect dark matter searches in the model. - Kenji Kadota, Tatsuo Kobayashi, Hajime Otsuka JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS (1) 2016年01月 [査読無し][通常論文]

We study the inflation scenarios, in the framework of superstring theory, where the inflaton is an axion producing the adiabatic curvature perturbations while there exists another light, axion producing the isocurvature perturbations. We discuss how the non-trivial couplings among string axions can generically arise, and calculate the consequent cross-correlations between the adiabatic and isocurvature modes through concrete examples. Based on the Planck analysis on the generally correlated isocurvature perturbations, we show that there is a preference for the existence of the correlated isocurvature modes for the axion monodromy inflation while the natural inflation disfavors such isocurvature modes. - Hironori Hattori, Tatsuo Kobayashi, Naoya Omoto, Osamu Seto PHYSICAL REVIEW D 92 (10) 2015年11月 [査読無し][通常論文]

We consider domain walls in the Z(3) symmetric next-to-minimal supersymmetric standard model. The spontaneous Z(3) discrete symmetry breaking produces domain walls, and the stable domain walls are problematic. Thus, we assume the Z(3) symmetry is slightly but explicitly broken and the domain walls decay. Such a decay causes a large late-time entropy production. We study its cosmological implications on unwanted relics such as the moduli, gravitino, lightest superparticle, and axion. - Hiroyuki Abe, Tatsuo Kobayashi, Hajime Otsuka, Yasufumi Takano JOURNAL OF HIGH ENERGY PHYSICS (9) 2015年09月 [査読無し][通常論文]

We search for realistic supersymmetric standard-like models from SO(32) heterotic string theory on factorizable tori with multiple magnetic fluxes. Three chiral ganerations of quarks and leptons are derived from the adjoint and vector representations of SO(12) gauge groups embedded in SO(32) adjoint representation. Massless spectra of our models also include Higgs fields, which have desired Yukawa couplings to quarks and leptons at the tree-level. - Yuta Hamada, Tatsuo Kobayashi, Shohei Uemura NUCLEAR PHYSICS B 897 563 -582 2015年08月 [査読無し][通常論文]

We systematically search intersecting D-brane models, which just realize the Standard Model chiral matter contents and gauge symmetry. We construct new classes of non-supersymmetric Standard Model-like models. We also study the gauge coupling constants of these models. The tree level gauge coupling is a function of the compactification moduli, the string scale, the string coupling and the winding numbers of D-branes. By tuning them, we examine whether the models can explain the experimental values of gauge couplings. As a result, we find that the string scale should be greater than 10(14-15) GeV if the compactification scale and the string scale are of the same order. (C) 2015 The Authors. Published by Elsevier B.V. - Hironori Hattori, Tatsuo Kobayashi, Naoya Omoto, Osamu Seto PHYSICAL REVIEW D 92 (2) 2015年07月 [査読無し][通常論文]

Axions in the Peccei-Quinn (PQ) mechanism provide a promising solution to the strong CP problem in the standard model of particle physics. Coherently generated PQ scalar fields could dominate the energy density in the early Universe and decay into relativistic axions, which would conflict with the current dark radiation constraints. We study the possibility that a thermal inflation driven by a U(1) gauged Higgs field dilutes such axions. A well-motivated extra gauged U(1) would be the local B - L symmetry. We also discuss the implication for the case of U(1)(B-L) and an available baryogenesis mechanism in such cosmology. - Hiroyuki Abe, Tatsuo Kobayashi, Yoshiyuki Tatsuta, Shohei Uemura PHYSICAL REVIEW D 92 (2) 2015年07月 [査読無し][通常論文]

We study the mu-term matrix of Higgs pairs induced by the D-brane instanton effects in intersecting D6-brane models compactified on T-6. It is found that the mu-term matrix has a certain permutation symmetry and its eigenvalues have large hierarchical structure without fine-tuning. - Tatsuo Kobayashi, Osamu Seto MODERN PHYSICS LETTERS A 30 (21) 2015年07月 [査読無し][通常論文]

Recent detection of B-mode polarization induced from tensor perturbations by the BICEP2 experiment implies the so-called large field inflation, where an inflaton field takes super-Planckian expectation value during inflation, at a high energy scale. We show however, if another inflation follows hybrid inflation, the hybrid inflation can generate a large tensor perturbation with not super-Planckian but Planckian field value. This scenario would relax the tension between BICEP2 and Planck concerning the tensor-to-scalar ratio, because a negative large running can also be obtained for a certain number of e-fold of the hybrid inflation. A natural interpretation of a large gravitational wave mode with or without the scalar spectral running might be multiple inflation in the early Universe. - Hiroyuki Abe, Tatsuo Kobayashi, Hajime Otsuka PROGRESS OF THEORETICAL AND EXPERIMENTAL PHYSICS (6) 2015年06月 [査読無し][通常論文]

We propose natural inflation from the heterotic string theory on the "Swiss-Cheese" Calabi-Yau manifold with multiple U(1) magnetic fluxes. Such multiple U(1) magnetic fluxes stabilize the same number of the linear combination of the universal axion and Kahler axions, and one of the Kahler axions is identified as the inflaton. This axion decay constant can be determined by the size of one-loop corrections to the gauge kinetic function of the hidden gauge groups, which leads effectively to the trans-Planckian axion decay constant consistent with the Planck data. During the inflation, the real parts of the moduli are also stabilized by employing the nature of the "Swiss-Cheese" Calabi-Yau manifold. - Tatsuo Kobayashi, Yuji Omura, Fumihiro Takayama, Daiki Yasuhara 2015年05月28日 [査読無し][通常論文]

Flavor symmetric model is one of the attractive Beyond Standard Models (BSMs)

to reveal the flavor structure of the Standard Model (SM). A lot of efforts

have been put into the model building and we find many kinds of flavor

symmetries and setups are able to explain the observed fermion mass matrices.

In this paper, we look for common predictions of physical observables among the

ones in flavor symmetric models, and try to understand how to test flavor

symmetry in experiments. Especially, we focus on the BSMs for leptons with

extra Higgs $SU(2)_L$ doublets charged under flavor symmetry. In many flavor

models for leptons, remnant symmetry is partially respected after the flavor

symmetry breaking, and it controls well the Flavor Changing Neutral Currents

(FCNCs) and suggests some crucial predictions against the flavor changing

process, although the remnant symmetry is not respected in the full lagrangian.

In fact, we see that $\tau^- \to e^+ \mu^- \mu^-$ $( \mu^+ e^- e^-)$ and $e^+

e^- \to \tau^+\tau^-$ $(\mu^-\mu^+)$ processes are the most important in the

flavor models that the extra Higgs doublets belong to triplet representation of

flavor symmetry. For instance, the stringent constraint from the $\mu \to e

\gamma$ process could be evaded according to the partial remnant symmetry. We

also investigate the breaking effect of the remnant symmetry mediated by the

Higgs scalars, and investigate the constraints from the flavor physics: the

flavor violating $\tau$ and $\mu$ decays, the electric dipole moments, and the

muon anomalous magnetic moment. We also discuss the correlation between FCNCs

and nonzero $\theta_{13}$, and point out the physical observables in the

charged lepton sector to test the BSMs for the neutrino mixing. - Tomo-hiro Abe, Yukihiro Fujimoto, Tatsuo Kobayashi, Takashi Miura, Kenji Nishiwaki, Makoto Sakamoto, Yoshiyuki Tatsuta NUCLEAR PHYSICS B 894 374 -406 2015年05月 [査読無し][通常論文]

We classify the combinations of parameters which lead three generations of quarks and leptons in the framework of magnetized twisted orbifolds on T-2/Z(2), T-2/Z(3), T-2/Z(4) and T-2/Z(6) with allowing nonzero discretized Wilson line phases and Scherk-Schwarz phases. We also analyze two actual examples with nonzero phases leading to one-pair Higgs and five-pair Higgses and discuss the difference from the results without nonzero phases studied previously. (Note: this article is registered under preprint number: arXiv:1501.02787 [hep-ph].) (C) 2015 The Authors. Published by Elsevier B.V. - Hiroyuki Abe, Tatsuo Kobayashi, Hajime Otsuka JOURNAL OF HIGH ENERGY PHYSICS (4) 2015年04月 [査読無し][通常論文]

We propose a mechanism for the natural inflation with and without modulation in the framework of type IIB string theory on toroidal orientifold or orbifold. We explicitly construct the stabilization potential of complex structure, dilaton and Kahler moduli, where one of the imaginary component of complex structure moduli becomes light which is identified as the inflaton. The inflaton potential is generated by the gaugino-condensation term which receives the one-loop threshold corrections determined by the field value of complex structure moduli and the axion decay constant of inflaton is enhanced by the inverse of one-loop factor. We also find the threshold corrections can also induce the modulations to the original scalar potential for the natural inflation. Depending on these modulations, we can predict several sizes of tensor-to-scalar ratio as well as the other cosmological observables reported by WMAP, Planck and/or BICEP2 collaborations. - Florian Beye, Tatsuo Kobayashi, Shogo Kuwakino JOURNAL OF HIGH ENERGY PHYSICS (3) 2015年03月 [査読無し][通常論文]

We investigate a gauge theory realization of non-Abelian discrete flavor symmetries and apply the gauge enhancement mechanism in heterotic orbifold models to field-theoretical model building. Several phenomenologically interesting non-Abelian discrete symmetries are realized effectively from a U(1) gauge theory with a permutation symmetry. We also construct a concrete model for the lepton sector based on a U(1)(2) x S-3 symmetry. - Tatsuo Kobayashi, Yuji Omura JOURNAL OF HIGH ENERGY PHYSICS (2) 1 -24 2015年02月 [査読無し][通常論文]

We build explicit supersymmetric unification models where grand unified gauge symmetry breaking and supersymmetry (SUSY) breaking are caused by the same sector. Besides, the SM-charged particles are also predicted by the symmetry breaking sector, and they give the soft SUSY breaking terms through the so-called gauge mediation. We investigate the mass spectrums in an explicit model with SU(5) and additional gauge groups, and discuss its phenomenological aspects. Especially, nonzero A-term and B-term are generated at one-loop level according to the mediation via the vector superfields, so that the electro-weak symmetry breaking and 125 GeV Higgs mass may be achieved by the large B-term and A-term even if the stop mass is around 1 TeV. - Tomo-hiro Abe, Yukihiro Fujimoto, Tatsuo Kobayashi, Takashi Miura, Kenji Nishiwaki, Makoto Sakamoto NUCLEAR PHYSICS B 890 442 -480 2015年01月 [査読無し][通常論文]

We discuss an effective way for analyzing the system on the magnetized twisted orbifolds in operator formalism, especially in the complicated cases T-2/Z(3), T-2/Z(4) and T-2/Z(6). We can obtain the exact and analytical results which can be applicable for any larger values of the quantized magnetic flux M, and show that the (non-diagonalized) kinetic terms are generated via our formalism and the number of the surviving physical states are calculable in a rigorous manner by simply following usual procedures in linear algebra in any case. Our approach is very powerful when we try to examine properties of the physical states on (complicated) magnetized orbifolds T-2/Z(3), T-2/Z(4), T-2/Z(6) (and would be in other cases on higher-dimensional torus) and could be an essential tool for actual realistic model construction based on these geometries. (Note: This article is registered under preprint number: arXiv:1409.5421.) (C) 2014 The Authors. Published by Elsevier B.V. - Hiroyuki Abe, Tatsuo Kobayashi, Keigo Sumita, Yoshiyuki Tatsuta PHYSICAL REVIEW D 90 (10) 2014年11月 [査読無し][通常論文]

We study the structure of Yukawa matrices derived from the supersymmetric Yang-Mills theory on magnetized orbifolds, which can realize the observed quark and charged lepton mass ratios as well as the Cabibbo-Kobayashi-Maswaka (CKM) mixing angles, even with a small number of tunable parameters and without any critical fine-tuning. As a reason behind this, we find that the obtained Yukawa matrices possess a Froggatt-Nielsen-like structure with Gaussian hierarchies, which provides a suitable texture for them favored by the experimental data. - Yuta Hamada, Tatsuo Kobayashi, Atsushi Ogasahara, Yuji Omura, Fumihiro Takayama, Daiki Yasuhara JHEP10(2014)183 (10) 2014年10月 [査読無し][通常論文]

We revisit the discrete dark matter model with $A_4$ flavor symmetry

originally introduced by M.Hirsch {\it et.al}. We show that radiative

corrections can lead to non-zero $\theta_{13}$ and non-zero mass for the

lightest neutrino. We find an interesting relation among neutrino mixing

parameters and it indicates the sizable deviation of $s_{23}$ from the maximal

angle $s_{23}^2=1/2$ and the degenerate mass spectrum for neutrinos. Also we

study the possibilities that the right-handed neutrino is a dark matter

candidate. Assuming the thermal freeze-out explains observed dark matter

abundance, TeV-scale right-handed neutrino and flavored scalar bosons are

required. In such a case, flavor symmetry plays an important role for the

suppression of lepton flavor violating processes as well as for the stability

of dark matter. We show that this scenario can be viable against currently

existing constraints from collider, low energy experiments and cosmological

observations. - Tetsutaro Higaki, Tatsuo Kobayashi, Osamu Seto, Yuya Yamaguchi JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS (10) 2014年10月 [査読無し][通常論文]

We study parameter space in the axion monodromy inflation corrected by nautically generated terms involving with the axion. The potential has the linear term with multiple sinusoidal functions, which play a role in generating modulations. We show that this potential leads both to a large tensor-to-scalar ratio r(T) similar to 0.16 and to a large negative running of spectral index alpha(s) similar to -(0.02 - 0.03), ameliorating the tension between the result of the PLANCK and that of the BICEP2. To realize these results, a small hierarchy among dynamical scales is required whereas the decay constants in sinusoidal functions remain sub-Planckian in this model. We discuss also reheating process after the inflation in a bottom-up approach. - Tatsuo Kobayashi, Osamu Seto, Yuya Yamaguchi PROGRESS OF THEORETICAL AND EXPERIMENTAL PHYSICS (10) 2014年10月 [査読無し][通常論文]

We study the axion monodromy inflation with a non-perturbatively generated sinusoidal term. The potential form is a mixture between the natural inflation and the axion monodromy inflation potentials. The sinusoidal term is subdominant in the potential, but leaves significant effects on the resultant fluctuation generated during inflation. A larger tensor-to-scalar ratio can be obtained in our model. We study two scenarios, the single inflation scenario and the double inflation scenario. In the first scenario, the axion monodromy inflation with a sufficient number of e-folds generates a larger tensor-to-scalar ratio of about 0.1-0.15 but also a tiny running of the spectral index. In the second scenario of double inflation, axion monodromy inflation is its first stage, and we assume another inflation follows. In this case, our model can realize a larger tensor-to-scalar ratio and a large negative running of the spectral index simultaneously. - Florian Beye, Tatsuo Kobayashi, Shogo Kuwakino PHYSICS LETTERS B 736 433 -437 2014年09月 [査読無し][通常論文]

We show that non-Abelian discrete symmetries in orbifold string models have a gauge origin. This can be understood when looking at the vicinity of a symmetry enhanced point in moduli space. At such an enhanced point, orbifold fixed points are characterized by an enhanced gauge symmetry. This gauge symmetry can be broken to a discrete subgroup by a nontrivial vacuum expectation value of the Kahler modulus T. Using this mechanism it is shown that the Delta (54) non-Abelian discrete symmetry group originates from a SU(3) gauge symmetry, whereas the D-4 symmetry group is obtained from a SU(2) gauge symmetry. (C) 2014 The Authors. Published by Elsevier B.V. - 上村 尚平, 小林 達夫, 濱田 雄太 日本物理学会講演概要集 69 (2) 16 -16 2014年08月22日
- Hiroyuki Abe, Tatsuo Kobayashi, Hiroshi Ohki, Keigo Sumita, Yoshiyuki Tatsuta JOURNAL OF HIGH ENERGY PHYSICS (6) 2014年06月 [査読無し][通常論文]

We study discrete flavor symmetries of the models based on a ten-dimensional supersymmetric Yang-Mills (10D SYM) theory compactified on magnetized tori. We assume non-vanishing non-factorizable fluxes as well as the orbifold projections. These setups allow model-building with more various flavor structures. Indeed, we show that there exist various classes of non-Abelian discrete flavor symmetries. In particular, we find that S-3 flavor symmetries can be realized in the framework of the magnetized 10D SYM theory for the first time. - Yuta Hamada, Tatsuo Kobayashi, Shohei Uemura JOURNAL OF HIGH ENERGY PHYSICS (5) 2014年05月 [査読無し][通常論文]

We study the flavor structure in intersecting D-brane models. We study anomalies of the discrete flavor symmetries. We analyze the Majorana neutrino masses, which can be generated by D-brane instanton effects. It is found that a certain pattern of mass matrix is obtained and the cyclic permutation symmetry remains unbroken. As a result, trimaximal mixing matrix can be realized if Dirac neutrino mass and charged lepton mass matrices are diagonal. - Tatsuo Kobayashi, Osamu Seto PHYSICAL REVIEW D 89 (10) 2014年05月 [査読無し][通常論文]

Large field inflation models are favored by the recent BICEP2 that has detected gravitational wave modes generated during inflation. We study general large field inflation models for which the potential contains (constant) quadratic and quartic terms of inflaton field. We show, in this framework, those inflation models can generate the fluctuation with the tensor-to-scalar ratio of 0.2 as well as the scalar spectral index of 0.96; those are very close to the center value of the tensor-to- scalar ratio reported by BICEP2 as well as Planck. Finally, we briefly discuss the particle physics model building of inflation. - Hiroyuki Abe, Tatsuo Kobayashi, Hiroshi Ohki, Keigo Sumita, Yoshiyuki Tatsuta JOURNAL OF HIGH ENERGY PHYSICS (4) 2014年04月 [査読無し][通常論文]

We study the flavor landscape of the visible sector of particle physics models based on a ten-dimensional super Yang-Mills theory compactified on magnetized tori preserving four-dimensional Nu = 1 supersymmetry. Recently, we constructed a semirealistic model which contains the minimal supersymmetric standard model (MSSM) using an ansatz of magnetic fluxes and orbifolding projections. However, we can consider more various configurations of magnetic fluxes and orbifolding projections preserving fourdimensional N = 1 supersymmetry. We research systematically such possibilities for leading to MSSM-like models and study their phenomenological aspects. - 上村 尚平, 小林 達夫, 濱田 雄太 日本物理学会講演概要集 69 (1) 12 -12 2014年03月05日
- 安倍博之, 小林達夫, 大木洋, 角田慶吾, 龍田佳幸 日本物理学会講演概要集 69 (1) 13 -13 2014年03月05日 [査読無し][通常論文]
- 阿部智大, 藤本教寛, 小林達夫, 三浦貴司, 西脇健二, 坂本眞人 日本物理学会講演概要集 69 (1) 12 -12 2014年03月05日 [査読無し][通常論文]
- 馬塲一晴, 小林達夫, 瀬戸治 日本物理学会講演概要集 69 (1) 124 -124 2014年03月05日 [査読無し][通常論文]
- Nana G. Cabo Bizet, Tatsuo Kobayashi, Damian K. Mayorga Pena, Susha L. Parameswaran, Matthias Schmitz, Ivonne Zavala JOURNAL OF HIGH ENERGY PHYSICS (2) 2014年02月 [査読無し][通常論文]

We study discrete R-symmetries, which appear in the 4D low energy effective field theory derived from heterotic orbifold models. We derive the R-symmetries directly from the geometrical symmetries of the orbifolds. In particular, we obtain the corresponding R-charges by requiring that the couplings be invariant under these symmetries. This allows for a more general treatment than the explicit computations of correlation functions made previously by the authors, including models with discrete Wilson lines, and orbifold symmetries beyond plane-by-plane rotational invariance. The R-charges obtained in this manner differ from those derived in earlier explicit computations. We study the anomalies associated with these R-symmetries, and comment on the results. - Tomo-hiro Abe, Yukihiro Fujimoto, Tatsuo Kobayashi, Takashi Miura, Kenji Nishiwaki, Makoto Sakamoto JOURNAL OF HIGH ENERGY PHYSICS (1) 2014年01月 [査読無し][通常論文]

We propose new backgrounds of extra dimensions to lead to four-dimensional chiral models with three generations of matter fermions, that is T (2) /Z (N) twisted orbifolds with magnetic fluxes. We consider gauge theory on six-dimensional space-time, which contains the T (2) /Z (N) orbifold with magnetic flux, Scherk-Schwarz phases and Wilson line phases. We classify all the possible Scherk-Schwarz and Wilson line phases on T (2) /Z (N) orbifolds with magnetic fluxes. The behavior of zero modes is studied. We derive the number of zero modes for each eigenvalue of the Z (N) twist, showing explicitly examples of wave functions. We also investigate Kaluza-Klein mode functions and mass spectra. - Florian Beye, Tatsuo Kobayashi, Shogo Kuwakino JOURNAL OF HIGH ENERGY PHYSICS (1) 2014年01月 [査読無し][通常論文]

Using Z(3) asymmetric orbifolds in heterotic string theory, we construct N = 1 SUSY three-generation models with the standard model gauge group SU(3)(C) x SU(2)(L) x U(1)(Y) and the left-right symmetric group SU(3)(C) x SU(2)(L) x SU(2)(R) x U(1)(B-L). One of the models possesses a gauge flavor symmetry for the Z(3) twisted matter. - Yuta Hamada, Kohei Kamada, Tatsuo Kobayashi, Yutaka Ookouchi JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS (1) 2014年01月 [査読無し][通常論文]

We study the spontaneous R-symmetry breaking model and investigate the cosmological constraints on this model due to the pseudo Nambu-Goldstone boson, R-axion. We consider the R-axion which has relatively heavy mass in order to complement our previous work. In this regime, model parameters, R-axions mass and R-symmetry breaking scale, are constrained by Big Bang Nucleosynthesis and overproduction of the gravitino produced from R-axion decay and thermal plasma. We find that the allowed parameter space is very small for high reheating temperature. For low reheating temperature, the U(1)(R) breaking scale f(a) is constrained as f(a) < 10(12-14) GeV regardless of the value of R-axion mass. - Takeshi Araki, Hiroyuki Ishida, Hajime Ishimori, Tatsuo Kobayashi, Atsushi Ogasahara PHYSICAL REVIEW D 88 (9) 2013年11月 [査読無し][通常論文]

