Researcher Database

Tatsuo Kobayashi
Faculty of Science Physics Nonlinear Physics
Professor

Researcher Profile and Settings

Affiliation

  • Faculty of Science Physics Nonlinear Physics

Job Title

  • Professor

Degree

  • Ph. D, Master of Science(Kanazawa University)

J-Global ID

Research Interests

  • 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   

Research Areas

  • Natural sciences / Theoretical studies related to particle-, nuclear-, cosmic ray and astro-physics
  • Natural sciences / Theoretical studies related to particle-, nuclear-, cosmic ray and astro-physics

Education

  •        - 1991  Kanazawa University
  •        - 1991  Kanazawa University  Graduate School, Division of National Science and Technology
  •        - 1986  Saitama University  Faculty of Science
  •        - 1986  Saitama University  Faculty of Science

Association Memberships

  • 日本物理学会   

Research Activities

Published Papers

  • Hiroyuki Abe, Tatsuo Kobayashi, Hiroshi Ohki, Keigo Sumita, Yoshiyuki Tatsuta
    JOURNAL OF HIGH ENERGY PHYSICS 1406 (6) 1029-8479 2014/06 [Refereed][Not invited]
     
    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 Seto
    PHYSICAL REVIEW D 89 (10) 2470-0010 2014/05 [Refereed][Not invited]
     
    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 Uemura
    JOURNAL OF HIGH ENERGY PHYSICS 1405 (5) 1029-8479 2014/05 [Refereed][Not invited]
     
    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 Tatsuta
    JOURNAL OF HIGH ENERGY PHYSICS 1404 (4) 1029-8479 2014/04 [Refereed][Not invited]
     
    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 Zavala
    JOURNAL OF HIGH ENERGY PHYSICS 1402 (2) 1029-8479 2014/02 [Refereed][Not invited]
     
    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 Ookouchi
    JOURNAL OF HIGH ENERGY PHYSICS 1401 (1) 1029-8479 2014/01 [Refereed][Not invited]
     
    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.
  • Yuta Hamada, Kohei Kamada, Tatsuo Kobayashi, Yutaka Ookouchi
    JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS (1) 1475-7516 2014/01 [Refereed][Not invited]
     
    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) 1550-7998 2013/11 [Refereed][Not invited]
     
    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 0550-3213 2013/10 [Refereed][Not invited]
     
    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 Zavala
    JOURNAL OF HIGH ENERGY PHYSICS (5) 1029-8479 2013/05 [Refereed][Not invited]
     
    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 0550-3213 2013/05 [Refereed][Not invited]
     
    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) 1029-8479 2013/05 [Refereed][Not invited]
     
    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) 1550-7998 2013/04/08 [Refereed][Not invited]
     
    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 Ookouchi
    JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS (4) 1475-7516 2013/04 [Refereed][Not invited]
     
    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 Ookouchi
    JOURNAL OF HIGH ENERGY PHYSICS (3) 1029-8479 2013/03 [Refereed][Not invited]
     
    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 (1) 1029-8479 2013/01 [Refereed][Not invited]
     
    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.
  • 2013 [Refereed][Not invited]
  • Tatsuo Azeyanagi, Tatsuo Kobayashi, Atsushi Ogasahara, Koichi Yoshioka
    PHYSICAL REVIEW D 86 (9) 1550-7998 2012/11 [Refereed][Not invited]
     
    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.
  • Hiroyuki Abe, Tatsuo Kobayashi, Hiroshi Ohki, Keigo Sumita
    NUCLEAR PHYSICS B 863 (1) 1 - 18 0550-3213 2012/10 [Refereed][Not invited]
     
    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) 2470-0010 2012/07 [Refereed][Not invited]
     
    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 [Refereed][Not invited]
  • Hajime Ishimori, Tatsuo Kobayashi
    JOURNAL OF HIGH ENERGY PHYSICS (10) 1029-8479 2011/10 [Refereed][Not invited]
     
    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 0217-751X 2011/09 [Refereed][Not invited]
     
    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) 1550-7998 2011/08 [Refereed][Not invited]
     