Following the way proposed recently by Hernandez and Smirnov, we seek possible residual symmetries in the quark sector with a focus on the von Dyck groups. We begin with two extreme cases in which both theta(13) and theta(23) or only theta(13) are set to zero. Then, cases where all the Cabibbo-Kobayashi-Maskawa parameters are allowed to take nonzero values are explored. The Z(7) symmetry is favorable to realize only the Cabibbo angle. On the other hand, larger groups are necessary in order to be consistent with all the mixing parameters. Possibilities of embedding the obtained residual symmetries into the Delta(6N(2)) series are also briefly discussed. - Florian Beye, Tatsuo Kobayashi, Shogo Kuwakino NUCLEAR PHYSICS B 875 (3) 599 -620 2013年10月 [査読無し][通常論文]

We study heterotic asymmetric orbifold models. By utilizing the lattice engineering technique, we classify (22,6)-dimensional Narain lattices with right-moving non-Abelian group factors which can be starting points for Z(3) asymmetric orbifold construction. We also calculate gauge symmetry breaking patterns. (C) 2013 Elsevier B.V. All rights reserved. - 安倍 博之, 小林 達夫, 大木 洋, 角田 慶吾, 龍田 佳幸 日本物理学会講演概要集 68 (2) 1 -1 2013年08月26日 [査読無し][通常論文]
- Nana G. Cabo Bizet, Tatsuo Kobayashi, Damian K. Mayorga Pena, Susha L. Parameswaran, Matthias Schmitz, Ivonne Zavala JOURNAL OF HIGH ENERGY PHYSICS (5) 2013年05月 [査読無し][通常論文]

We consider the string theory origin of R-charge conservation laws in heterotic orbifold compactifications, deriving the corresponding string coupling selection rule for factorizable and non-factorizable orbifolds, with prime ordered and non-prime ordered point groups. R-charge conservation arises due to symmetries among the worldsheet instantons that can mediate the couplings. Among our results is a previously missed non-trivial contribution to the conserved R-charges from the gamma-phases in non-prime orbifolds, which weakens the R-charge selection rule. Symmetries among the worldsheet instantons can also lead to additional selection rules for some couplings. We make a similar analysis for Rule 4 or the "torus lattice selection rule". Moreover, we identify a new string selection rule, that we call Rule 6 or the "coset vector selection rule". - Hiroyuki Abe, Tatsuo Kobayashi, Hiroshi Ohki, Akane Oikawa, Keigo Sumita NUCLEAR PHYSICS B 870 (1) 30 -54 2013年05月 [査読無し][通常論文]

We present a particle physics model based on a ten-dimensional (10D) super Yang-Mills (SYM) theory compactified on magnetized tori preserving four-dimensional N = 1 supersymmetry. The low-energy spectrum contains the minimal supersymmetric standard model with hierarchical Yukawa couplings caused by a wavefunction localization of the chiral matter fields due to the existence of magnetic fluxes, allowing a semi-realistic pattern of the quark and the lepton masses and mixings. We show supersymmetric flavor structures at low energies induced by a moduli-mediated and an anomaly-mediated supersymmetry breaking. (C) 2013 Elsevier B.V. All rights reserved. - Kohei Kamada, Tatsuo Kobayashi, Keisuke Ohashi, Yutaka Ookouchi JOURNAL OF HIGH ENERGY PHYSICS (5) 2013年05月 [査読無し][通常論文]

We study stabilization of an unstable cosmic string associated with spontaneously broken U(1)(R) symmetry, which otherwise causes a dangerous roll-over process. We demonstrate that in a gauge mediation model, messengers can receive enough corrections from the thermal plasma of the supersymmetric standard model particles to stabilize the unstable modes of the string. - Yukihiro Fujimoto, Tatsuo Kobayashi, Takashi Miura, Kenji Nishiwaki, Makoto Sakamoto Physical Review D - Particles, Fields, Gravitation and Cosmology 87 (8) 2013年04月08日 [査読無し][通常論文]

We propose a mechanism to obtain the generation of matter in the standard model. We start from the analysis of the T2/ZN shifted orbifold with magnetic flux, which imposes a ZN symmetry on torus. We also consider several orbifolds such as (T2×T2)/ZN, (T2×T2×T2)/ (ZN×ZN') and (T2×T2×T2)/(Z N×ZN'×ZN'). On such orbifolds, we study the behavior of fermions in two different means - the operator formalism and the explicit analysis of wave functions. For an interesting result, it is found that the number of zero-mode fermions is related to N of the ZN symmetry. In other words, the generation of matter relates to the type of orbifolds. Moreover, we find that shifted orbifold models are severely restricted from realizing three generations. For example, the three-generation model on the type of M4×(T2×T2)/ZN is unique. One can also construct other types of three-generation orbifold models with rich flavor structure. Those results may bring us a realistic model with desired Yukawa structure. © 2013 American Physical Society. - Takashi Hiramatsu, Minoru Eto, Kohei Kamada, Tatsuo Kobayashi, Yutaka Ookouchi 2013年04月02日 [査読無し][通常論文]

The breaking of $U(1)_R$ symmetry plays a crucial role in modeling the

breaking of supersymmetry (SUSY). In the models that possess both SUSY

preserving and SUSY breaking vacua, tube-like cosmic strings called R-tubes,

whose surfaces are constituted by domain walls interpolating a false and a true

vacuum with some winding numbers, can exist. Their (in)stability can strongly

constrain SUSY breaking models theirselves. In the present study, we

investigate the dynamical (in)stability of two colliding metastable tube-like

strings by field-theoretic simulations. From them, we find that the strings

become unstable, depending on the relative collision angle and speed of two

strings, and the false vacuum is eventually filled out by the true vacuum owing

to rapid expansion of the strings or unstable bubbles created as remnants of

the collision. - 藤本 教寛, 三浦 貴司, 坂本 眞人, 小林 達夫, 西脇 健二 日本物理学会講演概要集 68 (1) 7 -7 2013年03月26日 [査読無し][通常論文]
- 安倍 博之, 小林 達夫, 角田 慶吾, 龍田 佳幸 日本物理学会講演概要集 68 (1) 6 -6 2013年03月26日 [査読無し][通常論文]
- 藤本教寛, 三浦貴司, 坂本眞人, 小林達夫, 西脇健二 日本物理学会講演概要集 68 (1) 7 2013年03月05日 [査読無し][通常論文]
- Minoru Eto, Yuta Hamada, Kohei Kamada, Tatsuo Kobayashi, Keisuke Ohashi, Yutaka Ookouchi JOURNAL OF HIGH ENERGY PHYSICS (3) 2013年03月 [査読無し][通常論文]

We show that a cosmic string associated with spontaneous U(1)(R) symmetry breaking gives a constraint for supersymmetric model building. In some models, the string can be viewed as a tube-like domain wall with a winding number interpolating a false vacuum and a true vacuum. Such string causes inhomogeneous decay of the false vacuum to the true vacuum via rapid expansion of the radius of the tube and hence its formation would be inconsistent with the present Universe. However, we demonstrate that there exist metastable solutions which do not expand rapidly. Furthermore, when the true vacua are degenerate, the structure inside the tube becomes involved. As an example, we show a "bamboo"-like solution, which suggests a possibility observing an information of true vacua from outside of the tube through the shape and the tension of the tube. - Tatsuo Kobayashi, Hiroki Makino, Ken-Ichi Okumura, Takashi Shimomura, Tsubasa Takahashi Journal of High Energy Physics 2013 (1) 2013年 [査読無し][通常論文]

We study the next-to-minimal supersymmetric standard model. We consider soft supersymmetry breaking parameters, which are induced by the mirage mediation mechanism of supersymmetry breaking. We concentrate on the mirage mediation, where the so-called mirage scale is the TeV scale. In this scenario, we can realize the up-type Higgs soft mass of O(200) GeV, while other masses such as gaugino masses and stop masses are heavy such as 1 TeV or more. Cancellation between the effective μ-term and the down-type Higgs soft mass ameliorates the fine-tuning in the electroweak symmetry breaking even for μ =O(500) GeV. The mixingbetween the doublet and singlet Higgsbosons issuppressed by (κ/λ) tan-1 β. Then the lightest doublet Higgs mass naturally reaches 125 GeV lifted by the new quartic coupling. The higgsino and singlino are light and their linear combination is the lightest superparticle. © 2013 SISSA, Trieste, Italy. - Tatsuo Kobayashi, Hiroki Makino, Ken-Ichi Okumura, Takashi Shimomura, Tsubasa Takahashi Journal of High Energy Physics 2013 (1) 2013年 [査読無し][通常論文]

We study the next-to-minimal supersymmetric standard model. We consider soft supersymmetry breaking parameters, which are induced by the mirage mediation mechanism of supersymmetry breaking. We concentrate on the mirage mediation, where the so-called mirage scale is the TeV scale. In this scenario, we can realize the up-type Higgs soft mass of O(200) GeV, while other masses such as gaugino masses and stop masses are heavy such as 1 TeV or more. Cancellation between the effective μ-term and the down-type Higgs soft mass ameliorates the fine-tuning in the electroweak symmetry breaking even for μ =O(500) GeV. The mixingbetween the doublet and singlet Higgsbosons issuppressed by (κ/λ) tan-1 β. Then the lightest doublet Higgs mass naturally reaches 125 GeV lifted by the new quartic coupling. The higgsino and singlino are light and their linear combination is the lightest superparticle. © 2013 SISSA, Trieste, Italy. - Yuta Hamada, Kohei Kamada, Tatsuo Kobayashi, Yutaka Ookouchi JCAP 1304 (2013) 043 2012年11月24日 [査読無し][通常論文]

We study general constraints on spontaneous R-symmetry breaking models coming

from the cosmological effects of the pseudo Nambu-Goldstone bosons, R-axions.

They are substantially produced in the early Universe and may cause several

cosmological problems. We focus on relatively long-lived R-axions and find that

in a wide range of parameter space, models are severely constrained. In

particular, R-axions with mass less than 1 MeV are generally ruled out for

relatively high reheating temperature, $T_R>10$ GeV. - Tatsuo Azeyanagi, Tatsuo Kobayashi, Atsushi Ogasahara, Koichi Yoshioka PHYSICAL REVIEW D 86 (9) 2012年11月 [査読無し][通常論文]

We study the D-term effect on runaway directions of the F-term scalar potential. A minimal renormalizable model is presented where supersymmetry is broken without any pseudomoduli. The model is applied to the hidden sector of gauge mediation for spontaneously breaking R symmetry and generating nonvanishing gaugino masses at the one-loop order. - Yuta Hamada, Tatsuo Kobayashi Progress of Theoretical Physics 128 (5) 903 -923 2012年11月 [査読無し][通常論文]

We study higher dimensional models with magnetic fluxes, which can be derived from superstring theory. We study mass spectrum and wavefunctions of massless and massive modes for spinor, scalar and vector fields. We compute the 3-point couplings and higher order couplings among massless modes and massive modes in 4D low-energy effective field theory. These couplings have non-trivial behaviors, because wavefunctions of massless and massive modes are non-trivial. Copyright © Progress of Theoretical Physics 2012. - Hiroyuki Abe, Tatsuo Kobayashi, Hiroshi Ohki, Keigo Sumita NUCLEAR PHYSICS B 863 (1) 1 -18 2012年10月 [査読無し][通常論文]

We present a four-dimensional (4D) N = 1 superfield description of supersymmetric Yang-Mills (SYM) theory in ten-dimensional (10D) spacetime with certain magnetic fluxes in compactified extra dimensions preserving partial N = 1 supersymmetry out of full N = 4. We derive a 41) effective action in N = 1 superspace directly from the 10D superfield action via dimensional reduction, and identify its dependence on dilaton and geometric moduli superfields. A concrete model for three generations of quark and lepton superfields are also shown. Our formulation would be useful for building various phenomenological models based on magnetized SYM theories or D-branes. (C) 2012 Elsevier B.V. All rights reserved. - Tatsuo Kobayashi, Takashi Shimomura, Tsubasa Takahashi PHYSICAL REVIEW D 86 (1) 2012年07月 [査読無し][通常論文]

We study the mass, the mixing, and the coupling with the Z boson of the lightest Higgs boson in the next- to-minimal supersymmetric standard model with the Z(3) symmetry. The vacuum structure of the Higgs potential is analyzed, and the new false vacua are discussed. The significant parameter region can be excluded by requiring the realistic vacuum to be deeper than false vacua. In numerical analysis, we also require the couplings to be perturbative up to the Grand Unified Theory scale. Our analysis results in constraints on the properties of the lightest Higgs boson. - Hajime Ishimori, Tatsuo Kobayashi PHYSICAL REVIEW D 85 (12) 2012年06月 [査読無し][通常論文]

We study the lepton flavor models with the flavor symmetry (Z(N) x Z(N) x Z(N))xZ(3). Our models lead nonvanishing discrete values of theta(13) as well as (12) and theta(23) depending on N. For certain values, our models realize the tribimaximal mixing angles with theta(13) = 0. For other values, our models provide discrete deviation from the tribimaximal mixing angles. - Tatsuo Kobayashi, Susha L. Parameswaran, Saul Ramos-Sanchez, Ivonne Zavala JOURNAL OF HIGH ENERGY PHYSICS (5) 2012年05月 [査読無し][通常論文]

We study L-point couplings between twisted sector fields in heterotic orbifold compactifications, using conformal field theory. Selection rules provide an easy way to identify which couplings are non-vanishing. Those used in the current literature are gauge invariance, R-charge conservation and the space group selection rule, but they are not the whole story. We revive and refine a fourth selection rule, due to symmetries in the underlying torus lattice, and introduce a fifth one, due to the existence or not of classical worldsheet instanton solutions to mediate the couplings. We consider briefly the phenomenological consequences of the additional rules, in particular for recent orbifold constructions whose field content correspond to that of the MSSM. The structure of the exotic mass matrices is unaltered and many dimension-5 proton-decay operators vanish. - 安倍 博之, 及川 茜, 大木 洋, 小林 達夫, 角田 慶吾 日本物理学会講演概要集 67 (1) 9 -9 2012年03月05日 [査読無し][通常論文]
- 安倍 博之, 及川 茜, 大木 洋, 小林 達夫, 角田 慶吾, 野本 優樹 日本物理学会講演概要集 67 (1) 9 -9 2012年03月05日 [査読無し][通常論文]
- 安倍博之, 及川茜, 大木洋, 小林達夫, 角田慶吾, 野本優樹 日本物理学会講演概要集 67 (1) 9 -9 2012年03月05日 [査読無し][通常論文]
- Yoshimi Kanehata, Tatsuo Kobayashi, Yasufumi Konishi, Osamu Seto, Takashi Shimomura Progress of Theoretical Physics 126 (6) 1051 -1076 2011年12月 [査読無し][通常論文]

We study constraints to avoid deep unrealistic minima in the next-to-minimal supersymmetric standard model. We analyze the scalar potential along directions where all of and one of the three Higgs fields develop their vacuum expectation values, and find unrealistic minima deeper than the electroweak symmetry breaking (EWSB) vacuum. These unrealistic minima threaten the realization of successful EWSB and therefore should be avoided. Necessary conditions for avoiding these minima result in constraints on parameters. We show that a wide and significant region of the parameter space, especially a large λ, is ruled out by our constraints. - Hajime Ishimori, Tatsuo Kobayashi JOURNAL OF HIGH ENERGY PHYSICS (10) 2011年10月 [査読無し][通常論文]

We study lepton flavor models with the S-4 flavor symmetry. We construct simple models with smaller numbers of flavon fields and free parameters, such that we have predictions among lepton masses and mixing angles. The model with a S-4 triplet flavon is not realistic, but we can construct realistic models with two triplet flavons, or one triplet and one doublet flavons. - Hiroyuki Abe, Kang-Sin Choi, Tatsuo Kobayashi, Hiroshi Ohki, Manabu Sakai INTERNATIONAL JOURNAL OF MODERN PHYSICS A 26 (23) 4067 -4082 2011年09月 [査読無し][通常論文]

We study non-Abelian flavor symmetries on orbifolds, S-1/Z(2) and T-2/Z(3). Our extra dimensional models realize D-N, Sigma(2N(2)), Delta(3N(2)) and Delta(6N(2)) including A(4) and S-4. In addition, one can also realize their subgroups such as Q(N), T-7, etc. The S-3 flavor symmetry can be realized on both S-1/Z(2) and T-2/Z(3) orbifolds. - Tetsutaro Higaki, Tatsuo Kobayashi PHYSICAL REVIEW D 84 (4) 2011年08月 [査読無し][通常論文]

We study properties of moduli stabilization in the four dimensional N = 1 supergravity theory with heavy moduli and would-be saxion-axion multiplets, including light string-theoretic axions. We give general formulation for the scenario that heavy moduli and saxions are stabilized while axions remain light, assuming that moduli are stabilized near the supersymmetric solution. One can find stable vacuum, i.e. nontachyonic saxions, in the nonsupersymmetric Minkowski vacua. We also discuss the cases where the moduli are coupled to the supersymmetry breaking sector and/or moduli have contributions to supersymmetry breaking. Futhermore, we study the models with axions originating from matterlike fields. Our analysis on moduli stabilization is applicable even if there are not light axion multiplets. - Tatsuo Azeyanagi, Tatsuo Kobayashi, Atsushi Ogasahara, Koichi Yoshioka 2011年06月15日 [査読無し][通常論文]

We systematically study supersymmetry breaking with non-vanishing F and D

terms. We classify the models into two categories and find that a certain class

of models necessarily has runaway behavior of scalar potential, while the other

needs the Fayet-Iliopoulos term to break supersymmetry. The latter class is

useful to have a simple model of gauge mediation where the vacuum is stable

everywhere and the gaugino mass is generated at the one-loop order. - Tatsuo Kobayashi, Yuichiro Nakai, Manabu Sakai JOURNAL OF HIGH ENERGY PHYSICS (6) 2011年06月 [査読無し][通常論文]

We study anomaly mediation models with gauge mediation effects from messengers which have a general renormalizable mass matrix with a supersymmetry-breaking spurion. Our models lead to a rich structure of supersymmetry breaking terms in the visible sector. We derive sum rules among the soft scalar masses for each generation. Our sum rules for the first and second generations are the same as those in general gauge mediation, but the sum rule for the third generation is different because of the top Yukawa coupling. We find the parameter space where the tachyonic slepton problem is solved. We also explore the case in which gauge mediation causes the anomalously small gaugino masses. Since anomaly mediation effects on the gaugino masses exist, we can obtain viable mass spectrum of the visible sector fields. - Konishi Yasufumi, Kanehata Yoshimi, Kobayashi Tatsuo, Seto Osamu, Shimomura Takashi 素粒子論研究 119 (3) C32 2011年
- Tatsuo Kobayashia, Manabu Sakaib Journal of High Energy Physics 2011 (4) 2011年 [査読無し][通常論文]

We study the cosmological inflation from the viewpoint of the moduli stabilization. We study the scenario that the superpotential has a large value during the inflation era enough to stabilize moduli, but it is small in the true vacuum. This scenario is discussed by using a simple model, one type of hybrid models. © SISSA 2011. - Constraints from unrealistic vacua in the supersymmetric standard model with neutrino mass operatorsYoshimi Kanehata, Tatsuo Kobayashi, Yasufumi Konishi, Takashi Shimomura PHYSICAL REVIEW D 82 (7) 2010年10月 [査読無し][通常論文]

We analyze a scalar potential of the minimal supersymmetric standard model (MSSM) with neutrino mass operators along unbounded-from-below and color and/or charged breaking directions. We show necessary conditions to avoid the potential minima which can be deeper than the realistic vacuum. These conditions would constrain more strongly than conditions in the MSSM without taking into account neutrino mass operators and can improve the predictive power of supersymmetric models with neutrino mass operators. - Kang-Sin Choi, Tatsuo Kobayashi PHYSICS LETTERS B 693 (3) 330 -333 2010年10月 [査読無し][通常論文]

We study the MSSM in F-theory. Its group is the commutant to a structure group SU(5) x U(1)gamma of a gauge bundle in E(8). The spectrum contains three generations of quarks and leptons plus vectorlike electroweak and colored Higgses. The minimal MSSM Yukawa couplings with matter parity is obtained at the renormalizable level. (c) 2010 Elsevier B.V. All rights reserved. - Tatsuo Kobayashi, Yuichiro Nakai, Ryo Takahashi JOURNAL OF HIGH ENERGY PHYSICS (9) 2010年09月 [査読無し][通常論文]

We study superparticle spectra in the superconformal flavor scenario with non-universal gaugino masses. The non-universality of gaugino masses can lead to the wino-like or higgsino-like neutralino LSP. Furthermore, it is shown that the parameter space for the higgsino-like LSP includes the region where the fine-tuning problem can be improved. The degeneracy of soft scalar masses does not drastically change when ratios of gaugino masses are of O(1). The degeneracy of scalar masses for squarks and left-handed sleptons would be good enough to avoid the FCNC problem but that of right-handed slepton masses is weak. However, the overall size of right-handed slepton masses becomes larger when the bino becomes heavier. Thus, that would be favorable to avoid the FCNC problem due to soft scalar masses and A-terms. - 安倍博之, 大木洋, 久能秀幸, 小林達夫, 阪村豊, 角田慶吾 日本物理学会講演概要集 65 (2) 9 -9 2010年08月18日 [査読無し][通常論文]
- Tatsuo Kobayashi, Takashi Shimomura Phys.Rev.D82:035008,2010 82 (3) 2010年08月12日 [査読無し][通常論文]

We consider a model where right-handed neutrinos and sneutrinos are

introduced to the minimal supersymmetric standard model. In the scalar

potential of this model, there exist trilinear and quartic terms in scalar

potential that are proportional to Yukawa couplings of neutrinos. Due to these

trilinear and quartic terms, Color and/or Charge Breaking (CCB) and

Unbounded-From-Below (UFB) directions appear along which sneutrinos have a

vacuum expectation value, making the vacuum of the electroweak symmetry

breaking unstable. We analyze scalar potential of this model and derive

necessary conditions for CCB and UFB directions to vanish. - Kang-Sin Choi, Tatsuo Kobayashi, Ryosuke Maruyama, Masaki Murata, Yuichiro Nakai, Hiroshi Ohki, Manabu Sakai EUROPEAN PHYSICAL JOURNAL C 67 (1-2) 273 -282 2010年05月 [査読無し][通常論文]

We study 10D super Yang-Mills theory with the gauge groups E (6), E (7) and E (8). We consider the torus/orbifold compactification with magnetic fluxes and Wilson lines. They lead to 4D interesting models with three families of quarks and leptons, whose profiles in extra dimensions are quasi-localized because of magnetic fluxes. - 石森一, 岡田寛, 小林達夫, 清水勇介, 谷本盛光 日本物理学会講演概要集 65 (1) 7 -7 2010年03月01日 [査読無し][通常論文]
- Hiroyuki Abe, Kang-Sin Choi, Tatsuo Kobayashi, Hiroshi Ohki 2010年01月12日 [査読無し][通常論文]

We study the flavor structure of 4D effective theories, which are derived

from extra dimensional theories with magnetic fluxes and non-Abelian Wilson

lines. We study zero-mode wavefunctions and compute Yukawa couplings as well as

four-point couplings. In our models, we also discuss non-Abelian discrete

flavor symmetries such as $D_4$, $\Delta(27)$ and $\Delta(54)$. - Hajime Ishimori, Tatsuo Kobayashi, Hiroshi Ohki, Yusuke Shimizu, Hiroshi Okada, Morimitsu Tanimoto PROGRESS OF THEORETICAL PHYSICS SUPPLEMENT (183) 1 -163 2010年 [査読無し][通常論文]

We review pedagogically non-Abelian discrete groups, which play an important role in particle physics. We show group-theoretical aspects for many concrete groups, such as representations and their tensor products. We explain how to derive, conjugacy classes, characters, representations, and tensor products for these groups (with a finite number). We discuss them explicitly for S-N, A(N), T', D-N, Q(N), E(2N(2)), Delta(3N(2)), T-7, Sigma(3N(3)), and Delta(6N(2)), which have been applied for model building in particle physics. We also present typical flavor models by using A(4), S-4, and Delta(54) groups. Breaking patterns of discrete groups and decompositions of multiplets are important for applications of the non-Abelian discrete symmetry. We discuss these breaking patterns of the non-Abelian discrete group, which are a powerful tool for model buildings. We also review briefly anomalies of non-Abelian discrete symmetries by using the path integral approach. - Tatsuo Kobayashi, Ryosuke Maruyama, Masaki Murata, Hiroshi Ohki, Manabu Sakai Journal of High Energy Physics 2010 (5) 2010年 [査読無し][通常論文]

We study 10D super Yang-Mills E 8 theory on the 6D torus compactification with magnetic fluxes. We study systematically the possibilities for realizing 4D supersym-metric standard models with three generations of quarks and leptons. We also study quark mass matrices. © SISSA 2010. - Tatsuo Kobayashi, Yuichiro Nakai, Ryo Takahashi JOURNAL OF HIGH ENERGY PHYSICS (1) 2010年01月 [査読無し][通常論文]

We study the fine-tuning problem in the context of general gauge mediation. Numerical analyses toward for relaxing fine-tuning are presented. We analyse the problem in typical three cases of the messenger scale, that is, GUT (2 x 10(16) GeV), intermediate (10(10) GeV), and relatively low energy (10(6) GeV) scales. In each messenger scale, the parameter space reducing the degree of tuning as around 10% is found. Certain ratios among gluino mass, wino mass and soft scalar masses are favorable. It is shown that the favorable region becomes narrow as the messenger scale becomes lower, and tachyonic initial conditions of stop masses at the messenger scale are favored to relax the fine-tuning problem for the relatively low energy messenger scale. Our spectra would also be important from the viewpoint of the mu - B problem. - Hiroyuki Abe, Kang-Sin Choi, Tatsuo Kobayashi, Hiroshi Ohki PHYSICAL REVIEW D 80 (12) 2009年12月 [査読無し][通常論文]

We study torus/orbifold models with magnetic flux and Wilson line backgrounds. The number of zero modes and their profiles depend on those backgrounds. That has interesting implications from the viewpoint of particle phenomenology. - Hajime Ishimori, Tatsuo Kobayashi, Hiroshi Okada, Yusuke Shimizu, Morimitsu Tanimoto JOURNAL OF HIGH ENERGY PHYSICS (12) 2009年12月 [査読無し][通常論文]

We study a Delta(54) x Z(2) flavor model for leptons and sleptons. The tri-bimaximal mixing can be reproduced for arbitrary neutrino masses if certain vacuum alignments of scalar fields are realized. The deviation from the tri-bimaximal mixing of leptons is predicted. The predicted upper bound for sin theta(13) is 0.07. The value of sin theta(23) could be deviated from the maximal mixing considerably while sin theta(12) is hardly deviated from 1/root 3. We also study SUSY breaking terms in the slepton sector. Three families of left-handed and right-handed slepton masses are almost degenerate. Our model leads to smaller values of flavor changing neutral currents than the present experimental bounds. - Hiroyuki Abe, Kang-Sin Choi, Tatsuo Kobayashi, Hiroshi Ohki NUCLEAR PHYSICS B 820 (1-2) 317 -333 2009年10月 [査読無し][通常論文]

We study non-Abelian discrete flavor symmetries, which appear in magnetized brane models. For example, D-4, Delta(27) and Delta(54) flavor symmetries can appear with matter fields in several representations. We also study the orbifold background, where non-Abelian flavor symmetries are broken in a certain way. (C) 2009 Published by Elsevier B.V. - 石森一, 岡田寛, 小林達夫, 清水勇介, 谷本盛光 日本物理学会講演概要集 64 (2) 9 -9 2009年08月18日 [査読無し][通常論文]
- Hiroyuki Abe, Kang-Sin Choi, Tatsuo Kobayashi, Hiroshi Ohki JOURNAL OF HIGH ENERGY PHYSICS (6) 2009年06月 [査読無し][通常論文]

We compute three-point and higher order couplings in magnetized brane models. We show that higher order couplings are written as products of three-point couplings. This behavior is the same as higher order amplitudes by conformal field theory calculations e. g. in intersecting D-brane models. - Hiroyuki Abe, Kang-Sin Choi, Tatsuo Kobayashi, Hiroshi Ohki NUCLEAR PHYSICS B 814 (1-2) 265 -292 2009年06月 [査読無し][通常論文]

We study three generation models in the four-dimensional spacetime, which can be derived from the ten-dimensional N = 1 super-Yang-Mills theory on the orbifold background with a non-vanishing magnetic flux. We classify the flavor structures and show possible patterns of Yukawa matrices. Some examples of numerical studies are also shown. (C) 2009 Elsevier B.V. All rights reserved. - Hajime Ishimori, Tatsuo Kobayashi, Hiroshi Okada, Yusuke Shimizu, Morimitsu Tanimoto JOURNAL OF HIGH ENERGY PHYSICS (4) 2009年04月 [査読無し][通常論文]

We present the lepton flavor model with Delta(54), which appears typically in heterotic string models on the T-2/Z(3) orbifold. Our model reproduces the tri-bimaximal mixing in the parameter region around degenerate neutrino masses or two massless neutrinos. We predict the deviation from the tri-bimaximal mixing by putting the experimental data of neutrino masses in the normal hierarchy of neutrino masses. The upper bound of sin(2)theta(13) is 0.01. There is the strong correlation between theta(23) and theta(13). Unless theta(23) is deviated from the maximal mixing considerably, theta(13) remains to be tiny. - 石森一, 小林達夫, 大村雄司, 谷本盛光 日本物理学会講演概要集 64 (1) 18 -18 2009年03月03日 [査読無し][通常論文]
- Hiroyuki Abe, Tetsutaro Higaki, Tatsuo Kobayashi, Kazutoshi Ohta, Yuji Omura, Haruhiko Terao PHYSICAL REVIEW D 79 (4) 2009年02月 [査読無し][通常論文]

We study the duality cascade of softly broken supersymmetric theories. We investigate the renormalization group (RG) flow of SUSY breaking terms as well as supersymmetric couplings. It is found that the magnitudes of SUSY breaking terms are suppressed in most regimes of the RG flow through the duality cascade. At one stage of cascading, the gaugino mass of the strongly coupled sector and scalar masses converge to certain values, which are determined by the gauge coupling and the gaugino mass of the weakly coupled sector. At the next stage, the strongly and weakly coupled sectors are interchanged with each other. We also show the possibility that cascading would be terminated by the gauge symmetry breaking, which is induced by the so-called B-term. - Takeshi Araki, Tatsuo Kobayashi, Jisuke Kubo, Saul Ramos-Sanchez, Michael Ratz, Patrick K. S. Vaudrevange NUCLEAR PHYSICS B 805 (1-2) 124 -147 2008年12月 [査読無し][通常論文]

We derive anomaly constraints for Abelian and non-Abelian discrete symmetries using the path integral approach. We survey anomalies of discrete symmetries in heterotic orbifolds and find a new relation between such anomalies and the so-called 'anomalous' U(1). (C) 2008 Elsevier B.V. All rights reserved. - Tatsuo Kobayashi, Yuji Omura, Koichi Yoshioka PHYSICAL REVIEW D 78 (11) 2008年12月 [査読無し][通常論文]

Flavor symmetry has been widely studied for figuring out the masses and mixing angles of standard model fermions. In this paper we present a framework for handling flavor symmetry breaking where the symmetry breaking is triggered by boundary conditions of scalar fields in extra-dimensional space. The alignment of scalar expectation values is achieved without referring to any details of scalar potential and its minimization procedure. As applications to non-Abelian discrete flavor symmetries, illustrative lepton mass models are constructed where the S-3 and A(4) flavor symmetries are broken down to the directions leading to the tribimaximal form of lepton mixing and realistic mass patterns. - Hiroyuki Abe, Tatsuo Kobayashi, Hiroshi Ohki Journal of High Energy Physics 2008 (9) 2008年09月01日 [査読無し][通常論文]

We study (4+2n)-dimensional N=1 super Yang-Mills theory on the orbifold background with non-vanishing magnetic fluxes. In particular, we study zero-modes of spinor fields. The flavor structure of our models is different from one in magnetized torus models, and would be interesting in realistic model building. © 2008 SISSA. - 小林 達夫, 奥村 健一 日本物理學會誌 63 (8) 603 -611 2008年08月05日 [査読無し][通常論文]

10次元で定式化される超弦理論を4次元にコンパクト化するとモジュライとよばれる場が現れる.これらは適当な真空期待値に固定される必要があるが,最近になってKKLTと呼ばれる新たな固定シナリオが提案された.ここでは,このシナリオをレビューし,この種のモジュライ固定に伴なって現れる超対称標準理論の特徴的な質量スペクトラムを解説する.特にヒッグスセクターの微調整問題との関連を議論する. - 小林達夫, 奥村健一 日本物理学会誌 63 (8) 603 -611 2008年08月05日 [査読無し][通常論文]

10次元で定式化される超弦理論を4次元にコンパクト化するとモジュライとよばれる場が現れる.これらは適当な真空期待値に固定される必要があるが,最近になってKKLTと呼ばれる新たな固定シナリオが提案された.ここでは,このシナリオをレビューし,この種のモジュライ固定に伴なって現れる超対称標準理論の特徴的な質量スペクトラムを解説する.特にヒッグスセクターの微調整問題との関連を議論する. - Hajime Ishimori, Tatsuo Kobayashi, Yuji Omura, Morimitsu Tanimoto JHEP 0812:082,2008 2008年07月29日 [査読無し][通常論文]

We study the supersymmetric model with the A4 lepton flavor symmetry, in

particular soft supersymmetry breaking terms, which are predicted from the A4

lepton flavor symmetry. We evaluate soft slepton masses and A-terms within the

framework of supergravity theory. Constraints due to experiments of flavor

changing neutral current processes are examined. - Hiroyuki Abe, Tetsutaro Higaki, Tatsuo Kobayashi, Osamu Seto PHYSICAL REVIEW D 78 (2) 2008年07月 [査読無し][通常論文]

We study nonperturbative moduli superpotentials with positive exponents, i.e. the form like Ae(aT) with a positive constant a and the modulus T. These effects can be generated, e.g., by D-branes which have negative Ramond-Ramond charge of the lower-dimensional D-brane. The scalar potentials including such terms have quite a rich structure. There are several local minima with different potential energies and a high barrier, whose height is of O(M-p(4)). We discuss their implications from the viewpoints of cosmology and particle phenomenology, e.g. the realization of inflation models, avoiding the overshooting problem. This type of potential would be useful to realize the inflation and low-energy supersymmetry breaking. - Kang-Sin Choi, Tatsuo Kobayashi NUCLEAR PHYSICS B 797 (1-2) 295 -321 2008年07月 [査読無し][通常論文]

We calculate couplings of arbitrary order from correlation functions among twisted strings, using conformal field theory. Twisted strings arise in heterotic string compactified on orbifolds yielding matter fields in the low energy limit. We calculate completely the classical and the quantum amplitude including normalization, up to a contribution from Kahler potential. The classical action has saddle points which are interpreted as worldsheet instantons described by metastable untwisted strings, formed by twisted strings distributed at certain fixed points. This understanding generalizes the area rule, in the case that the locations of twisted strings do not form a polygon, and provides a general rule for calculating these kinds of instanton corrections. An interpretation of couplings involving linearly combined states is given, which commonly appear in non-prime order orbifolds. The quantum part of the amplitude is given by ratios of gamma functions with order one arguments. (C) 2008 Elsevier B.V. All rights reserved. - Hajime Ishimori, Tatsuo Kobayashi, Hiroshi Ohki, Yuji Omura, Ryo Takahashi, Morimitsu Tanimoto PHYSICAL REVIEW D 77 (11) 2008年06月 [査読無し][通常論文]

We study the supersymmetric model with D(4) x Z(2) lepton flavor symmetry. We evaluate soft supersymmetry breaking terms, i.e., soft slepton masses and A-terms, which are predicted in the D(4) flavor model. We consider constraints due to experiments of flavor-changing neutral current processes. - Hajime Ishimoria, Tatsuo Kobayashi, Hiroshi Ohki, Yuji Omura, Ryo Takahashi, Morimitsu Tanimoto PHYSICS LETTERS B 662 (2) 178 -184 2008年04月 [査読無し][通常論文]

We present the D-4 x Z(2) flavor symmetry, which is different from the previous work by Grimus and Lavoura. Our model reduces to the standard model in the low energy and there is no FCNC at the tree level. Putting the experimental data, parameters are fixed, and then the implication of our model is discussed. The condition to realize the tri-bimaximal mixing is presented. The possibility for stringy realization of our model is also discussed. (C) 2008 Elsevier B.V. All rights reserved. - Hiroyuki Abe, Tetsutaro Higaki, Tatsuo Kobayashi, Yuji Omura Journal of High Energy Physics 2008 (4) 2008年04月01日 [査読無し][通常論文]

We study moduli stabilization, the dynamical supersymmetry (SUSY) breaking, the uplifting of SUSY anti-de Sitter (AdS) vacuum and the sequestering of hidden sector in a five-dimensional supergravity model, where all modes of the visible sector and the hidden sector are originated from bulk fields. We clarify couplings between the visible and hidden sectors. The expressions for the visible sector soft SUSY breaking terms as well as the hidden sector potential are shown explicitly in our model. The sequestering is achieved dynamically by a wavefunction localization in extra dimension. We find that the tree-level soft scalar mass and the A-term can be suppressed at a SUSY breaking Minkowski minimum where the radius modulus is stabilized, while gaugino masses would be a mirage type. - 石森一, 小林達夫, 大木洋, 大村雄司, 高橋亮, 谷本盛光 日本物理学会講演概要集 63 (1) 10 -10 2008年02月29日 [査読無し][通常論文]
- Hiroyuki Abe, Tatsuo Kobayashi, Yuji Omura Phys.Rev.D77:065001,2008 2007年12月16日 [査読無し][通常論文]

We study the scenario that conformal dynamics leads to metastable

supersymmetry breaking vacua. At a high energy scale, the superpotential is not

R-symmetric, and has a supersymmetric minimum. However, conformal dynamics

suppresses several operators along renormalization group flow toward the

infrared fixed point. Then we can find an approximately R-symmetric

superpotential, which has a metastable supersymmetry breaking vacuum, and the

supersymmetric vacuum moves far away from the metastable supersymmetry breaking

vacuum. We show a 4D simple model. Furthermore, we can construct 5D models with

the same behavior, because of the AdS/CFT dual. - Hiroyuki Abe, Tetsutaro Higaki, Tatsuo Kobayashi PHYSICAL REVIEW D 76 (10) 2007年11月 [査読無し][通常論文]

We study moduli stabilization and a realization of de Sitter vacua in generalized F-term uplifting scenarios of the Kachru-Kallosh-Linde-Trivedi-type anti-de Sitter vacuum, where the uplifting sector X directly couples to the light Kahler modulus T in the superpotential through, e.g., stringy instanton effects. F-term uplifting can be achieved by a spontaneous supersymmetry breaking sector, e.g., the Polonyi model, the O'Raifeartaigh model, and the Intriligator-Seiberg-Shih model. Several models with the X-T mixing are examined, and qualitative features in most models even with such mixing are almost the same as those in the Kachru-Kallosh-Linde-Trivedi scenario. One of the quantitative changes, which are relevant to the phenomenology, is a larger hierarchy between the modulus mass m(T) and the gravitino mass m(3/2), i.e., m(T)/m(3/2)=O(a(2)), where a similar to 4 pi(2). In spite of such a large mass, the modulus F term is suppressed not like F-T=O(m(3/2)/a(2)), but like F-T=O(m(3/2)/a) for ln(M-Pl/m(3/2))similar to a, because of an enhancement factor coming from the X-T mixing. Then we typically find a mirage-mediation pattern of gaugino masses of O(m(3/2)/a), while the scalar masses would be generically of O(m(3/2)). - Takeshi Araki, Kang-Sin Choi, Tatsuo Kobayashi, Jisuke Kubo, Hiroshi Ohki PHYSICAL REVIEW D 76 (6) 2007年09月 [査読無し][通常論文]

Anomalies of discrete R symmetries appearing in heterotic orbifold models are studied. We find that the mixed anomalies for different gauge groups satisfy the universal Green-Schwarz condition, indicating that these anomalies are canceled by the Green-Schwarz mechanism. An exact relation between the anomaly coefficients of the discrete R symmetries and one-loop beta-function coefficients is obtained. We also find that the discrete R symmetries have a good chance to be unbroken down to the supersymmetry breaking scale. Even below this scale a Z(2) subgroup is unbroken, and it may be an origin of the R parity of the minimal supersymmetric standard model. Relations between the R-symmetry anomalies and T-duality anomalies are also investigated. - Hiroyuki Abe, Yeong Gyun Kim, Tatsuo Kobayashi, Yasuhiro Shimizu Journal of High Energy Physics 2007 (9) 2007年09月01日 [査読無し][通常論文]

We study the TeV scale partial mirage unification scenario, where the gluino and wino masses are degenerate around a TeV scale, but the bino mass is not degenerate. This scenario has phenomenologically interesting aspects. First, because of the degeneracy between the gluino and wino masses, this scenario does not have the little hierarchy problem, that is, the higgisino mass is around 150 GeV. The lightest superparticle is a mixture of the bino and higgsino, and can lead to a right amount of thermal relic density as a dark matter candidate. © SISSA 2007. - Pyungwon Ko, Tatsuo Kobayashi, Jae-hyeon Park, Stuart Raby PHYSICAL REVIEW D 76 (5) 2007年09月 [査読無し][通常論文]

In this paper we show how some flavor symmetries may be derived from the

heterotic string, when compactified on a 6D orbifold. In the body of the paper

we focus on the $D_4$ family symmetry, recently obtained in $Z_3 \times Z_2$

orbifold constructions. We show how this flavor symmetry constrains fermion

masses, as well as the soft SUSY breaking mass terms. Flavor symmetry breaking

can generate the hierarchy of fermion masses and at the same time the flavor

symmetry suppresses large flavor changing neutral current processes. - Hiroyuki Abe, Tatsuo Kobayashi, Yuji Omura JHEP 0711:044,2007 2007年08月23日 [査読無し][通常論文]

We study N=1 global and local supersymmetric theories with a continuous

global $U(1)_R$ symmetry as models of dynamical supersymmetry (SUSY) breaking.