    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 Sakai
    JOURNAL OF HIGH ENERGY PHYSICS (6) 1029-8479 2011/06 [Refereed][Not invited]
     
    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 Kobayashi
    PHYSICS LETTERS B 693 (3) 330 - 333 0370-2693 2010/10 [Refereed][Not invited]
     
    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.
  • Yoshimi Kanehata, Tatsuo Kobayashi, Yasufumi Konishi, Takashi Shimomura
    PHYSICAL REVIEW D 82 (7) 1550-7998 2010/10 [Refereed][Not invited]
     
    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.
  • 2010 [Refereed][Not invited]
  • 2010 [Refereed][Not invited]
  • Hiroyuki Abe, Kang-Sin Choi, Tatsuo Kobayashi, Hiroshi Ohki
    PHYSICAL REVIEW D 80 (12) 2470-0010 2009/12 [Refereed][Not invited]
     
    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) 1029-8479 2009/12 [Refereed][Not invited]
     
    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 814 (1-2) 265 - 292 0550-3213 2009/06 [Refereed][Not invited]
     
    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 Ohki
    JOURNAL OF HIGH ENERGY PHYSICS (6) 1029-8479 2009/06 [Refereed][Not invited]
     
    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.
  • 2009 [Refereed][Not invited]
  • Tatsuo Kobayashi, Yuji Omura, Koichi Yoshioka
    PHYSICAL REVIEW D 78 (11) 1550-7998 2008/12 [Refereed][Not invited]
     
    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. Vaudrevange
    NUCLEAR PHYSICS B 805 (1-2) 124 - 147 0550-3213 2008/12 [Refereed][Not invited]
     
    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.
  • Kang-Sin Choi, Tatsuo Kobayashi
    NUCLEAR PHYSICS B 797 (1-2) 295 - 321 0550-3213 2008/07 [Refereed][Not invited]
     
    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, Tatsuo Kobayashi, Yuji Omura
    PHYSICAL REVIEW D 77 (6) 1550-7998 2008/03 [Refereed][Not invited]
     
    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.
  • 2008 [Refereed][Not invited]
  • Hiroyuki Abe, Tetsutaro Higaki, Tatsuo Kobayashi
    PHYSICAL REVIEW D 76 (10) 2470-0010 2007/11 [Refereed][Not invited]
     
    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) 1550-7998 2007/09 [Refereed][Not invited]
     
    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.
  • 2007 [Refereed][Not invited]
  • Hiroyuki Abe, Tetsutaro Higaki, Tatsuo Kobayashi, Yuji Omura
    PHYSICAL REVIEW D 75 (2) 1550-7998 2007/01 [Refereed][Not invited]
     
    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) 1550-7998 2006/09 [Refereed][Not invited]
     
    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) 2470-0010 2006/07 [Refereed][Not invited]
     
    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 0550-3213 2006/05 [Refereed][Not invited]
     
    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 Okumura
    PHYSICS LETTERS B 633 (2-3) 355 - 361 0370-2693 2006/02 [Refereed][Not invited]
     
    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 Higaki, N Kitazawa, T Kobayashi, K Takahashi
    PHYSICAL REVIEW D 72 (8) 1550-7998 2005/10 [Refereed][Not invited]
     
    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) 2470-0010 2005/06 [Refereed][Not invited]
     
    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.
  • P Ko, T Kobayashi, JH Park
    PHYSICAL REVIEW D 71 (9) 2470-0010 2005/05 [Refereed][Not invited]
     
    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 1434-6044 2005/04 [Refereed][Not invited]
     
    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.
  • Orbifold grand uni#12;ed theories from heterotic string compacti#12;cation on a Z6 orbifold 2005 [Refereed][Not invited]
  • P Ko, T Kobayashi, JH Park
    PHYSICS LETTERS B 598 (3-4) 263 - 272 0370-2693 2004/09 [Refereed][Not invited]
     
    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 0370-2693 2004/07 [Refereed][Not invited]
     
    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 1434-6044 2004/06 [Refereed][Not invited]
     