We introduce explicit R-symmetry breaking terms into such models, in particular

a generalized O'Raifeartaigh model. Such explicit R-symmetry breaking terms can

lead to a SUSY preserving minimum. We classify explicit R-symmetry breaking

terms by the structure of newly appeared SUSY stationary points as a

consequence of the R-breaking effect, which could make the SUSY breaking vacuum

metastable. We show that the R-breaking terms are basically divided into two

categories. One of them does not generate a SUSY solution, or yields SUSY

solutions that disappear in the case of supergravity when we tune a parameter

so that the original SUSY breaking minimum becomes a Minkowski vacuum. We also

show that the general argument by Nelson and Seiberg for a dynamical SUSY

breaking still holds with a local SUSY except for a certain nontrivial case,

and present concrete examples of the exception. - Hiroyuki Abe, Tatsuo Kobayashi, Yuji Omura PHYSICAL REVIEW D 76 (1) 2007年07月 [査読無し][通常論文]

We study, in a bottom-up approach, the fine-tuning problem between soft supersymmetry (SUSY) breaking parameters and the mu term for the successful electroweak symmetry breaking in the minimal supersymmetric standard model. It is shown that certain ratios among nonuniversal gaugino masses at the grand unified theory (GUT) scale are necessary to reduce fine-tuning. In addition, when all the gaugino masses are regarded as independent ones in their origins, a small gluino mass M-3 less than or similar to 120 GeV and a nonvanishing A term A(t)similar to O(M-3) associated with top squarks are also required at the GUT scale as well as the nonuniversality. On the other hand, when we consider some UV theory, which fixes ratios of soft SUSY breaking parameters as certain values aside from the overall magnitude, heavier spectra are allowed. It is favored that the gluino and W-ino masses are almost degenerate at the weak scale, while a wider region of b-ino mass is favorable. - Kiwoon Choi, Kwang Sik Jeong, Tatsuo Kobayashi, Ken-ichi Okumura PHYSICAL REVIEW D 75 (9) 2007年05月 [査読無し][通常論文]

TeV scale mirage mediation has been proposed as a supersymmetry-breaking scheme reducing the fine-tuning for electroweak symmetry breaking in the minimal supersymmetric extension of the standard model. We discuss a moduli stabilization setup for TeV scale mirage mediation which allows an extradimensional interpretation for the origin of supersymmetry breaking and naturally gives a weak-scale size of the Higgs B parameter. The setup utilizes the holomorphic gauge kinetic functions depending on both the heavy dilaton and the light volume modulus whose axion partners are assumed to be periodic fields. We also examine the low-energy phenomenology of TeV scale mirage mediation, particularly the constraints from electroweak symmetry breaking and flavor changing neutral current processes. - Tatsuo Kobayashi, Hans Peter Nilles, Felix Ploeger, Stuart Raby, Michael Ratz NUCLEAR PHYSICS B 768 (1-2) 135 -156 2007年04月 [査読無し][通常論文]

We study the origin of non-Abelian discrete flavor symmetries in superstring theory. We classify all possible non-Abelian discrete flavor symmetries which can appear in heterotic orbifold models. These symmetries include D-4 and Delta(54). We find that the symmetries of the couplings are always larger than the symmetries of the compact space. This is because they are a consequence of the geometry of the orbifold combined with the space group selection rules of the string. We also study possible breaking patterns. Our analysis yields a simple geometric understanding of the realization of non-Abelian flavor symmetries. (c) 2007 Elsevier B.V. All rights reserved. - Stefan Förste, Tatsuo Kobayashi, Hiroshi Ohki, Kei-Jiro Takahashi Journal of High Energy Physics 2007 (3) 2007年03月01日 [査読無し][通常論文]

We classify compactification lattices for supersymmetric 2 × 2 orbifolds. These lattices include factorisable as well as non-factorisable six-tori. Different models lead to different numbers of fixed points/tori. A lower bound on the number of fixed tori per twisted sector is given by four, whereas an upper bound consists of 16 fixed tori per twisted sector. Thus, these models have a variety of generation numbers. For example, in the standard embedding, the smallest number of net generations among these classes of models is equal to six, while the largest number is 48. Conditions for allowed Wilson lines and Yukawa couplings are derived. © SISSA 2007. - Takeshi Chiba, Tatsuo Kobayashi, Masahide Yamaguchi, Jun'ichi Yokoyama PHYSICAL REVIEW D 75 (4) 2007年02月 [査読無し][通常論文]

Recent astrophysical observations indicate that the proton-electron mass ratio and the fine structure constant have gone through nontrivial time evolution. We discuss their time variation in the context of a dilaton runaway scenario with gauge coupling unification at the string scale M-s. We show that the choice of adjustable parameters allows them to fit the same order magnitude of both variations and their (opposite) signs in such a scenario. - Hiroyuki Abe, Tetsutaro Higaki, Tatsuo Kobayashi, Yuji Omura PHYSICAL REVIEW D 75 (2) 2007年01月 [査読無し][通常論文]

We study moduli stabilization with F-term uplifting. As a source of uplifting F-term, we consider spontaneous supersymmetry breaking models, e.g. the Polonyi model and the Intriligator-Seiberg-Shih model. We analyze potential minima by requiring almost vanishing vacuum energy and evaluate the size of modulus F-term. We also study soft SUSY-breaking terms. In our scenario, the mirage mediation is dominant in gaugino masses. Scalar masses can be comparable with gaugino masses or much heavier, depending on couplings with spontaneous supersymmetry breaking sector. - Tatsuo Kobayashi, Yuji Omura, Haruhiko Terao PHYSICAL REVIEW D 74 (5) 2006年09月 [査読無し][通常論文]

We study realization of the democratic form of Yukawa matrices by infrared fixed points. We investigate renormalization-group flows of Yukawa couplings in models with a single Yukawa matrix for three families, and up and down-sector Yukawa matrices. It is found that each model has its certain pattern of renormalization-group flows of Yukawa matrices. We apply them to the charged lepton sector and quark sector, and examine in which situation our class of models can lead to realistic results for the mass ratios and mixing angles between the second and third families. We also study corresponding A-terms. The A-terms approach toward the universal form with no physical CP-violating phase. Thus, constraints due to various neutral flavor changing processes except for mu -> e gamma are found to be satisfied by this dynamics. In order to suppress the electric dipole moments as well as mu -> e gamma sufficiently, more alignment of the A-terms with some reason is required. - Tatsuo Kobayashi, Haruhiko Terao, Akito Tsuchiya PHYSICAL REVIEW D 74 (1) 2006年07月 [査読無し][通常論文]

It is shown that fine-tuning of the Higgs parameters stronger than a few percent is required at best in the models with gauge mediated supersymmetry breaking. With the aim of solving this problem, we consider a new type of models in which the top Yukawa coupling is induced at TeV scale through mass mixing with unknown matter fields. Then it is found that the fine-tuning problem can be eliminated essentially. We discuss some phenomenological features of this model and also consider the extension to the next-to-minimal models. - H Abe, T Higaki, T Kobayashi NUCLEAR PHYSICS B 742 187 -207 2006年05月 [査読無し][通常論文]

We study supersymmetric models with double gaugino condensations in the hidden sector, where the gauge couplings depend on two light moduli of superstring theory. We perform a detailed analysis of this class of model and show that there is no stable supersymmetric minimum with finite vacuum values of moduli fields. Instead, we find that the supersymmetry breaking occurs with moduli stabilized and negative vacuum energy. That yields moduli-dominated soft supersymmetry breaking terms. To realize slightly positive (or vanishing) vacuum energy, we add uplifting potential. We discuss uplifting does not change qualitatively the vacuum expectation values of moduli and the above feature of supersymmetry breaking. (c) 2006 Elsevier B.V. All rights reserved. - 小林 達夫, 寺尾 治彦, 土谷 暁人 日本物理学会講演概要集 61 (1) 10 -10 2006年03月04日
- K Choi, KS Jeong, T Kobayashi, K Okumura PHYSICS LETTERS B 633 (2-3) 355 -361 2006年02月 [査読無し][通常論文]

Motivated by the KKLT string compactification involving a supersymmetry-breaking uplifting potential, we examine 4D effective supergravity with a generic form of uplifting potential, focusing on the possibility that the resulting mixed modulus-anomaly mediated soft terms realize the little hierarchy between the Higgs boson masses m(H) and the sparticle masses m(SUSY). It is noted that for some type of uplifting potential, the anomaly-mediated contribution to m(H)(2) at M-GUT can cancel the subsequent renormalization group evolution of m(H)(2) down to TeV scale, thereby the model can naturally realize the little hierarchy m(H)(2) similar to m(SUSY)(2)/8 pi(2) which is desirable for the lightest Higgs boson mass to satisfy the experimental bound. In such models, the other Higgs mass parameters mu and B can have the desirable size mu similar to B similar to mH without severe fine-tuning of parameters, although the gravitino is much heavier than the Higgs boson. Those models for the little hierarchy avoid naturally the dangerous SUSY flavor and CP violations, and predict nearly degenerate low energy gaugino masses, pure Higgsino LSP and also a specific relation between the stop and gaugino masses. (c) 2005 Elsevier B.V. All rights reserved. - H Abe, T Higaki, T Kobayashi PHYSICAL REVIEW D 73 (4) 2006年02月 [査読無し][通常論文]

We study KKLT type models with moduli-mixing superpotential. In several string models, gauge kinetic functions are written as linear combinations of two or more moduli fields. Their gluino condensation generates moduli-mixing superpotential. We assume one of moduli fields is frozen already around the string scale. It is found that Kahler modulus can be stabilized at a realistic value without tuning 3-form fluxes because of gluino condensation on (non-)magnetized D-brane. Furthermore, we do not need to highly tune parameters in order to realize a weak gauge coupling and a large hierarchy between the gravitino mass and the Planck scale, when there exists nonperturbative effects on D3-brane. SUSY breaking patterns in our models have a rich structure. Also, some of our models have cosmologically important implications, e.g., on the overshooting problem and the destabilization problem due to finite temperature effects as well as the gravitino problem and the moduli problem. - Hiroyuki Abe, Tetsutaro Higaki, Tatsuo Kobayashi Physical Review D - Particles, Fields, Gravitation and Cosmology 74 (4) 2006年 [査読無し][通常論文]

We study two steps of moduli stabilization in type IIB flux compactification with gaugino condensations. We consider the condition that one can integrate out heavy moduli first with light moduli remaining. We give appendix, where detail study is carried out for potential minima of the model with a six dimensional compact space with h1,1=h2,1=1, including the model, whose respective moduli with h1,1,h2,1≠1 are identified. © 2006 The American Physical Society. - Tatsuo Kobayashi, Carlos Mũoz Journal of High Energy Physics (1) 1107 -1117 2006年 [査読無し][通常論文]

In realistic four-dimensional string constructions the presence of anomalous U(1)'s is generic. In addition, the associated Fayet-Iliopoulos contribution to the D-term can break the extra gauge symmetries. As a consequence, physical particles can appear combined with other states. We show that even if a three-generation standard-like model has originally flavour-independent soft scalar masses, the particle mixing contribution may generate non-universality among them. Thus FCNC effects which were apparently absent reappear. We also discuss the size of these contributions in an explicit model, and how they can be suppressed. © SISSA 2006. - T Higaki, N Kitazawa, T Kobayashi, K Takahashi PHYSICAL REVIEW D 72 (8) 2005年10月 [査読無し][通常論文]

We study flavor structure and the coupling selection rule in intersecting D-brane configurations on T(2)xT(2)xT(2) as well as its orbifold/orientifold compactifications. We formulate the selection rule for Yukawa couplings and its extensions to generic n-point couplings. We investigate the possible flavor structure, which can appear from intersecting D-brane configuration. Then, we show there is a certain rule among intersecting numbers for states corresponding to allowed Yukawa couplings, and also it is found that their couplings are determined by discrete Abelian symmetries. Our studies on the flavor structure and the coupling selection rule in this class of models show that there is no solution to derive realistic Yukawa matrices only from stringy 3-point couplings at the tree-level within the framework of the minimal matter content of the supersymmetric standard model. - T Kobayashi, H Nakano, H Terao PHYSICAL REVIEW D 71 (11) 2005年06月 [査読無し][通常論文]

In the scenarios with heavy top squarks, mass parameters of the Higgs field must be fine-tuned due to a large logarithmic correction to the soft scalar mass. We consider a new possibility that the top-Yukawa coupling is small above TeV scale. The large top mass is induced from strong Yukawa interaction of the Higgs with another gauge sector, in which supersymmetry breaking parameters are given to be small. Then it is found that the logarithmic correction to the Higgs soft scalar mass is suppressed in spite of the strong coupling, and the fine-tuning is ameliorated. We propose an explicit model coupled to a superconformal gauge theory which realizes the above situation. - Tatsuo Kobayashi, Hiroaki Shirano, Haruhiko Terao Progress of Theoretical Physics 113 (5) 1077 -1099 2005年05月 [査読無し][通常論文]

We consider the dynamical realization of the democratic type of Yukawa coupling matrices as the Pendelton-Ross infrared fixed points. Such fixed points of the Yukawa couplings become possible through the introduction of many Higgs fields, which are made superheavy, but one massless mode. Explicitly, we consider a strongly coupled supersymmetric GUT based on SU(5) × SU(5), in which rapid convergence to the infrared fixed point generates sufficiently large mass hierarchy for quarks and leptons. In particular, it is found that the remarkable difference between mixing angles in the quark and lepton sectors can be explained as a simple dynamical consequence. We also discuss a possible scenario leading to a realistic mass spectra and mixing angles for quarks and leptons. In this scheme, the Yukawa couplings not only for top but also for bottom appear close to their quasi-fixed points at low energy and, therefore, tan β should be large. - P Ko, T Kobayashi, JH Park PHYSICAL REVIEW D 71 (9) 2005年05月 [査読無し][通常論文]

We systematically study the possibility for realizing realistic values of lepton mass ratios and mixing angles by using only renormalizable Yukawa couplings derived from the heterotic Z(6)-I orbifold. We assume one pair of up and down sector Higgs fields. We consider both the Dirac neutrino mass scenario and the seesaw scenario with degenerate right-handed majorana neutrino masses. It is found that realistic values of the charged lepton mass ratios, m(e)/m(tau) and m(mu)/m(tau), the neutrino mass squared difference ratio, Delta m(31)(2)/Delta m(21)(2), and the lepton mixing angles can be obtained in certain cases. - N Kitazawa, T Kobayashi, N Maru, N Okada EUROPEAN PHYSICAL JOURNAL C 40 (4) 579 -587 2005年04月 [査読無し][通常論文]

The structure of Yukawa coupling matrices is investigated in type IIA T-6/(Z(2) x Z(2)) orientifold models with intersecting D-branes. Yukawa coupling matrices are difficult to be made realistic in conventional models in which the generation structure emerges by the multiple intersection of D-branes in the factorized T-6 = T-2 x T-2 x T-2. We study the new type of favor structure, where Yukawa couplings are dynamically generated, and show this type of models lead to non-trivial structures of Yukawa coupling matrices, which can be realistic. - Tatsuo Kobayashi, Hiroaki Nakano, Haruhiko Terao, Yoshihisa Yamada Progress of Theoretical Physics 113 (2) 413 -427 2005年02月 [査読無し][通常論文]

We consider a mechanism for realizing the desired decoupling of the strongly-coupled sector which is supposed to generate hierarchical structure of the Yukawa couplings. In our mechanism, the same strongly-coupled sector is responsible for generating a sufficiently flat potential and a large vacuum expectation value (VEV) of a gauge-singlet scalar field by suppressing its soft scalar mass and self-coupling. A vacuum instability is caused by a supersymmetry-breaking A-term of order 10 TeV. We explicitly demonstrate the infrared convergence of soft scalar masses due to strongly-coupled dynamics and show that the soft mass of the singlet is at most of the same order as the soft masses of squarks and sleptons, which are much more strongly suppressed than the A-term. The physical mass scale of the decoupling is calculated in a self-consistent way. We also reinterpret the result in terms of an RG-improved effective potential. - T Kobayashi, S Raby, RJ Zhang NUCLEAR PHYSICS B 704 (1-2) 3 -55 2005年01月 [査読無し][通常論文]

Motivated by orbifold grand unified theories, we construct a class of three-family Pati-Salam models in a Z(6) Abelian symmetric orbifold with two discrete Wilson lines. These models have marked differences from previously-constructed three-family models in prime-order orbifolds. In the limit where one of the six compactified dimensions (which lies in a Z(2) sub-orbifold) is large compared to the string length scale, our models reproduce the supersymmetry and gauge symmetry breaking pattern of 5d orbifold grand unified theories on an S-1/Z(2) orbicircle. We find a horizontal 2 divided by 1 splitting in the chiral matter spectra-2 families of matter are localized on the Z(2) orbifold fixed points. and 1 family propagates in the 5d bulk-and identify them as the first-two and third families. Remarkably. the first two families enjoy a non-Abelian dihedral D-4 family symmetry. due to the geometric setup of the compactified space. In all our models there are always some color triplets. i.e.. (6.1.1) representations of the Pati-Salam group, survive orbifold projections. They could be utilized to spontaneously break the Pati-Salam symmetry to that of the Standard Model. One model. with a 5d E-6 symmetry. may give rise to interesting low energy phenomenology. We study gauge coupling unification. allowed Yukawa couplings and some of their phenomenological consequences. The E-6 model has a renormalizable Yukawa coupling only for the third family. It predicts a gauge-Yukawa unification relation at the 5d compactification scale, and is capable of generating reasonable quark/lepton masses and mixings. Potential problems are also addressed, they may point to the direction for refining our models. (C) 2004 Elsevier B.V. All rights reserved. - Tatsuo Kobayashi, Koichi Yoshioka JHEP 0411:024,2004 2004年09月30日 [査読無し][通常論文]

Higher-dimensional field theory has been applied to explore various issues in

recent particle physics such as the gauge hierarchy problem. In order for such

approaches to be viable, a crucial ingredient is to fix the sizes of extra

dimensions at some finite values, which sizes are generically free parameters

in the theory. In this paper, we present several schemes to determine the

radius of extra dimension in warped five-dimensional theory. In every case, a

non-vanishing Fayet-Iliopoulos term for abelian gauge factor plays a crucial

role for the radius stabilization. It is radiatively generated in the presence

of charged matter fields and the compactification is therefore spontaneous, not

forced by selected operators. The low-energy supersymmetry is broken or

unbroken, and the radius can be fixed to give a small or large scale hierarchy

without any fine tuning of parameters. We also discuss a model of the radius

stabilization correlated with Yukawa hierarchy and supersymmetry breaking. - P Ko, T Kobayashi, JH Park PHYSICS LETTERS B 598 (3-4) 263 -272 2004年09月 [査読無し][通常論文]

We study systematically the possibility for realizing realistic values of quark mass ratios m(c)/m(t) and m(s)/m(b) and the mixing angle V-cb by using only renormalizable Yukawa couplings derived from heterotic orbifold models. We assume one pair of up and down sector Higgs fields. We find many realistic Yukawa matrices. (C) 2004 Published by Elsevier B.V. - T Kobayashi, S Raby, RJ Zhang PHYSICS LETTERS B 593 262 -270 2004年07月 [査読無し][通常論文]

A three-generation Pati-Salam model is constructed by compactifying the heterotic string on a particular T-6/Z(6) Abelian symmetric orbifold with two discrete Wilson lines. The compactified space is taken to be the Lie algebra lattice G(2) circle plus SU(3) circle plus SO(4). When one dimension of the SO(4) lattice is large compared to the string scale, this model reproduces many features of a 5d SO(10) grand unified theory compactified on an S-1/Z(2) orbifold. (Of course, with two large extra dimensions we can obtain a 6d SO(10) grand unified theory.) We identify the orbifold parities and other ingredients of the orbifold grand unified theories in the string model. Our construction provides a UV completion of orbifold grand unified theories, and gives new insights into both field theoretical and string theoretical constructions. (C) 2004 Elsevier B.V. All rights reserved. - K Choi, DY Kim, IW Kim, T Kobayashi EUROPEAN PHYSICAL JOURNAL C 35 (2) 267 -275 2004年06月 [査読無し][通常論文]

We examine the soft supersymmetry breaking parameters in supersymmetric theories on a slice of AdS(5) which generate the hierarchical Yukawa couplings by dynamically quasi-localizing the bulk matter fields in an extra dimension. Such models can be regarded as the AdS dual of the recently studied 4-dimensional models which contain a supersymmetric CFT to generate the hierarchical Yukawa couplings. In such models, if supersymmetry breaking is mediated by the bulk radion superfield and/or some brane chiral superfields, potentially dangerous flavor-violating soft parameters are suppressed with an appropriate correlation with the Yukawa coupling suppression, thereby avoiding the SUSY flavor problem in a natural manner. We present some models of radion-dominated supersymmetry breaking which yield a highly predictive form of soft parameters in this framework, and discuss the constraints from flavor-changing rare processes. Most of the discussions in this paper can be applied also to models with a flat extra dimension in which the Yukawa hierarchy is generated by quasi-localizing the bulk matter fields in the extra dimension. - T Higaki, Y Kawamura, T Kobayashi, H Nakano PHYSICAL REVIEW D 69 (8) 2004年04月 [査読無し][通常論文]

We study the D-term contribution for anomalous U(1) symmetries in type I string models and derive a general formula for the D-term contribution, assuming that the dominant source of supersymmetry breaking is given by the F terms of the dilaton, (overall) moduli, or twisted moduli fields. On the basis of the formula, we also point out that there are several features different from the case of heterotic string models. The differences originate from the different forms of the Kahler potential between twisted moduli fields in type I string models and the dilaton field in heterotic string models. - Tatsuo Kobayashi, Haruhiko Terao JHEP 0407 (2004) 026 2004年03月31日 [査読無し][通常論文]

We study the little hierarchy between mass parameters in the Higgs sector and

other SUSY breaking masses. This type of spectrum can relieve the fine-tuning

problem in the MSSM Higgs sector. Our scenario can be realized by

superconformal dynamics. The spectrum in our scenario has significant

implications in other phenomenological aspects like the relic abundance of the

lightest neutralino and relaxation of the unbounded-from-below constraints. - Tetsutaro Higaki, Yoshiharu Kawamura, Tatsuo Kobayashi, Hiroaki Nakano Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics 582 (3-4) 257 -262 2004年03月04日 [査読無し][通常論文]

We study the dilaton stabilization in models with anomalous U(1) symmetry by adding specific string-motivated, non-perturbative corrections to the tree-level dilaton Kähler potential. We find that the non-perturbative effects can stabilize the dilaton at a desirably large value. We also observe that the size of Fayet-Iliopoulos term is reduced at the stabilized point. © 2004 Published by Elsevier B.V. - Tetsutaro Higaki, Tatsuo Kobayashi, Osamu Seto JHEP 0407 (2004) 035 2004年02月19日 [査読無し][通常論文]

We study the $D$-term inflation scenario with a nonperturbative K\"ahler

potential of the dilaton field. Although the FI term which leads an

inflationary expansion is given by the derivative of the K\"ahler potential

with respect to the dilaton in heterotic string models with anomalous U(1), the

too large magnitude is problematic for a viable $D$-term inflation. In this

paper, we point out that the K\"ahler potential with a nonperturbative term can

reduce the magnitude of FI term to desired values while both the dilaton

stabilization and $D$-term domination in the potential are realized by

nonperturbative superpotential. - SG Nibbelink, M Hillenbach, T Kobayashi, MGA Walter PHYSICAL REVIEW D 69 (4) 2004年02月 [査読無し][通常論文]

We investigate the structure of local anomalies of heterotic E(8)xE'(8) theory on T-6/Z(4). We show that the untwisted states lead to anomalies in 10, four, and in particular six dimensions. At each of the six dimensional fixed spaces of this orbifold the twisted states ensure, that the anomalies factorize separately. As some of these twisted states live on T-2/Z(2), they give rise to four dimensional anomalies as well. At all four dimensional fixed points at worst a single Abelian anomaly can arise. Additionally we show that the anomalies in all these dimensions factorize in a universal way and can be canceled simultaneously. - O Lebedev STRING PHENOMENOLOGY 2003 256 -259 2004年 [査読無し][通常論文]

I discuss effects of the Wilson lines on heterotic Yukawa couplings with the focus on their relation to CP violation. - T Kobayashi, O Seto PHYSICAL REVIEW D 69 (2) 2004年01月 [査読無し][通常論文]

We investigate the possibility of D-term inflation within the framework of type-I string-inspired models. Although the D-term inflation model has the excellent property that it is free of the so-called eta problem, two serious problems appear when we embed D-term inflation in string theory: the magnitude of the FI term and the rolling motion of the dilaton. In the present paper, we analyze the potential of D-term inflation in type-I-inspired models and study the behavior of dilaton and twisted moduli fields. Adopting the nonperturbative superpotential induced by gaugino condensation, the twisted moduli can be stabilized. If the dilaton is in a certain range, it evolves very slowly and does not run away to infinity. Thus D-term-dominated vacuum energy becomes available for driving inflation. By studying the density perturbation generated by the inflation model, we derive the constraints on model parameters and give some implications on D-term inflation in type-I-inspired models. - Tatsuo Kobayashi, Jisuke Kubo, Haruhiko Terao Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics 568 (1-2) 83 -91 2003年08月21日 [査読無し][通常論文]

The exact discrete flavor symmetry, S3 in the hadronic sector and S3 x Z2 in the leptonic sector, which has been recently found, is introduced in a supersymmetric extension of the standard model. We investigate the supersymmetric flavor problem, and explicitly find that thanks to the flavor symmetry the dangerous FCNC processes and CP-violating phases are sufficiently suppressed. © 2003 Elsevier B.V. All rights reserved. - T Higaki, T Kobayashi PHYSICAL REVIEW D 68 (4) 046006 2003年08月 [査読無し][通常論文]

We consider a model with dilaton and twisted moduli fields, which is inspired by type I string models. The stabilization of their vacuum expectation values is studied. We find that the stabilization of the twisted moduli field has different aspects from dilaton stabilization. - T Kobayashi, J Kubo, H Terao PHYSICS LETTERS B 568 (1-2) 83 -91 2003年08月 [査読無し][通常論文]

The exact discrete flavor symmetry, S-3 in the hadronic sector and S-3 x Z(2) in the leptonic sector, which has been recently found, is introduced in a supersymmetric extension of the standard model. We investigate the supersymmetric flavor problem, and explicitly find that thanks to the flavor symmetry the dangerous FCNC processes and CP-violating phases are sufficiently suppressed. (C) 2003 Elsevier B.V. All rights reserved. - T Kobayashi, H Nakano, H Terao, K Yoshioka PROGRESS OF THEORETICAL PHYSICS 110 (2) 247 -267 2003年08月 [査読無し][通常論文]

In this paper, we study flavor violation in supersymmetric models with gauged flavor symmetries. There are several sources of flavor violation in these theories. The dominant flavor violation is the tree-level D-term contribution to scalar masses generated by flavor symmetry breaking. We present a new approach for suppressing these phenomenologically dangerous effects by separating the flavor-breaking sector from the supersymmetry-breaking sector. This separation can be achieved either in geometrical setups or in a dynamical way. We also point out that radiative corrections from the gauginos of gauged flavor symmetries can yield sizable generation-dependent masses of scalars. Generically the gaugino mass effects are not suppressed, even if the dominant D-term contribution is suppressed. We also analyze the constraints on the flavor symmetry sector resulting from these flavor-violating corrections. - T Kobayashi, O Lebedev PHYSICS LETTERS B 565 (1-4) 193 -200 2003年07月 [査読無し][通常論文]

In the framework of Z(N) orbifolds, we discuss effects of heterotic string backgrounds including discrete Wilson lines on the Yukawa matrices and their connection to CP violation. (C) 2003 Published by Elsevier B.V. - T Kobayashi, O Lebedev PHYSICS LETTERS B 566 (1-2) 164 -170 2003年07月 [査読無し][通常論文]

Realistic heterotic string models require the presence of the Wilson lines. We study the effect of continuous Wilson lines on the heterotic Yukawa couplings for Abelian orbifold compactifications and find that the presence of continuous Wilson lines affects the magnitude of the twisted Yukawa couplings resulting in their stronger hierarchy. (C) 2003 Published by Elsevier B.V. - T Kobayashi, N Maru, K Yoshioka EUROPEAN PHYSICAL JOURNAL C 29 (2) 277 -284 2003年07月 [査読無し][通常論文]

In this letter, we discuss a four-dimensional model with modulus fields which are responsible for supersymmetry breaking. Given a non-trivial moduli dependence of the action, the model is found to give a proper description of higher-dimensional supersymmetry breaking. We explicitly calculate the gaugino and scalar mass spectrum and show that several classes of scenarios proposed in the literature are described in certain regions of the parameter space of the moduli vacuum expectation values. The model in other generic regions of the moduli space gives unexplored scenarios (mass spectra) of supersymmetry breaking in four dimensions. - Tatsuo Kobayashi, Oleg Lebedev Phys.Lett. B565 (2003) 193-200 2003年04月25日 [査読無し][通常論文]

In the framework of $Z_N$ orbifolds, we discuss effects of heterotic string

backgrounds including discrete Wilson lines on the Yukawa matrices and their

connection to CP violation. - KS Babu, T Kobayashi, J Kubo PHYSICAL REVIEW D 67 (7) 075018 2003年04月 [査読無し][通常論文]

We study a finite SU(5) grand unified model based on the non-Abelian discrete symmetry A(4). This model leads to the democratic structure of the mass matrices for the quarks and leptons. In the soft supersymmetry breaking sector, the scalar trilinear couplings are aligned and the soft scalar masses are degenerate, thus solving the SUSY flavor problem. - T Kobayashi, H Nakano, T Noguchi, H Terao JOURNAL OF HIGH ENERGY PHYSICS 0302 (2) 022 2003年02月 [査読無し][通常論文]

We propose a scenario that leads to hierarchical Yukawa couplings and degenerate sfermion masses at the same time, in the context of extra-dimensional models, which can be naturally embedded in a wide class of string models. The hierarchy of Yukawa couplings and degeneracy of sfermion masses can be realized thanks to superconformal gauge dynamics. The sfermion mass degeneracy is guaranteed by taking the superconformal fixed point to be family independent. In our scenario, the origin of Yukawa hierarchy is attributed to geometry of compactified dimensions and the consequent volume dependence of gauge couplings in the superconformal sectors. The difference in these gauge couplings is dynamically transferred to the hierarchy of the Yukawa couplings. Thus, our scenario combines a new dynamical approach and the conventional geometrical approach of the supersymmetric flavor problem. - H Abe, T Kobayashi, N Maru, K Yoshioka PHYSICAL REVIEW D 67 (4) 045019 2003年02月 [査読無し][通常論文]

We present four-dimensional gauge theories that describe physics on five-dimensional curved (warped) backgrounds, which includes bulk fields with various spins (vectors, spinors, and scalars). Field theory on the AdS(5) geometry is examined as a simple example of our formulation. Various properties of bulk fields on this background, e.g., the mass spectrum and field localization behavior, can be achieved within a fully four-dimensional framework. Moreover, that gives a localization mechanism for massless vector fields. We also consider supersymmetric cases, and show in particular that the conditions on bulk masses imposed by supersymmetry on warped backgrounds are derived from a four-dimensional supersymmetric theory on the flat background. As a phenomenological application, models are shown to generate hierarchical Yukawa couplings. Finally, we discuss possible underlying mechanisms which dynamically realize the required couplings to generate curved geometries. - H Abe, T Kobayashi, N Maru, K Yoshioka PHYSICAL REVIEW D 67 (4) 2003年02月 [査読無し][通常論文]

We present four-dimensional gauge theories that describe physics on five-dimensional curved (warped) backgrounds, which includes bulk fields with various spins (vectors, spinors, and scalars). Field theory on the AdS(5) geometry is examined as a simple example of our formulation. Various properties of bulk fields on this background, e.g., the mass spectrum and field localization behavior, can be achieved within a fully four-dimensional framework. Moreover, that gives a localization mechanism for massless vector fields. We also consider supersymmetric cases, and show in particular that the conditions on bulk masses imposed by supersymmetry on warped backgrounds are derived from a four-dimensional supersymmetric theory on the flat background. As a phenomenological application, models are shown to generate hierarchical Yukawa couplings. Finally, we discuss possible underlying mechanisms which dynamically realize the required couplings to generate curved geometries. - Kiwoon Choi, Do Young Kim, Ian-Woo Kim, Tatsuo Kobayashi 2003年01月18日 [査読無し][通常論文]

We point out that supersymmetric warped geometry can provide a solution to

the SUSY flavor problem, while generating hierarchical Yukawa couplings. In

supersymmetric theories in a slice of AdS_5 with the Kaluza-Klein scale M_KK

much higher than the weak scale, if all visible fields originate from 5D bulk

fields and supersymmetry breaking is mediated by the bulk radion superfield

and/or some brane chiral superfields, potentially dangerous soft scalar masses

and trilinear $A$ parameters at M_KK can be naturally suppressed compared to

the gaugino masses by small warp factor. We present simple models yielding

phenomenologically interesting patterns of soft parameters in this framework. - Hiroyuki Abe, Tetsutaro Higaki, Tatsuo Kobayashi Progress of Theoretical Physics 109 (5) 809 -830 2003年 [査読無し][通常論文]

We systematically study VEVs of a gauge scalar field Σ in a bulk U(1) vector multiplet and scalar fields in brane/bulk hypermultiplets charged under U(1) in the 5D S1/Z2 orbifold model with generic FI terms. A non-trivial VEV of Σ generates bulk mass terms for U(1) charged fields, and their zero modes have non-trivial profiles. In particular, in the SUSY-breaking case, bosonic and fermionic zero modes have Gaussian profiles. Such non-trivial profiles are useful to explain hierarchical couplings. A toy model for SUSY breaking is studied, and it yields sizable D-term contributions to scalar masses. Because the overall magnitude of D-term contributions is the same everywhere in the bulk and also on both branes, we have to take into account these contributions and other SUSY-breaking terms to obtain a realistic description. We also give profiles and mass eigenvalues of higher modes. - Tatsuo Kobayashi, Hiroaki Nakano, Haruhiko Terao, Koichi Yoshioka Progress of Theoretical Physics 110 (2) 247 -267 2003年 [査読無し][通常論文]

In this paper, we study flavor violation in supersymmetric models with gauged flavor symmetries. There are several sources of flavor violation in these theories. The dominant flavor violation is the tree-level D-term contribution to scalar masses generated by flavor symmetry breaking. We present a new approach for suppressing these phenomenologically dangerous effects by separating the flavor-breaking sector from the supersymmetry-breaking sector. This separation can be achieved either in geometrical setups or in a dynamical way. We also point out that radiative corrections from the gauginos of gauged flavor symmetries can yield sizable generation-dependent masses of scalars. Generically the gaugino mass effects are not suppressed, even if the dominant D-term contribution is suppressed. We also analyze the constraints on the flavor symmetry sector resulting from these flavor-violating corrections. - Hiroyuki Abe, Tetsutaro Higaki, Tatsuo Kobayashi Progress of Theoretical Physics 109 (5) 809 -830 2003年 [査読無し][通常論文]

We systematically study VEVs of a gauge scalar field Σ in a bulk U(1) vector multiplet and scalar fields in brane/bulk hypermultiplets charged under U(1) in the 5D S1/Z2 orbifold model with generic FI terms. A non-trivial VEV of Σ generates bulk mass terms for U(1) charged fields, and their zero modes have non-trivial profiles. In particular, in the SUSY-breaking case, bosonic and fermionic zero modes have Gaussian profiles. Such non-trivial profiles are useful to explain hierarchical couplings. A toy model for SUSY breaking is studied, and it yields sizable D-term contributions to scalar masses. Because the overall magnitude of D-term contributions is the same everywhere in the bulk and also on both branes, we have to take into account these contributions and other SUSY-breaking terms to obtain a realistic description. We also give profiles and mass eigenvalues of higher modes. - Tetsutaro Higaki, Tatsuo Kobayashi Physical Review D 68 (4) 2003年 [査読無し][通常論文]

We consider a model with dilaton and twisted moduli fields, which is inspired by type I string models. The stabilization of their vacuum expectation values is studied. We find that the stabilization of the twisted moduli field has different aspects from dilaton stabilization. © 2003 The American Physical Society. - T Kobayashi, H Nakano, T Noguchi, H Terao PHYSICAL REVIEW D 66 (9) 095011 2002年11月 [査読無し][通常論文]

We discuss issues in a scenario where hierarchical Yukawa couplings are generated through the strong dynamics of superconformal field theories (SCFTs). Independently of the mediation mechanism of supersymmetry breaking, the infrared convergence property of SCFTs can provide an interesting solution to the supersymmetric flavor problem; sfermion masses are suppressed around the decoupling scale of SCFTs and eventually become degenerate to some degree, thanks to family-independent radiative corrections governed by the gaugino masses of the minimal supersymmetric standard model (MSSM). We discuss under what conditions the degeneracy of the sfermion mass can be estimated in a simple manner. We also discuss the constraints from lepton flavor violations. We then explicitly study sfermion mass degeneracy within the framework of grand unified theories coupled to SCFTs. It is found that the degeneracy for right-handed sleptons becomes worse in the conventional SU(5) model than in the MSSM. On the other hand, in the flipped SU(5)xU(1) model, each right-handed lepton is still an SU(5) singlet, whereas the B-ino mass M-1 is determined by two independent gaugino masses of SU(5)xU(1). These two properties enable us to have an improved degeneracy for the right-handed sleptons. We also speculate on how further improvement can be obtained in the SCFT approach. - T Kobayashi, H Nakano, T Noguchi, H Terao PHYSICAL REVIEW D 66 (9) 2002年11月 [査読無し][通常論文]

We discuss issues in a scenario where hierarchical Yukawa couplings are generated through the strong dynamics of superconformal field theories (SCFTs). Independently of the mediation mechanism of supersymmetry breaking, the infrared convergence property of SCFTs can provide an interesting solution to the supersymmetric flavor problem; sfermion masses are suppressed around the decoupling scale of SCFTs and eventually become degenerate to some degree, thanks to family-independent radiative corrections governed by the gaugino masses of the minimal supersymmetric standard model (MSSM). We discuss under what conditions the degeneracy of the sfermion mass can be estimated in a simple manner. We also discuss the constraints from lepton flavor violations. We then explicitly study sfermion mass degeneracy within the framework of grand unified theories coupled to SCFTs. It is found that the degeneracy for right-handed sleptons becomes worse in the conventional SU(5) model than in the MSSM. On the other hand, in the flipped SU(5)xU(1) model, each right-handed lepton is still an SU(5) singlet, whereas the B-ino mass M-1 is determined by two independent gaugino masses of SU(5)xU(1). These two properties enable us to have an improved degeneracy for the right-handed sleptons. We also speculate on how further improvement can be obtained in the SCFT approach. - Tatsuo Kobayashi, Hiroaki Nakano, Tatsuya Noguchi, Haruhiko Terao JHEP 0302:022,2003 2002年05月08日 [査読無し][通常論文]

We propose a scenario that leads to hierarchical Yukawa couplings and

degenerate sfermion masses at the same time, in the context of

extra-dimensional models, which can be naturally embedded in a wide class of

string models. The hierarchy of Yukawa couplings and degeneracy of sfermion

masses can be realized thanks to superconformal gauge dynamics. The sfermion

mass degeneracy is guaranteed by taking the superconformal fixed point to be

family independent. In our scenario, the origin of Yukawa hierarchy is

attributed to geometry of compactified dimensions and the consequent volume

dependence of gauge couplings in the superconformal sectors. The difference in

these gauge couplings is dynamically transferred to the hierarchy of the Yukawa

couplings. Thus, our scenario combines a new dynamical approach and the

conventional geometrical approach to the supersymmetric flavor problem. - Tatsuo Kobayashi, Haruhiko Terao Progress of Theoretical Physics 107 (4) 785 -792 2002年04月 [査読無し][通常論文]

We study the so-called "Kaluza-Klein regularization". We calculate one-loop corrections to the Higgs mass due to Kaluza-Klein modes explicitly in a model with SUSY breaking mass splitting between bosonic and fermionic modes. We perform the proper time cutoff at 1/Λ2 and the KK level truncation at ℓ. It is shown that a finite result is obtained as long as ℓ ≫ RΛ for the compactification radius R. - T Kobayashi, H Nakano, H Terao PHYSICAL REVIEW D 65 (1) 015006 2002年01月 [査読無し][通常論文]

We propose a new type of supersymmetric model coupled to superconformal (SC) field theories, leading simultaneously to hierarchical Yukawa couplings and completely degenerate sfermion masses. We consider models with an extra Abelian gauge symmetry to generate hierarchical structure for couplings between the standard model (SM) sector and the SC sector. Interestingly, this hierarchy is inversely transferred to the Yukawa couplings in the SM sector. In this type of model, the flavor-independent structure of the superconformal fixed point guarantees that the sfermion masses of the first and the second generations are completely degenerate at low energy. - T Kobayashi, H Nakano, H Terao PHYSICAL REVIEW D 65 (1) 2002年01月 [査読無し][通常論文]

We propose a new type of supersymmetric model coupled to superconformal (SC) field theories, leading simultaneously to hierarchical Yukawa couplings and completely degenerate sfermion masses. We consider models with an extra Abelian gauge symmetry to generate hierarchical structure for couplings between the standard model (SM) sector and the SC sector. Interestingly, this hierarchy is inversely transferred to the Yukawa couplings in the SM sector. In this type of model, the flavor-independent structure of the superconformal fixed point guarantees that the sfermion masses of the first and the second generations are completely degenerate at low energy. - 小林 達夫 素粒子論研究 103 (6) F43 -F46 2001年09月20日 [査読無し][通常論文]
- M Drees, YG Kim, T Kobayashi, MM Nojiri PHYSICAL REVIEW D 63 (11) 115009 2001年06月 [査読無し][通常論文]

We compare predictions for the spin-independent contribution to the neutralino-proton scattering cross section sigma (xp) and for the anomalous magnetic dipole moment of the muon, a(mu)= (g(mu) - 2)/2, in models with gravity-mediated supersymmetry breaking. We nearly always find a positive correlation between these two measurables; i.e., scenarios with larger a(mu) also tend to have larger sigma (xp), but the detailed prediction differs greatly between models. Tn particular, we find that for the popular MSUGRA scenario with universal soft breaking masses at the scale of grand unification, measurements of a(mu) currently seem more promising. On the other hand, if scalar soft breaking masses at the GUT scale receive sizable contributions from SO(10) D terms, one often finds scenarios with large sigma (xp) but a(mu) below the currently foreseen sensitivity. A string-inspired model with a nonuniversal scalar spectrum at the GUT scale falls between these two cases. - M Bando, T Kobayashi, T Noguchi, K Yoshioka PHYSICAL REVIEW D 63 (11) 113017 2001年06月 [査読無し][通常論文]

In this paper we study renormalization-group evolutions of Yukawa matrices enhanced by Kaluza-Klein excited modes and analyze their infrared fixed-point structure. We derive the necessary conditions to obtain hierarchies between generations on the fixed point. These conditions restrict how the fields in the models can extend to higher dimensions. Several specific mechanisms to realize the conditions are presented. We also take into account the generation mixing effects and find a scenario where the mixing angles become small at low energy even with large initial values at a high-energy scale. A toy model is shown to lead to realistic quark mass matrices. - Tatsuo Kobayashi Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics 506 (1-2) 187 -192 2001年05月03日 [査読無し][通常論文]

We study soft SUSY breaking terms in heterotic M-theory. We show that both weakly and strongly coupled heterotic string models lead to the same relations of soft SUSY breaking terms, A = -M and m2 = M2/3, up to O((αT/S)2). © 2001 Published by Elsevier Science B.V. - T Kobayashi, O Vives PHYSICS LETTERS B 506 (3-4) 323 -330 2001年05月 [査読無し][通常論文]

We analyze the flavor structure of the trilinear couplings in the different theoretical models of SUSY breaking. We generically obtain A(ij) = A(i)(L) + A(j)(R) in all the models examined. In fact, this is the rigorous form when SUSY breaking effects appear through the Kahler metric of chiral fields or through wave-function renormalization. Indeed, the low-energy phenomenological requirements from the absence of charge and color breaking minima and the measurements in flavor changing neutral current (FCNC) observables strongly favor this restricted form of the trilinear matrices. As a straightforward consequence the number of unknown parameters associated with the trilinear couplings is decreased in a factor of 2. (C) 2001 Elsevier Science B.V. All rights reserved. - 小林 達夫 日本物理学会講演概要集 56 (1) 7 -7 2001年03月09日
- Phys. Rev. D64 075003 2001年 [査読無し][通常論文]
**Sfermion masses in Nelson-Strassler-Type models : SUSY standard models coupled with SCFTs(??)** - Manuel Drees, Yeong Gyun Kirn, Tatsuo Kobayashi, Mihoko M. Nojiri Physical Review D 63 (11) 2001年 [査読無し][通常論文]

We compare predictions for the spin-independent contribution to the neutralino-proton scattering cross section σχp and for the anomalous magnetic djpole moment of the muon, aμ=(gμ- 2)12, in models with gravity-mediated supersymmetry breaking. We nearly always find a positive correlation between these two measurables i.e.,' 'scenarios with larger aμ also tend to have larger σχp, but the detailed prediction differs greatly between models. In particular, we find that for the popular MSUGRA scenario with universal soft breaking masses at the scale of grand unification, measurements of aμ currently seem more promising. On the other hand, if scalar soft breaking masses at the GUT scale receive sizable contributions from SO(10) D terms, one often finds scenarios with large σχp but aμ below the currently foreseen sensitivity. A string-inspired model with a nonuniversal scalar spectrum at the GUT scale falls between these two cases. ©2001 The American Physical Society. - Masako Bando, Tatsuo Kobayashi, Tatsuya Noguchi, Koichi Yoshioka Physical Review D 63 (11) 2001年 [査読無し][通常論文]

In this paper we study renormalization-group evolutions of Yukawa matrices enhanced by Kaluza-Klein excited modes and analyze their infrared fixed-point structure. We derive the necessary conditions to obtain hierarchies between generations on the fixed point. These conditions restrict how the fields in the models can extend to higher dimensions. Several specific mechanisms to realize the conditions are presented. We also take into account the generation mixing effects and find a scenario where the mixing angles become small at low energy even with large initial values at a high-energy scale. A toy model is shown to lead to realistic quark mass matrices. © 2001 The American Physical Society. - Tatsuo Kobayashi, Haruhiko Terao Physical Review D 64 (7) 2001年 [査読無し][通常論文]

We study soft supersymmetric (SUSY) breaking parameters in the Nelson-Strassler-type models: SUSY standard models coupled with superconformal field theories (SCFT's). In this type of model, soft SUSY breaking parameters including sfermion masses can be suppressed around the decoupling scale of SCFT's. We clarify the condition to derive exponential suppression of sfermion masses within the framework of pure SCFT's. Such behavior is favorable for degeneracy of sfermion masses. However, the realistic sfermion masses are not quite degenerate due to the gauge couplings and the gaugino masses in the standard model sector. We show the sfermion mass spectrum obtained in such models. The aspect of suppression for the soft SUSY breaking parameters is also demonstrated in an explicit model. We also give a mechanism generating the μ term of the electroweak scale by a singlet field coupled with the SCFT. ©2001 The American Physical Society. - T Kobayashi, K Yoshioka PHYSICAL REVIEW LETTERS 85 (26) 5527 -5530 2000年12月 [査読無し][通常論文]

The four-dimensional effective theories with compactified extra dimensions are considered. We then discuss a new framework for communicating supersymmetry breaking to the visible sector where the radius modulus, which determines the size of extra dimensions, has an auxiliary vacuum expectation value. The modulus couplings generate mass splitting in Kaluza-Klein supermultiplets and they act as messengers of supersymmetry breaking. The soft mas ses are expressed in terms of renormalization-group functions and the sparticle spectrum is determined by what kind of field propagates in the bulk. - Tatsuo Kobayashi, Haruhiko Terao Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics 489 (1-2) 233 -242 2000年09月14日 [査読無し][通常論文]

We examine the (softly broken) supersymmetric gauge-Higgs-Yukawa theories satisfying the compositeness conditions at a certain scale. In these theories the Higgs superfields can be regarded as the chiral composite fields. It is found that there are the fundamental theories, which contain the dimension 5 interactions and the hard SUSY breaking in perturbation, turn to be renormalizable and also softly broken theories in the nonperturbative framework. The soft SUSY breaking parameters as well as the Yukawa coupling in the corresponding gauge-Higgs-Yukawa theories are restricted by the renormalization group invariant relations. (C) 2000 Published by Elsevier Science B.V. - Tatsuo Kobayashi, Koichi Yoshioka Phys.Rev.Lett. 85 (2000) 5527-5530 2000年08月08日 [査読無し][通常論文]

We discuss a framework for communicating supersymmetry breaking to the

visible sector where the radius modulus, which determines the size of extra

dimensions, has an auxiliary vacuum expectation value. The modulus couplings

generate mass splitting in Kaluza-Klein supermultiplets and they act as

messengers of supersymmetry breaking. The soft masses are expressed in terms of

renormalization-group functions and the sparticle spectrum is determined by

what kind of field propagates in the bulk. This framework also provides new

possibilities for solving the supersymmetric flavor problem. - Tatsuo Kobayashi, Koichi Yoshioka Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics 486 (1-2) 223 -227 2000年07月27日 [査読無し][通常論文]

We study new renormalization-group invariant quantities of soft supersymmetry breaking parameters other than the ratio of gaugino mass to gauge coupling squared by using the spurion method. The obtained invariants are useful to probe supersymmetry breaking and μ-term generation mechanisms at high-energy scale. We also discuss the convergence behavior of fixed points of supersymmetry breaking parameters. (C) 2000 Elsevier Science B.V. - S Khalil, T Kobayashi, O Vives NUCLEAR PHYSICS B 580 (1-2) 275 -288 2000年07月 [査読無し][通常論文]

Non-universality in the soft breaking terms is a common feature in most superstring inspired SUSY models. This property is required to obtain sizeable CP violation effects from SUSY and, on the other hand, can be used to avoid the electric dipole moment (EDM) constraints. We take advantage of these qualities and explore a class of SUSY models based on type I string theory where scalar masses, gaugino masses and trilinear couplings are non-universal. In this framework, we show that, in the presence of large SUSY phases, the bounds on the electric dipole moments can be controlled without fine-tuning. At the same time, we find that these phases, free from EDM constraints, lead to large contributions to the observed CP phenomena in Kaon system and, in particular, to direct CP violation in epsilon'/epsilon. (C) 2000 Elsevier Science B.V. All rights reserved. - T Kobayashi, J Kubo, H Shimabukuro NUCLEAR PHYSICS B 580 (1-2) 3 -28 2000年07月 [査読無し][通常論文]

We study soft SUSY breaking parameters in M-theory with and without 5-brane moduli fields. We investigate successful electroweak symmetry breaking and the positivity of stau mass squared. A value of alpha(T + (T) over bar) is phenomenologically constrained. Also we have phenomenological constraints on magnitudes of derivatives of the Kahler potential for the 5-brane moduli field. We study mass spectra in the allowed regions. (C) 2000 Elsevier Science B.V. All rights reserved. - Tatsuo Kobayashi, Haruhiko Terao Phys.Lett. B489 (2000) 233-242 2000年05月26日 [査読無し][通常論文]

We examine the (softly broken) supersymmetric gauge-Higgs-Yukawa theories

satisfying the compositeness conditions at a certain scale. In these theories

the Higgs superfields can be regarded as the chiral composite fields. It is

found that there are the fundamental theories, which contain the dimension 5

interactions and the hard SUSY breaking in perturbation, turn to be

renormalizable and also softly broken theories in the nonperturbative

framework. The soft SUSY breaking parameters as well as the Yukawa coupling in

the corresponding gauge-Higgs-Yukawa theories are restricted by the

renormalization group invariant relations. - 小林 達夫 素粒子論研究 101 (2) B25 -B26 2000年05月20日 [査読無し][通常論文]
- Masako Bando, Tatsuo Kobayashi, Tatsuya Noguchi, Koichi Yoshioka Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics 480 (1-2) 187 -192 2000年05月04日 [査読無し][通常論文]

We discuss the existence of hierarchy of Yukawa couplings in the models with extra spatial dimensions. The hierarchical structure is induced by the power behavior of the cutoff dependence of the evolution equations which yield large suppressions of couplings at the compactification scale. The values of coupling constants at this scale can be made stable almost independently of the initial input parameters by utilizing the infrared fixed point. We find that the Yukawa couplings converge to the fixed points very quickly because of the enhanced energy dependence of the suppression factor from extra dimensions as well as in the Case of large gauge couplings at high-energy scale. (C) 2000 Published by Elsevier Science B.V. - M Bando, T Kobayashi, T Noguchi, K Yoshioka PHYSICS LETTERS B 480 (1-2) 187 -192 2000年05月 [査読無し][通常論文]

We discuss the existence of hierarchy of Yukawa couplings in the models with extra spatial dimensions. The hierarchical structure is induced by the power behavior of the cutoff dependence of the evolution equations which yield large suppressions of couplings at the compactification scale. The values of coupling constants at this scale can be made stable almost independently of the initial input parameters by utilizing the infrared fixed point. We find that the Yukawa couplings converge to the fixed points very quickly because of the enhanced energy dependence of the suppression factor from extra dimensions as well as in the case of large gauge couplings at high-energy scale. (C) 2000 Published by Elsevier Science B.V. All rights reserved. - Marcela Carena, Katri Huitu, Tatsuo Kobayashi Nucl.Phys.B592:164-182,2001 2000年03月20日 [査読無し][通常論文]

We study a generalization of anomaly-mediated supersymmetry breaking (AMSB)

scenarios, under the assumption that the effects of the high-scale theory do

not completely decouple and D-term type contributions can be therefore present.

We investigate the effect of such possible D-term additional contributions to

soft scalar masses by requiring that, for non-vanishing, renormalizable Yukawa

couplings $Y^{ijk} $, the sum of squared soft supersymmetry breaking mass

parameters, $M^2_{ijk} \equiv m_i^2+m_j^2+m_k^2$, is RG-invariant, in the sense

that it becomes independent of the specific ultraviolet boundary conditions as

it occurs in the AMSB models. These type of models can avoid the problem of

tachyonic solutions for the slepton mass spectrum present in AMSB scenarios. We

implement the electroweak symmetry breaking condition and explore the sparticle

spectrum associated with this framework. To show the possible diversity of the

sparticle spectrum, we consider two examples, one in which the D-terms induce a

common soft supersymmetry breaking mass term for all sfermion masses, and

another one in which a light stop can be present in the spectrum. - Shaaban Khalil, Tatsuo Kobayashi, Oscar Vives Nucl.Phys. B580 (2000) 275-288 2000年03月10日 [査読無し][通常論文]

Non-universality in the soft breaking terms is a common feature in most

superstring inspired SUSY models. This property is required to obtain sizeable

CP violation effects from SUSY and, on the other hand, can be used to avoid the

Electric Dipole Moment constraints. We take advantage of these qualities and

explore a class of SUSY models based on type I string theory where scalar

masses, gaugino masses and trilinear couplings are non-universal. In this

framework, we show that, in the presence of large SUSY phases, the bounds on

the Electric Dipole Moments can be controled without fine-tuning. At the same

time, we find that these phases, free from EDM constraints, lead to large

contributions to the observed CP phenomena in Kaon system and, in particular,

to direct CP violation in epsilon'/epsilon. - K Huitu, Y Kawamura, T Kobayashi, K Puolamaki PHYSICAL REVIEW D 61 (3) 035001 2000年02月 [査読無し][通常論文]

We study the phenomenological aspects of supersymmetric SU(5) grand unified theories with nonuniversal gaugino masses. For large tan beta, we investigate constraints fi-om the requirement of successful electroweak symmetry breaking, the positivity of stau mass squared, and the b-->s gamma decay rate. In the allowed region, the nature of the lightest supersymmetric particle is determined. Examples of mass spectra are given. We also calculate loop corrections to the bottom mass due to superpartners. - Nucl. Phys. B592 164 -182 2000年 [査読無し][通常論文]
**"RG-invariant Sum Rule in a Generalization of Anomaly-Mediated SUSY Breaking Models"(jointly worked)** - Phys. Rev. D62 115003 2000年 [査読無し][通常論文]
**"Infrared alignment of SUSY flavor structures"(jointly worked)** - Y Kawamura, T Kobayashi, M Watanabe INTERNATIONAL JOURNAL OF MODERN PHYSICS A 15 (1) 81 -103 2000年01月 [査読無し][通常論文]

We study mu and B-parameters in the minimal supersymmetric standard model (MSSM) based on the radiative electroweak symmetry breaking scenario using "bottom-up" approach and show how useful our approach is to select a phenomenologically viable model beyond the MSSM under the assumption that the underlying theory is a string model or a gauge-Yukawa unified gauge model. - K Huitu, Y Kawamura, T Kobayashi, K Puolamaki HIGH ENERGY PHYSICS 99, PROCEEDINGS 786 -788 2000年 [査読無し][通常論文]

We study phenomenological aspects of supersymmetric SU(5) grand unified theories with non-universal gaugino masses. For large tan beta, we investigate constraints from the requirement of successful electroweak symmetry breaking, the positivity of stau mass squared and the b --> s gamma decay rate. We also calculate loop corrections to the bottom mass due to superpartners. - Tatsuo Kobayashi Physical Review D - Particles, Fields, Gravitation and Cosmology 62 (11) 8 2000年 [査読無し][通常論文]

The various experimental bounds on flavor-changing interactions severely restrict the low-energy flavor structures of soft supersymmetry breaking parameters. In this work, we show that with a particular assumption of Yukawa couplings, the fermion mass and sfermion soft mass matrices are simultaneously diagonalized by common mixing matrices and we then obtain an alignment solution for the flavor problems. The required condition is generated by renormalization group evolutions and achieved at low-energy scale independently of high-energy structures of couplings. In this case, the diagonal entries of the soft scalar mass matrices are determined by gaugino and Higgs boson soft masses. We also discuss possible realizations of this scenario and the characteristic sparticle spectrum in the models. © 2000 The American Physical Society. - K Huitu, Y Kawamura, T Kobayashi, K Puolamaki PHYSICS LETTERS B 468 (1-2) 111 -117 1999年11月 [査読無し][通常論文]

We study non-universal corrections to the gauge couplings due to higher dimensional operators in supersymmetric SU(5) grand unified theories. The corrections are, in general, parametrized by three components originating from 24, 75 and 200 representations. We consider the prediction of alpha(3)(M-z) along each 24, 75 and 200 direction, and their linear combinations. The magnitude of GUT scale and its effects on proton decay are discussed. Non-SUSY case is also examined. (C) 1999 Published by Elsevier Science B.V. All rights reserved. - SM Harun-or-Rashid, T Kobayashi, H Shimabukuro PHYSICS LETTERS B 466 (2-4) 95 -99 1999年11月 [査読無し][通常論文]

We study F-term inflation in M-theory with and without five-brane moduli fields. We show the slow rolling condition is not satisfied in M-theory without five-brane moduli fields, but it can be satisfied in the case with non-vanishing F-terms of five-brane moduli fields. (C) 1999 Elsevier Science B.V. All rights reserved. - S Khalil, T Kobayashi, A Masiero PHYSICAL REVIEW D 60 (7) 075003 1999年10月 [査読無し][通常論文]

We study the CP phases of the soft supersymmetry breaking terms in string-inspired models with nonuniversal trilinear couplings. We show that such nonuniversality plays an important role on all CP violating processes. In particular these new supersymmetric sources of CP violation may significantly contribute to the observed CP phenomena in kaon physics while respecting the severe bound on the electric dipole moment of the neutron. [S0556-2821(99)03217-8]. - Katri Huitu, Yoshiharu Kawamura, Tatsuo Kobayashi, Kai Puolamaki Phys.Lett. B468 (1999) 111-117 1999年09月02日 [査読無し][通常論文]

We study non-universal corrections to the gauge couplings due to higher

dimensional operators in supersymmetric SU(5) grand unified theories. The

corrections are, in general, parametrized by three components originating from

{\bf 24}, {\bf 75} and {\bf 200} representations. We consider the prediction of

$\alpha_3(M_Z)$ along each {\bf 24}, {\bf 75} and {\bf 200} direction, and

their linear combinations. The magnitude of GUT scale and its effects on proton

decay are discussed. Non-SUSY case is also examined. - Katri Huitu, Tatsuo Kobayashi Phys.Lett. B470 (1999) 90-94 470 (1/4) 90 -94 1999年06月21日 [査読無し][通常論文]

We study the gauge coupling unification with extra dimensions. We take into

account corrections due to the higher dimensional operators. We show the

prediction of $\alpha_3(M_Z)$ is sensitive to such corrections, even if

$c<\Phi>/M=O(0.01)$. We also discuss the $b-\tau$ Yukawa unification. - Shaaban Khalil, Tatsuo Kobayashi Phys.Lett.B460:341-347,1999 B460 341 -347 1999年06月16日 [査読無し][通常論文]

We study contributions of supersymmetric CP phases to the CP violation

$\epsilon'/\epsilon$ in models with asymmetric $A$-matrices. We consider

asymmetric $A$-matrices, which are obtained from string-inspired supergravity.

We show that a certain type of asymmetry of $A$-matrices enhances

supersymmetric contributions to the CP violation $\epsilon'/\epsilon$ and the

supersymmetric contribution to $\epsilon'/\epsilon$ can be of order of the KTeV

result, $\epsilon'/\epsilon \sim 10^{-3}$. - Tatsuo Kobayashi, Jisuke Kubo, Myriam Mondragón, George Zoupanos Nuclear Physics B 550 (1-2) 99 -122 1999年06月14日 [査読無し][通常論文]

The evolution of the parameters including those in the soft supersymmetry-breaking (SSB) sector is studied in the minimal supersymmetric standard model (MSSM) with a certain set of Kaluza-Klein towers which has been recently considered by Dienes et al. We use the continuous Wilson renormalization group technique to derive the one-loop matching condition between the effective, renormalizable and original, unrenormalizable theories. We investigate whether the assumption of a large compactification radius in the model is consistent with the gauge coupling unification, the b-τ unification and the radiative breaking of the electroweak gauge symmetry with the universal SSB terms. We calculate the superpartner spectrum under the assumption of the universal SSB parameters to find differences between the model and the MSSM. © 1999 Elsevier Science B.V. All rights reserved. - T Kobayashi, J Kubo, M Mondragon, G Zoupanos NUCLEAR PHYSICS B 550 (1-2) 99 -122 1999年06月 [査読無し][通常論文]

The evolution of the parameters including those in the soft supersymmetry-breaking (SSB) sector is studied in the minimal supersymmetric standard model (MSSM) with a certain set of Kaluza-Klein towers which has been recently considered by Dienes et al. We use the continuous Wilson renormalization group technique to derive the one-loop matching condition between the effective, renormalizable and original, unrenormalizable theories. We investigate whether the assumption of a large compactification radius in the model is consistent with the gauge coupling unification, the b-tau unification and the radiative breaking of the electroweak gauge symmetry with the universal SSB terms. We calculate the superpartner spectrum under the assumption of the universal SSB parameters to find differences between the model and the MSSM. (C) 1999 Elsevier Science B.V. All rights reserved. - Tatsuo Kobayashi, Jisuke Kubo, Hitoshi Shimabukuro Nucl.Phys. B580 (2000) 3-28 1999年04月01日 [査読無し][通常論文]

We study soft SUSY breaking parameters in M-theory with and without 5-brane

moduli fields. We investigate successful electroweak symmetry breaking and the

positivity of stau mass squared. We study mass spectra in allowed regions. - Shaaban Khalil, Tatsuo Kobayashi, Antonio Masiero Phys.Rev. D60 (1999) 075003 1999年04月01日 [査読無し][通常論文]

We study the CP phases of the soft supersymmetry breaking terms in

string-inspired models with non-universal trilinear couplings. We show that

such non-universality plays an important role on all CP violating processes. In

particular these new supersymmetric sources of CP violation may significantly

contribute to the observed CP phenomena in kaon physics while respecting the

severe bound on the electric dipole moment of the neutron. - Masud Chaichian, Tatsuo Kobayashi Phys.Lett. B481 (2000) 26-32 481 (1) 26 -32 1999年03月12日 [査読無し][通常論文]

We compare two approaches in supersymmetric confinement, the Seiberg

formulation and the superconvergence rule. For the latter, the critical point

is $\gamma_{00}=0$ in the Landau gauge. We find $4\gamma_{00}=\beta_0$ is the

critical point for most of confining theories without a tree-level

superpotential in the Seiberg formulation, in particular, in the large $N_c$

and $N_f$ limit. We show how confining theories with a discrete symmetry and a

tree-level superpotential connect these two critical points: the large and

small discrete symmetry limits correspond to the critical points

$4\gamma_{00}=\beta_0$ and $\gamma_{00}=0$, respectively. - S. M. Harun-or-Rashid, Tatsuo Kobayashi, Hitoshi Shimabukuro Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics 466 (2-4) 95 -99 1999年 [査読無し][通常論文]

We study F-term inflation in M-theory with and without five-brane moduli fields. We show the slow rolling condition is not satisfied in M-theory without five-brane moduli fields, but it can be satisfied in the case with non-vanishing F-terms of five-brane moduli fields. © 1999 Elsevier Science B.V. All rights reserved. - T Kobayashi, J Kubo, M Mondragon, G Zoupanos INTERNATIONAL EUROPHYSICS CONFERENCE ON HIGH ENERGY PHYSICS 877 -882 1999年 [査読無し][通常論文]

A new solution to the requirement of two-loop finiteness of the soft supersymmetry breaking terms (SSB) parameters is found in Finite-Gauge-Yukawa unified theories. The new solution has the form of a sum rule for the relevant scaler masses, relaxing the universality required by the previously known solution, which leads to models with unpleasant phenomenological consequences. Using the sum rule we investigate two Finite-Gange-Yukawa unified models and we determine their spectrum in terms of few parameters. Some characteristic features of the models are that a) the old agreement of the top quark mass prediction with the measured value remains unchanged, b) the lightest Higgs mass is predicted around 120 GeV, c) the s-spectrum starts above 200 GeV. - 久保治輔, 小林達夫, 島袋均 日本物理学会北陸支部・応用物理学会北陸・信越支部合同講演会講演予稿集 1998 17 1998年12月 [査読無し][通常論文]
- A Karch, T Kobayashi, J Kubo, G Zoupanos PHYSICS LETTERS B 441 (1-4) 235 -242 1998年11月 [査読無し][通常論文]

Applying the recently obtained results on the renormalization of soft supersymmetry-breaking parameters, we investigate the infrared behaviour of the softly broken supersymmetric QCD as well as its dual theory in the conformal window. Under general assumptions on beta-functions, it is shown that the soft supersymmetry-breaking parameters asymptotically vanish in the infrared limit so that superconformal symmetry in softly broken supersymmetric QCD and in its dual theory revives at the infrared fixed point, provided the soft scalar masses satisfy certain renormalization group invariant relations. If these relations are not satisfied, there exist marginal operators in both theories that lead to the breaking of supersymmetry and also colour symmetry. (C) 1998 Elsevier Science B.V. All rights reserved. - Y Kawamura, T Kobayashi, H Shimabukuro PHYSICS LETTERS B 436 (1-2) 108 -116 1998年09月 [査読無し][通常論文]

We study phenomenological aspects of supersymmetric SO(10) GUTs with sum rules among soft SUSY breaking parameters, In particular, the sum rule related to the stau mass leads to the constraints from the requirements of successful electroweak breaking and the positivity of stau mass squared. The bottom quark mass is also estimated. (C) 1998 Elsevier Science B.V. All rights reserved. - Andreas Karch, Tatsuo Kobayashi, Jisuke Kubo, George Zoupanos Phys.Lett. B441 (1998) 235-242 1998年08月29日 [査読無し][通常論文]

Applying the recently obtained results on the renormalization of soft

supersymmetry-breaking parameters, we investigate the infrared behaviour of the

softly broken supersymmetric QCD as well as its dual theory in the conformal

window. Under general assumptions on $\beta$-functions, it is shown that the

soft supersymmetry-breaking parameters asymptotically vanish in the infrared

limit so that superconformal symmetry in softly broken supersymmetric QCD and

in its dual theory revives at the infrared fixed point, provided the soft

scalar masses satisfy certain renormalization group invariant relations. If

these relations are not satisfied, there exist marginal operators in both

theories that lead to the breaking of supersymmetry and also colour symmetry. - Shaaban Khalil, Tatsuo Kobayashi Nuclear Physics B 526 (1-3) 99 -114 1998年08月24日 [査読無し][通常論文]

We study Yukawa unification in string models with moduli-dominant SUSY breaking. This type of SUSY breaking in general leads to non-universal soft masses, i.e. soft scalar masses and gaugino masses. Such non-universality is important for phenomenological aspects of Yukawa unification, i.e., successful electroweak breaking, SUSY corrections to the bottom mass and the branching ratio of b → sγ, We show three regions in the whole parameter space that lead to successful electroweak breaking and allow small SUSY corrections to the bottom mass. For these three regions we investigate the b → sγ decay and mass spectra. © 1998 Elsevier Science B.V. - Masud Chaichian, Tatsuo Kobayashi Phys.Lett. B442 (1998) 192-198 1998年08月19日 [査読無し][通常論文]

We study softly broken N=1 supersymmetric QCD with the gauge group $SU(N_c)$

and $N_f$ flavors of quark pairs. We investigate vacuum structure of the theory

with generic soft supersymmetry breaking terms. Trilinear soft breaking terms

play an essential role in determining vacua. For $N_f=N_c+1$, chiral symmetry

is broken for a sufficiently large magnitude of trilinear couplings, while it

is unbroken in the case with only soft masses. In the case where appearance of

trilinear coupling terms is allowed, i.e. for $N_f \geq N_c+1$, we have two

possible vacua, the trivial and non-trivial ones. Otherwise, we only have the

non-trivial vacuum, which corresponds to the non-trivial vacuum in the $N_f

\geq N_c+1$ theory. - S Khalil, T Kobayashi NUCLEAR PHYSICS B 526 (1-3) 99 -114 1998年08月 [査読無し][通常論文]

We study Yukawa unification in string models with moduli-dominant SUSY breaking. This type of SUSY breaking in general leads to non-universal soft masses, i.e. soft scalar masses and gaugino masses. Such non-universality is important for phenomenological aspects of Yukawa unification, i.e., successful electroweak breaking, SUSY corrections to the bottom mass and the branching ratio of b --> s gamma, We show three regions in the whole parameter space that lead to successful electroweak breaking and allow small SUSY corrections to the bottom mass. For these three regions we investigate the b --> s gamma decay and mass spectra, (C) 1998 Elsevier Science B.V. - 小林 達夫 素粒子論研究 97 (4) D31 -D33 1998年07月20日 [査読無し][通常論文]
- M Chaichian, WF Chen, T Kobayashi PHYSICS LETTERS B 432 (1-2) 120 -127 1998年07月 [査読無し][通常論文]

We study softly broken N=1 supersymmetric QCD with the gauge group SU(N-c) and Nf flavours of quarks for N-f > N-c + 1. We investigate the phase structure of its dual theory adding generic soft supersymmetry breking terms, i.e. soft scalar masses, trilinear coupling terms of scalar fields and gaugino masses. It is found that the trilinear coupling terms play an important role in determining the potential minima. Also we compare softly broken original and dual theories in the broken phase. (C) 1998 Elsevier Science B.V. All rights reserved. - Y Kawamura, T Kobayashi, J Kubo PHYSICS LETTERS B 432 (1-2) 108 -113 1998年07月 [査読無し][通常論文]

Assuming that the universal sum rules for soft supersymmetry-breaking terms, which have been recently found both in four-dimensional superstring models and in supersymmertic gauge-Yukawa unified gauge models, are satisfied above and at the grand unification scale, we investigate their low energy consequences and derive sum rules in the superpartner spectrum of the minimal supersymmetric standard model (MSSM). These sum rules contain only observable parameters of the MSSM, and could be experimentally verified in future. (C) 1998 Elsevier Science B.V. All rights reserved. - Yoshiharu Kawamura, Tatsuo Kobayashi, Hitoshi Shimabukuro Phys.Lett.B436:108-116,1998 1998年05月15日 [査読無し][通常論文]

We study phenomenological aspects of supersymmetric SO(10) GUTs with sum

rules among soft SUSY breaking parameters. In particular, the sum rule related

to the stau mass leads to the constraints from the requirements of successful

electroweak breaking and the positivity of stau mass squared. The bottom quark

mass is also estimated. - T Kobayashi, J Kubo, G Zoupanos PHYSICS LETTERS B 427 (3-4) 291 -299 1998年05月 [査読無し][通常論文]

It is proven that the recently found, renormalization-group invariant sum rule for the soft scalar masses in softly-broken N=l supersymmetric gauge-Yukawa unified theories can be extended to all orders in perturbation theory. Ln the case of finite unified theories, the sum rule ensures the all-loop finiteness in the soft supersymmetry breaking sector. As a byproduct the exact beta function for the soft scalar masses in the Novikov-Shifman-Vainstein-Zakharov (NSVZ) scheme for softly-broken supersymmetric QCD is obtained. It is also found that the singularity appearing in the sum rule in the NSVZ scheme exactly coincides with that which has been previously found in a certain class of superstring models in which the massive string states are organized into N=4 supermultiplets. (C) 1998 Elsevier Science B.V. All rights reserved. - Masud Chaichian, Wen-Feng Chen, Tatsuo Kobayashi Phys.Lett. B432 (1998) 120-127 1998年03月18日 [査読無し][通常論文]

We study softly broken N=1 supersymmetric QCD with the gauge group $SU(N_c)$

and $N_f$ flavours of quarks for $N_f > N_c+1$. We investigate the phase

structure of its dual theory adding generic soft supersymmetry breking terms,

i.e. soft scalar masses, trilinear coupling terms of scalar fields and gaugino

masses. It is found that the trilinear coupling terms play an improtant role in

determining the potential minima. Also we compare softly broken original and

dual theories in the broken phase. - Yoshiharu Kawamura, Tatsuo Kobayashi, Manabu Watanabe Int.J.Mod.Phys. A15 (2000) 81-104 1998年02月16日 [査読無し][通常論文]

We study $\mu$ and $B$-parameters in the minimal supersymmetric standard

model (MSSM) based on the radiative electroweak symmetry breaking scenario

using $\lq\lq$bottom-up" approach and show how useful our approach is to select

a phenomenologically viable model beyond the MSSM under the assumption that the

underlying theory is a string model or a gauge-Yukawa unified gauge model. - Yoshiharu Kawamura, Tatsuo Kobayashi, Manabu Watanabe Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics 419 (1-4) 157 -166 1998年02月11日 [査読無し][通常論文]

We give a generic method to select a realistic μ-term generation mechanism based on the radiative electroweak symmetry breaking scenario and study which type is hopeful within the framework of string theory. We discuss effects of the moduli F-term condensation and D-term contribution to soft scalar masses. © 1998 Elsevier Science B.V. - Tatsuo Kobayashi, Jisuke Kubo, George Zoupanos Phys.Lett.B427:291-299,1998 1998年02月07日 [査読無し][通常論文]

It is proven that the recently found, renormalization-group invariant sum

rule for the soft scalar masses in softly-broken N=1 supersymmetric

gauge-Yukawa unified theories can be extended to all orders in perturbation

theory. In the case of finite unified theories, the sum rule ensures the

all-loop finiteness in the soft supersymmetry breaking sector. As a byproduct

the exact $\beta$ function for the soft scalar masses in the

Novikov-Shifman-Vainstein-Zakharov (NSVZ) scheme for softly-broken

supersymmetric QCD is obtained. It is also found that the singularity appearing

in the sum rule in the NSVZ scheme exactly coincides with that which has been

previously found in a certain class of superstring models in which the massive

string states are organized into $N = 4$ supermultiplets. - T Kobayashi, J Kubo, M Mondragon, G Zoupanos NUCLEAR PHYSICS B 511 (1-2) 45 -68 1998年02月 [査読無し][通常論文]

Requiring the soft supersymmetry-breaking (SSB) parameters in finite gauge-Yukawa unified models to be finite up to and including two-loop order, we derive a two-loop sum rule for the soft scalar masses, It is shown that this sum rule coincides with that of a certain class of string models in which the massive string states are organized into N = 4 supermultiplets. We investigate the SSB sector of two finite SU(5) models. Using the sum rule which allows non-universality of the SSB terms and requiring that the lightest superparticle is neutral, we constrain the parameter space of the SSB sector in each model. (C) 1998 Elsevier Science B.V. - Phys. Lett. 442 (1/4) 192 -198 1998年 [査読無し][通常論文]
- Int. J. Mod. Phys. 13 (13) 2201 -2215 1998年 [査読無し][通常論文]
- N Aizawa, T Kobayashi, HT Sato INTERNATIONAL JOURNAL OF MODERN PHYSICS A 12 (32) 5867 -5883 1997年12月 [査読無し][通常論文]

Based on the quantum superspace construction of q-deformed algebra, we discuss a supersymmetric extension of the deformed Virasoro algebra, which is a subset of the q-W-infinity algebra which recently appeared in the context of two-dimensional string theory. We analyze two types of deformed super Virasoro algebra as well as their osp(1,2) subalgebras. Applying our quantum superspace structure to conformal field theory, we find the same type of deformation of affine sl(2) algebra. - 久保治輔, 小林達夫, 島袋均 日本物理学会北陸支部・応用物理学会北陸・信越支部合同講演会講演予稿集 1997 146 1997年12月 [査読無し][通常論文]
- Yoshiharu Kawamura, Tatsuo Kobayashi, Jisuke Kubo Phys.Lett. B432 (1998) 108-113 1997年10月24日 [査読無し][通常論文]

Assuming that the string inspired, universal sum rules for soft

supersymmetry-breaking terms, which have been recently found both in a wide

class of four-dimensional superstrings and in supersymmertic gauge-Yukawa

unified gauge models, are satisfied above and at the grand unification scale,

we investigate theirlow energy consequences and derive sum rules in the

superpartner spectrum of the minimal supersymmetric standard model. - Yoshiharu Kawamura, Shaaban Khalil, Tatsuo Kobayashi Nuclear Physics B 502 (1-2) 37 -58 1997年09月29日 [査読無し][通常論文]

We study moduli-dominated SUSY breaking within the framework of string models. This type of SUSY breaking in general leads to non-universal soft masses, i.e. soft scalar masses and gaugino masses. Further gauginos are lighter than sfermions. This non-universality has phenomenologically important implications. We investigate radiative electroweak symmetry breaking in the mass spectrum derived from moduli-dominated SUSY breaking, where the lightest chargino and neutralino are almost gauginos. Moreover, constraints from the branching ratio of b → sγ and the relic abundance of the LSP are also considered. The mass spectrum of moduli-dominated SUSY breaking is favorable to the experimental bound of the b → sγ decay decreasing its branching ratio. We obtain an upper bound for the gravitino mass from the cosmological constraint. © 1997 Elsevier Science B.V. - Y Kawamura, S Khalil, T Kobayashi NUCLEAR PHYSICS B 502 (1-2) 37 -58 1997年09月 [査読無し][通常論文]

We study moduli-dominated SUSY breaking within the framework of string models. This type of SUSY breaking in general leads to non-universal soft masses, i.e. soft scalar masses and gaugino masses. Further gauginos are Lighter than sfermions. This non-universality has phenomenologically important implications. We investigate radiative electroweak symmetry breaking in the mass spectrum derived from moduli-dominated SUSY breaking, where the lightest chargino and neutralino are almost gauginos. Moreover, constraints from the branching ratio of b --> s gamma and the relic abundance of the LSP are also considered. The mass spectrum of moduli-dominated SUSY breaking is favorable to the experimental bound of the b --> s gamma decay decreasing its branching ratio. We obtain an upper bound for the gravitino mass from the cosmological constraint. (C) 1997 Elsevier Science B.V. - Tatsuo Kobayashi, Hiroaki Nakano Nuclear Physics B 496 (1-2) 103 -131 1997年07月07日 [査読無し][通常論文]

"Anomalous" U(1) gauge symmetry with a Green-Schwarz anomaly cancellation mechanism is discussed in the orbifold construction of four-dimensional heterotic string models. Some conditions are given as criteria to have "anomalous" U(1) in orbifold string models. In particular, "anomalous" U(1) is absent if the massless twisted matter has no mixing between visible and hidden sectors or if a certain type of discrete symmetries are found. We then give a general procedure for classifying orbifold models with "anomalous" U(1) and for identifying the "anomalous" U(1) basis. We illustrate our procedure in Z3 and Z4 orbifold models. According to our procedure, the classification of "anomalous" U(1) can be reduced to classification in the absence of a Wilson line. We also discuss discrete symmetries left unbroken after the "anomalous" U(1) breaking. This includes a possible relation between "anomalous" U(1) and discrete R symmetries. © 1997 Elsevier Science B.V. - Y Kawamura, T Kobayashi, J Kubo PHYSICS LETTERS B 405 (1-2) 64 -70 1997年07月 [査読無し][通常論文]

It is shown that a certain sum rule for soft supersymmetry-breaking scalar masses, which has been recently found in a certain class of superstring models, is universal for gauge-Yukawa unified models. To explain this coincidence, we argue that the low-energy remnant of the target-space duality invariance in the effective supergravity of compactified superstrings can be identified with the (broken) scale invariance in gauge-Yukawa unified models, and that gauge-Yukawa unification which is indispensable for the sum rule to be satisfied follows from the matching of anomalies. (C) 1997 Published by Elsevier Science B.V. - Naruhiko Aizawa, Tatsuo Kobayashi, Haru-Tada Sato Int.J.Mod.Phys. A12 (1997) 5867-5883 1997年06月25日 [査読無し][通常論文]

Based on the quantum superspace construction of $q$-deformed algebra, we

discuss a supersymmetric extension of the deformed Virasoro algebra, which is a

subset of the $q$-$W_{\infty}$ algebra recently appeared in the context of

two-dimensional string theory. We analyze two types of deformed super-Virasoro

algebra as well as their $osp(1,2)$ subalgebras. Applying our quantum

superspace structure to conformal field theory, we find the same type of

deformation of affine $sl(2)$ algebra. - 川村嘉春, 小林達夫 素粒子論研究 95 (1) A60-A61 -A61 1997年04月 [査読無し][通常論文]
- Yoshiharu Kawamura, Tatsuo Kobayashi, Jisuke Kubo Phys.Lett. B405 (1997) 64-70 1997年03月14日 [査読無し][通常論文]

It is shown that a certain sum rule for soft supersymmetry-breaking scalar

masses, which has been recently found in a certain class of superstring models,

is universal for gauge-Yukawa unified models. To explain this coincidence, we

argue that the low-energy remnant of the target-space duality invariance in the

effective supergravity of compactified superstrings can be identified with the

(broken) scale invariance in gauge-Yukawa unified models, and that gauge-Yukawa

unification which is indispensable for the sum rule to be satisfied follows

from the matching of anomalies. - T Kobayashi, ZZ Xing MODERN PHYSICS LETTERS A 12 (8) 561 -571 1997年03月 [査読無し][通常論文]

We propose a simple but realistic pattern of quark mass matrices at the string scale, which can be derived from orbifold models of superstring theory with no use of gauge symmetries. This pattern is left-right symmetric and preserves the structural parallelism between up and down quark sectors. Its phenomenological consequences on flavor mixing and CP-violation are confronted with current experiments at the weak scale by using the renormalization group equations in the framework of minimal supersymmetric standard model. We find that good agreement is achievable without fine-tuning. - Nucl. Phys. B496 103 -131 1997年 [査読無し][通常論文]
**"Anomalous U(1) Symmetry in Orbifold String Models"(jointly worked)** - Tatsuo Kobayashi, Zhi-zhong Xing 1996年09月26日We propose a simple but realistic pattern of quark mass matrices at the string scale, which can be derived from orbifold models of superstring theory with no use of gauge symmetries. This pattern is left-right symmetric and preserves the structural parallelism between up and down quark sectors. Its phenomenological consequences on flavor mixing and $CP$ violation are confronted with current experiments at the weak scale by use of the renormalization-group equations in the framework of the minimal supersymmetric standard model. We find that good agreement is achievable without fine-tuning.
- Tatsuo Kobayashi, Zhi-zhong Xing Mod.Phys.Lett. A12 (1997) 561-572 1996年09月26日 [査読無し][通常論文]

We propose a simple but realistic pattern of quark mass matrices at the

string scale, which can be derived from orbifold models of superstring theory

with no use of gauge symmetries. This pattern is left-right symmetric and

preserves the structural parallelism between up and down quark sectors. Its

phenomenological consequences on flavor mixing and $CP$ violation are

confronted with current experiments at the weak scale by use of the

renormalization-group equations in the framework of the minimal supersymmetric

standard model. We find that good agreement is achievable without fine-tuning. - Yoshiharu Kawamura, Tatsuo Kobayashi, Tohru Komatsu 1996年09月25日We study flat directions and soft scalar masses using a $Z_3$ orbifold model with $SU(3) \times SU(2) \times U(1)$ gauge group and extra gauge symmetries including an anomalous $U(1)$ symmetry. Soft scalar masses contain $D$-term contributions and particle mixing effects after symmetry breaking and they are parametrized by a few number of parameters. Some specific relations among scalar masses are obtained.
- Yoshiharu Kawamura, Tatsuo Kobayashi, Tohru Komatsu Phys.Lett. B400 (1997) 284-294 400 (3/4) 284 -294 1996年09月25日 [査読無し][通常論文]

We study flat directions and soft scalar masses using a $Z_3$ orbifold model

with $SU(3) \times SU(2) \times U(1)$ gauge group and extra gauge symmetries

including an anomalous $U(1)$ symmetry. Soft scalar masses contain $D$-term

contributions and particle mixing effects after symmetry breaking and they are

parametrized by a few number of parameters. Some specific relations among

scalar masses are obtained. - Yoshiharu Kawamura, Tatsuo Kobayashi 1996年08月06日We derive formula of soft supersymmetry breaking scalar masses from 4-dimensional string models within a more generic framework. We consider effects of extra gauge symmetry breakings including an anomalous $U(1)$ breaking through flat directions, that is, $D$-term and $F$-term contributions, particle mixing effects and heavy-light mass mixing effects. Some phenomenological implications are discussed based on our mass formula.
- Yoshiharu Kawamura, Tatsuo Kobayashi Phys.Rev. D56 (1997) 3844-3859 56 (7) 3844 -3859 1996年08月06日 [査読無し][通常論文]

We derive formula of soft supersymmetry breaking scalar masses from

4-dimensional string models within a more generic framework. We consider

effects of extra gauge symmetry breakings including an anomalous $U(1)$

breaking through flat directions, that is, $D$-term and $F$-term contributions,

particle mixing effects and heavy-light mass mixing effects. Some

phenomenological implications are discussed based on our mass formula. - T Kobayashi, Y Yamagishi PHYSICS LETTERS B 381 (1-3) 169 -176 1996年07月 [査読無し][通常論文]

We study effects of SUSY particle decouplings on a quasi fixed point (QFP) of Yukawa coupling. From renormalization group analysis it is shown that if the SUSY breaking scale M(S) is large (greater than or similar to 1 TeV), effects of decoupling of Higgsinos and squarks raise the top Yukawa QFP. Tnis tendency is enhanced in most cases of non-universal SUSY breaking. For the case of M(S) less than or similar to 1 TeV, the decoupling of gluinos lowers m(t)(QFP). We checked some parameter dependencies for the top Yukawa QFP. The bottom-top Yukawa unified case is also studied. When top quark mass is measured more precisely, some patterns of soft mass spectra could be excluded if a rather large initial top Yukawa coupling is realized by the underlying theory. - Yoshiharu Kawamura, Tatsuo Kobayashi Nucl.Phys. B481 (1996) 539-576 1996年07月01日 [査読無し][通常論文]

We study generic features related to matter contents and flat directions in

$Z_{2n}$ orbifold models. It is shown that $Z_{2n}$ orbifold models have

massless conjugate pairs, $R$ and $\overline R$, in certain twisted sectors as

well as one of untwisted subsectors. Using these twisted sectors, $Z_{2n}$

orbifold models are classified into two types. Conjugate pairs, $R$ and

$\overline R$, lead to $D$-flatness as $\langle R \rangle = \langle \overline R

\rangle \neq 0$. We investigate generic superpotentials derived from orbifold

models so as to show that this direction is indeed a flat direction. - Y Kawamura, T Kobayashi PHYSICS LETTERS B 375 (1-4) 141 -148 1996年05月 [査読無し][通常論文]

We obtain the low-energy effective theory from string models with anomalous U(1) symmetry. The feature of soft supersymmetry breaking scalar masses and some phenomenological implications are discussed. We show that it is, in general, difficult to keep the degeneracy and the positivity of squared soft scalar masses at the Planck scale. - R Altendorfer, T Kobayashi INTERNATIONAL JOURNAL OF MODERN PHYSICS A 11 (5) 903 -920 1996年02月 [査読無し][通常論文]

We study the gauge coupling unification of the minimal supersymmetric standard model with nonuniversal soft scalar and gaugino masses. The unification scale of the gauge couplings is estimated for nonuniversal cases. It is sensitive to the nonuniversality. It turns out that these cases can be combined with the assumption of string unification, which leads to a prediction of sin(2) theta(W)(M(Z)) and k(1), the normalization of the U(1)(Y) generator. String unification predicts that k(1) = 1.3-1.4. These values have nontrivial implications for string model building. Two-loop corrections are also calculated. Some of these cases exhibit a large discrepancy between experiment and string unification. We calculate string threshold corrections to explain the discrepancy. - Tatsuo Kobayashi, Yoshio Yamagishi Phys.Lett. B381 (1996) 169-176 1996年01月31日 [査読無し][通常論文]

We study effects of SUSY particle decouplings on a quasi fixed point (QFP) of

Yukawa coupling. From renormalization group analysis it is shown that if the

SUSY breaking scale $M_S$ is large ($\simg 1$TeV), effects of decoupling of

Higgsinos and squarks raise the top Yukawa QFP. This tendency is enhanced in

most cases of non-universal SUSY breaking. For the case of $M_S\siml 1$TeV, the

decoupling of gluinos lowers $m_t^{\rm QFP}$. We checked some parameter

dependencies for the top Yukawa QFP. The bottom-top Yukawa unified case is also

studied. When top quark mass is measured more precisely, some patterns of soft

mass spectra could be excluded if rather large initial top Yukawa coupling is

realized by underlying theory. - Yoshiharu Kawamura, Tatsuo Kobayashi 1996年01月30日We obtain the low-energy effective theory from string models with anomalous $U(1)$ symmetry. The feature of soft supersymmetry breaking scalar masses and some phenomenological implications are discussed. We show that it is, in general, difficult to keep the degeneracy and the positivity of squared soft scalar masses at the Planck scale.
- Yoshiharu Kawamura, Tatsuo Kobayashi Phys.Lett.B375:141-148,1996; ERRATUM-ibid.B388:867,1996 1996年01月30日 [査読無し][通常論文]

We obtain the low-energy effective theory from string models with anomalous

$U(1)$ symmetry. The feature of soft supersymmetry breaking scalar masses and

some phenomenological implications are discussed. We show that it is, in

general, difficult to keep the degeneracy and the positivity of squared soft

scalar masses at the Planck scale. - Nucl. Phys. B481 539 -576 1996年 [査読無し][通常論文]
- Tatsuo Kobayashi, Zhi-Zhong Xing Modern Physics Letters A 11 (31) 2531 -2537 1996年 [査読無し][通常論文]

We study the Kielanowski parametrization of the Kobayashi-Maskawa (KM) matrix V. A new two-angle parametrization is investigated explicitly and compared with the Kielanowski ansatz. Both of them are symmetric matrices and lead to |V13/V23| = 0.129. Necessary corrections to the off-diagonal symmetry of V are also discussed. - Mod. Lett. 11 (7) 535 -543 1996年 [査読無し][通常論文]
- 小林達夫, 山岸芳夫 日本物理学会北陸支部・応用物理学会北陸・信越支部合同講演会講演予稿集 1995 18 1995年12月 [査読無し][通常論文]
- 小林 達夫, 紺村 昌彦, 末松 大二郎, 山田 聖典, 山岸 芳夫 素粒子論研究 92 (1) A46 1995年10月20日 [査読無し][通常論文]
- 小林 達夫, 山岸 芳夫 日本物理学会講演概要集. 秋の分科会 1995 (1) 14 -14 1995年09月12日 [査読無し][通常論文]
- T KOBAYASHI, M KONMURA, D SUEMATSU, K YAMADA, Y YAMAGISHI PROGRESS OF THEORETICAL PHYSICS 94 (3) 417 -433 1995年09月 [査読無し][通常論文]

The electric dipole moment of the neutron (EDMN) is re-examined in the framework of non-universal soft supersymmetry breaking. We review some features of the relation between the EDMN and non-universal soft supersymmetry breaking terms. It is shown that the constraints on the soft scalar masses and soft CP phases have a rather strong dependence on the non-universality of soft breaking terms. We also show that the soft CP phase phi(B) which has no natural suppression mechanism may not contribute significantly to the EDMN in a certain parameter region where the radiative symmetry breaking occurs successfully. If this is the case, phi(B) may not need to be so small. - 小林達夫, 山岸芳夫 日本物理学会講演概要集(分科会) 1995 (Autumn Pt 1) 14 -14 1995年09月 [査読無し][通常論文]
- T KOBAYASHI, D SUEMATSU, K YAMADA, Y YAMAGISHI PHYSICS LETTERS B 348 (3-4) 402 -410 1995年04月 [査読無し][通常論文]

We study soft scalar masses in comparison with gaugino masses in 4-dimensional string models. In general non-universal soft masses are derived in orbifold models. We give conditions on modular weights which lead to a large non-degeneracy in the soft scalar mass spectrum. This non-universality is applied to the unification of gauge coupling constants in the minimal string model. - 小林 達夫, 末松 大二郎, 山田 聖典, 山岸 芳夫 日本物理学会講演概要集. 年会 50 (1) 17 -17 1995年03月16日 [査読無し][通常論文]
- 小林達夫, 末松大二郎, 山田聖典, 山岸芳夫 日本物理学会講演概要集(年会) 50th (1) 17 1995年03月 [査読無し][通常論文]
- Phys. Lett. 354 (3/4) 264 -270 1995年 [査読無し][通常論文]
- Int. J. Mod. Phys. 10 (10) 1393 -1411 1995年 [査読無し][通常論文]
- H KAWABE, T KOBAYASHI, N OHTSUBO NUCLEAR PHYSICS B 434 (1-2) 210 -230 1995年01月 [査読無し][通常論文]

We examine whether a minimal string model possessing the same massless spectra as the MSSM can be obtained from Z(4), Z(6) and Z(8) orbifold constructions. Using an anomaly cancellation condition of the target space duality symmetry, we derive allowable values of a level k(1) of U(1)(Y) for the minimal string model on the orbifolds through computer analyses. We investigate threshold corrections of the gauge coupling constants of SU(3), SU(2) and U(1)Y and examine consistencies of the model with the LEP experiments. It is found that Z(4) and Z(8)-II cannot derive the minimal string model but Z(6)-I, Z(6)-II and Z(8)-I are possible to derive it with k(1) = 29/21, 1 less than or equal to k(1) less than or equal to 32/21 and 1 less than or equal to k(1) less than or equal to 41/21, respectively, We obtain explicitly allowed combinations of modular weights. The minimum values of the moduli on unrotated planes are estimated within the ranges of the levels. Further we investigate what kinds of hidden sectors are consistent with the minimal string models. Also their gauge coupling constants of the hidden groups are estimated, We discuss Yukawa couplings of the models. - 小林 達夫 素粒子論研究 90 (3) C81 -C85 1994年12月20日 [査読無し][通常論文]
- 小林達夫, 紺村昌彦, 末松大二郎, 山田聖典, 山岸芳夫 日本物理学会北陸支部・応用物理学会北陸・信越支部合同講演会講演予稿集 1994 11 1994年11月 [査読無し][通常論文]
- 小林 達夫 素粒子論研究 89 (4) D92 -D94 1994年07月20日 [査読無し][通常論文]
- T KOBAYASHI, D SUEMATSU, Y YAMAGISHI PHYSICS LETTERS B 329 (1) 27 -32 1994年06月 [査読無し][通常論文]

Gauge coupling unification is studied in the MSSM with non-universal soft supersymmetry breaking terms. If gaugino masses are sufficiently smaller than scalar soft masses and the scalar soft masses have also certain types of non-universality, the gauge coupling unification scale can be larger than 3 x 10(16) GeV even within the MSSM contents. String unification may not need a large threshold correction or a large modulus value. We also discuss the relation to the string model building. - T KOBAYASHI PHYSICS LETTERS B 326 (3-4) 231 -237 1994年05月 [査読無し][通常論文]

We study the minimal supersymmetric standard model derived from the Z8 orbifold models and its hidden sectors. We use a target-space duality anomaly cancellation so as to investigate hidden sectors consistent with the MSSM unification. For the allowed hidden sectors, we estimate running gauge coupling constants making use of threshold connections due to higher massive modes. The calculation is important from the viewpoint of a gaugino condensation, which is one of the most promising mechanisms to break the supersymmetry. - H KAWABE, T KOBAYASHI, N OHTSUBO PHYSICS LETTERS B 325 (1-2) 77 -84 1994年04月 [査読無し][通常論文]

We study a minimal string model possessing the same massless spectra as the MSSM on Z(N) x Z(M) orbifolds. Threshold corrections of the gauge coupling constants of SU(3), SU(2) and U(1)Y are investigated in a case of an overall modulus. Using computer analyses, we search ranges of levels of U(1)Y allowed by the LEP experiments. It is found that Z3 x Z3 can not derive the minimal string model for a M(Z) SUSY breaking scale. The minimum values of the overall moduli are estimated within the ranges of the levels. - Phys. Lett. 322 (4) 331 -339 1994年 [査読無し][通常論文]
- T KOBAYASHI, N OHTSUBO INTERNATIONAL JOURNAL OF MODERN PHYSICS A 9 (1) 87 -125 1994年01月 [査読無し][通常論文]

We study the geometrical and phenomenological aspects of Z(N) orbifold models. Conditions on-the background moduli and Wilson lines are clarified. We investigate the explicit Z(N)-invariant states through a modified GSO projection and calculate Yukawa coupling conditions due to space-group and SO(10) invariance. We estimate a contribution of a holomorphic instanton to the couplings. - Tatsuo Kobayashi Mod.Phys.Lett.A8:3741-3748,1993 8 (39) 3741 -3748 1993年08月10日 [査読無し][通常論文]

We study differential calculus on h-deformed bosonic and fermionic quantum

space. It is shown that the fermionic quantum space involves a parafermionic

variable as well as a classical fermionic one. Further we construct the

classical $su(2)$ algebra on the fermionic quantum space and discuss a mapping

between the classical $su(2)$ and the h-deformed $su(2)$ algebras. - T KOBAYASHI, N OHTSUBO, K TANIOKA INTERNATIONAL JOURNAL OF MODERN PHYSICS A 8 (20) 3553 -3563 1993年08月 [査読無し][通常論文]

We construct a ''shifted'' Z4 x Z4 orbifold model equivalent to a twisted Z4 X Z4 orbifold model at an enhancement point. It is proved that the N = 2 algebra of the 2(6) model is coincident with that of the ''shifted'' orbifold model. The correspondence between the Z4 x Z4 orbifold model and the 2(6) model is investigated with respect to massless spectra and Yukawa couplings. Also studied are Yukawa couplings and some other aspects of the models away from the enhancement point. - Tatsuo Kobayashi Zeitschrift für Physik C Particles and Fields 60 (1) 101 -109 1993年03月 [査読無し][通常論文]

We study a deformed su(m/n) algebra on a quantum superspace. Some interesting aspects of the deformed algebra are shown. As an application of the deformed algebra we construct a deformed superconformal algebra. From the deformed su(1/4) algebra, we derive deformed Lorentz, translation of Minkowski space, iso(2, 2) and its supersymmetric algebras as closed subalgebras with consistent automorphisms. © 1993 Springer-Verlag. - Tatsuo Kobayashi, Tsuneo Uematsu Phys.Lett. B306 (1993) 27-33 1993年02月15日 [査読無し][通常論文]

A quantum deformation of 4-dimensional superconformal algebra realized on

quantum superspace is investigated. We study the differential calculus and the

action of the quantum generators corresponding to $sl_q(1|4)$ which act on the

quantum superspace. We derive deformed $su(1|2,2)$ algebras from the deformed

$sl(1|4)$ algebra. Through a contraction procedure we obtain a deformed

super-Poincar{\'e} algebra. - Phys. Lett. 306 (1/2) 27 -33 1993年 [査読無し][通常論文]
- Z. Phys. 58 (4) 559 -565 1993年 [査読無し][通常論文]
- Tatsuo Kobayashi, Tsuneo Uematsu Z.Phys. C58 (1993) 559-566 1992年10月30日 [査読無し][通常論文]

We investigate quantum deformation of conformal algebras by constructing the

quantum space for $sl_q(4,C)$. The differential calculus on the quantum space

and the action of the quantum generators are studied. We derive deformed

$su(4)$ and $su(2,2)$ algebras from the deformed $sl(4)$ algebra using the

quantum 4-spinor and its conjugate spinor. The 6-vector in $so_q(4,2)$ is

constructed as a tensor product of two sets of 4-spinors. The reality condition

for the 6-vector and that for the generators are found. The q-deformed

Poincar{\'e} algebra is extracted as a closed subalgebra. - 小林 達夫 秋の分科会講演予稿集 1992 (1) 2 -2 1992年09月14日 [査読無し][通常論文]
- 小林 達夫 素粒子論研究 85 (4) D3 -D5 1992年07月20日 [査読無し][通常論文]
- Tatsuo Kobayashi, Tsuneo Uematsu Zeitschrift für Physik C Particles and Fields 56 (2) 193 -199 1992年06月 [査読無し][通常論文]

We investigate non-commutative differential calculus on the supersymmetric version of quantum space in which quantum supergroups are realized. Multiparametric quantum deformation of the general linear super-group, GLq(m|n), is studied and the explicit form for the {Mathematical expression} is presented. We apply these results to the quantum phase-space construction of OSpq(2n|2m) and calculate their {Mathematical expression}. © 1992 Springer-Verlag. - Prog. Theor. Phys 88 (2) 431 -442 1992年 [査読無し][通常論文]
- Suppl. Prog. Theor. Phys. 110 277 -290 1992年 [査読無し][通常論文]
**"Correspondence of Z**_{4}×Z_{4}Orbifold Model and 2^{6}Model" - T KOBAYASHI, N OHTSUBO PHYSICS LETTERS B 262 (4) 425 -431 1991年06月 [査読無し][通常論文]

SO(10) momenta and fixed points are studied for obtaining non-vanishing Yukawa couplings of Z(N) x Z(M) orbifold models. As an example, we investigate explicity three-point couplings of Z4 x Z4 and Z3 x Z3 orbifolds with the standard embedding and no discrete torsion. Comparing with the 2(6) and 1(9) model by Gepner's coset construction, respectively, we find discrepancies with respect to the numbers of (27) (27) (27) couplings, while the numbers of 27 are identical. - T KOBAYASHI, N OHTSUBO PHYSICS LETTERS B 257 (1-2) 56 -62 1991年03月 [査読無し][通常論文]

The Wilson lines of Z(N) orbifold models are studied. From analyses on the six-dimensional orbifolds, we obtain some features and restrictions on the Wilson lines, which are not found in the prime order orbifolds. Following the generalized GSO projection, the Z(N) invariant physical states are constructed. It is shown how to obtain all possible Z(N) orbifold models with Wilson lines. - Y KATSUKI, Y KAWAMURA, T KOBAYASHI, N OHTSUBO, Y ONO, K TANIOKA NUCLEAR PHYSICS B 341 (3) 611 -640 1990年09月 [査読無し][通常論文]
- Phys. Lett. B245 441 -446 1990年 [査読無し][通常論文]
**"Yukawa Coupling Condition of Z**_{N}Orbifold Models", (jointly worked) - Y KATSUKI, Y KAWAMURA, T KOBAYASHI, N OHTSUBO, Y ONO, K TANIOKA PHYSICS LETTERS B 227 (3-4) 381 -386 1989年08月 [査読無し][通常論文]
- Y KATSUKI, Y KAWAMURA, T KOBAYASHI, N OHTSUBO, K TANIOKA PROGRESS OF THEORETICAL PHYSICS 82 (1) 171 -182 1989年07月 [査読無し][通常論文]
- Y KATSUKI, Y KAWAMURA, T KOBAYASHI, Y ONO, K TANIOKA, N OHTSUBO PHYSICS LETTERS B 218 (2) 169 -175 1989年02月 [査読無し][通常論文]
- Y KATSUKI, Y KAWAMURA, T KOBAYASHI, N OHTSUBO PHYSICS LETTERS B 212 (3) 339 -342 1988年09月 [査読無し][通常論文]
- Phys. Rev. D [査読無し][通常論文]
**Localication of Heterotic Anomalies on Various Hyper Surfaces of T^6/Z_4**

- 文部科学省：科学研究費補助金(基盤研究(C))研究期間 : 2008年 -2011年代表者 : 小林 達夫超弦理論から導かれる4次元低エネルギー有効理論の研究を行った。超弦理論から標準模型を導出するのに有望と思われるオービファルド上のヘテロ型弦理論と背景磁場をもつブレーン模型を出発点にその4次元低エネルギー有効理論の解析を行った。特に、その有効理論に現れる離散対称性を調べ、特定の非可換離散フレーバー対称性が現れることを示した。そして離散対称性のアノマリーに関しても普遍的な関係式があることを示した。さらに、弦理論の振幅の計算を行い、場の理論の対称性の言葉では簡単に理解することができない弦理論特有の結合の選択則があることをしめした。
- 文部科学省：科学研究費補助金(基盤研究(C))研究期間 : 2005年 -2007年代表者 : 小林 達夫この研究の目的は、超弦理論のどのようなフレーバー構造が導出できるかを研究し、そのフレーバー構造に関する現象論的な性質を明らかにすることであった。現実的な湯川行列を導くために場の理論の模型として、離散的なnon-Abelianフレーバー対称性が最近注目されている。その起源については4次元の場の理論の立場からは、明らかではない。平成18年度に我々は、ヘテロ型オービフォルド模型において、コンパクト空間の幾何学的構造がこの離散的なnon-Abelianフレーバー対称性の起源になる可能性を示し、この4次元弦模型から導くことが可能な離散的なnon-Abelianフレーバー対称性を分類した。その典型的な対称性は、D4,Δ(54)などである。平成19年度では、D4フレーバー対称性のもとで、どのようなクォーク、レプトンの現実的な質量行列が導出可能なのかを研究した。更に、超対称性理論の枠組みでは、フレーバー対称性はクォーク、レプトンの質量行列を支配するのみでなく、そのスーパーパートナーであるスクォーク、スレプトンの質量行列(超対称性の破れの項)をも制限し、将来の加速器で検証できる可能性のある予言を与える。そのようなスーパーパートナーの質量行列の特徴的なパターンについて解析を行った。その他、湯川結合を含む高次の結合項のモジュライ依存性の計算の研究やモジュライの値の決定機構、およびそれにともなう超対称性の破れとその現象論的な効果についての研究を行った。
- 文部科学省：科学研究費補助金(特定領域研究)研究期間 : 2004年 -2005年代表者 : 小林 達夫今年度は、超弦理論から導かれるフレーバー構造についての研究を行いました。超弦理論は、重力相互作用を含んだ統一理論の有力な候補です。そしてその弦理論は、我々の4次元時空以外に6次元コンパクト空間の存在を予言します。このコンパクト空間の幾何学的性質がフェルミオンのフレーバー構造を決定しています。しかし、これまではどのようなフレーバー構造が弦模型から導けるのか、どのようなパターンの湯川行列が導けるかといったことの系統的な解析はなされていませんでした。今年度は、heterotic Z6 orbifold models, intersecting D-brane modelsにおいて、そのような系統的な解析の第一歩を踏み出しました。前者についは現実的なquark/lepton質量行列が得られる可能性があるものの、後者の単純な模型では現実的な質量行列を導くことが難しいことが分かりました。さらに、上述のような弦模型の枠組みでは、湯川結合の強さ自体は、コンパクト空間の大きさや形を決めるようなモジュライ場の真空期待値の関数として与えられます。そしてその値が適当な値になったときに、現実的な質量行列が得られるわけです。したがって、モジュライの真空期待値の決定機構は、非常に重要です。このような目的で今年度はモジュライの真空期待値の様々な決定機構について研究しました。さらに、その際現れる超対称性の破れについても研究をしました。
- 文部科学省：科学研究費補助金(基盤研究(C))研究期間 : 2002年 -2003年代表者 : 小林 達夫Quark, Leptonの質量や混合角の起源を探ることは、素粒子論の中で重要な課題の1つです。また、超対称模型の枠組みでは、現実的な湯川行列を生み出す機構は、超対称性の破れの項への影響を考えながら議論すべきです。更に、超対称性の破れの項については、フレーバー間の縮退度に関して大きな実験的制限があります。上述のようなことを踏まえ、縮退したsfermionの質量を導きつつ湯川結合の階層構造を与えるような模型を研究しました。まず、超対称標準模型に加えて超共形固定点をもつようなSCセクターのある模型を研究しました。この超共形ダイナミックスがquark, leptonに大きなanomalous dimensionを生成するので、この模型では湯川結合の階層構造を出しつつ、縮退したsfermion質量を導くことが可能です。また、似たような結果を導く別の模型としては、余剰次元がwarp背景幾何上の模型を研究しました。この模型では、湯川結合の階層構造の生成に役立つのは、5次元bulk massによる、bulk場の局在化です。更に、ラディオンにより超対称性の破れが起る場合には、フレーバー問題も改善されることが分かりました。また世代間にS_3離散対称性を課すような模型を調べました。このような対称性はこれまで提案されてきましたが、我々の模型の新しい点はHiggsセクターにもこの対称性を課せるようにHiggsセクターを拡張した点で、そのため、超対称性の破れの項もこのような対称性をもつと仮定でき、FCNCへの効果を実験の制限程度に下げることに役立ちます。更に、弦理論の枠内で湯川結合の計算をしました。特に、興味があるのはorbifold模型で、なぜなら階層的な湯川結合が導けることが知られているからです。これまで代表的なmoduliへの依存性は分かっていましたが、今回我々は様々なmoduliへの依存性を計算しました。我々の結果の中で、現象論的に重要なことの1つは、湯川結合での物理的なCPの破れに関しては、ある特定のmoduliしか関与しないことを示した点です。
**超対称性の破れと超弦理論の現象論的性質****Supersymmetry Breaking and Superstring Phenomenology**

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