    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) 2470-0010 2004/04 [Refereed][Not invited]
     
    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.
  • SG Nibbelink, M Hillenbach, T Kobayashi, MGA Walter
    PHYSICAL REVIEW D 69 (4) 0556-2821 2004/02 [Refereed][Not invited]
     
    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.
  • 2004 [Refereed][Not invited]
  • T Kobayashi, O Seto
    PHYSICAL REVIEW D 69 (2) 1550-7998 2004/01 [Refereed][Not invited]
     
    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 0370-2693 2003/08/21 [Refereed][Not invited]
     
    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) 2470-0010 2003/08 [Refereed][Not invited]
     
    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, O Lebedev
    PHYSICS LETTERS B 566 (1-2) 164 - 170 0370-2693 2003/07 [Refereed][Not invited]
     
    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 Lebedev
    PHYSICS LETTERS B 565 (1-4) 193 - 200 0370-2693 2003/07 [Refereed][Not invited]
     
    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.
  • H Abe, T Kobayashi, N Maru, K Yoshioka
    PHYSICAL REVIEW D 67 (4) 0556-2821 2003/02 [Refereed][Not invited]
     
    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.
  • 2003 [Refereed][Not invited]
  • T Kobayashi, H Nakano, T Noguchi, H Terao
    PHYSICAL REVIEW D 66 (9) 2470-0010 2002/11 [Refereed][Not invited]
     
    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, H Terao
    PHYSICAL REVIEW D 65 (1) 2470-0010 2002/01 [Refereed][Not invited]
     
    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.
  • 2002 [Refereed][Not invited]
  • SUSY standard models coupled with SCFTs 2001 [Refereed][Not invited]
  • 2000 [Refereed][Not invited]
  • 2000 [Refereed][Not invited]
  • 1998 [Refereed][Not invited]
  • 1998 [Refereed][Not invited]
  • 1996 [Refereed][Not invited]
  • 1995 [Refereed][Not invited]
  • 1995 [Refereed][Not invited]
  • 1992 [Refereed][Not invited]
  • 1990 [Refereed][Not invited]
  • 1989 [Refereed][Not invited]
  • 1989 [Refereed][Not invited]
  • 1989 [Refereed][Not invited]
  • 1988 [Refereed][Not invited]

Books etc

MISC

  • Junichiro Kawamura, Tatsuo Kobayashi, Natsumi Nagata  2018/08/24  [Not refereed][Not invited]
     
    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  2018/08/09  [Not refereed][Not invited]
     
    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)  2018/05/18  [Not refereed][Not invited]
     
    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, Satoshi Nagamoto, Shintaro Takada, Shio Tamba, Takuya H. Tatsuishi  Phys. Rev. D 97, 116002 (2018)  2018/04/18  [Not refereed][Not invited]
     
    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)  2018/04/01  [Not refereed][Not invited]
     
    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)  2018/03/28  [Not refereed][Not invited]
     
    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.
  • Tatsuo Kobayashi, Naoya Omoto, Hajime Otsuka, Takuya H. Tatsuishi  Phys. Rev. D 97, 106006 (2018)  2017/11/28  [Not refereed][Not invited]
     
    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.
  • Kenji Kadota, Tatsuo Kobayashi, Keigo Sumita  Journal of Cosmology and Astroparticle Physics  2017-  (11)  2017/11/21  [Not refereed][Not invited]
     
    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)  2017/11  [Not refereed][Not invited]
     
    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)  2017/11  [Not refereed][Not invited]
     
    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)  2017/09  [Not refereed][Not invited]
     
    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)  2017/08  [Not refereed][Not invited]
     
    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.
  • Tatsuo Kobayashi, Shohei Uemura, Junji Yamamoto  PHYSICAL REVIEW D  96-  (2)  2017/07  [Not refereed][Not invited]
     
    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)  2017/04  [Not refereed][Not invited]
     
    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)  2017/04  [Not refereed][Not invited]
     
    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  2017/03/29  [Not refereed][Not invited]
     
    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.
  • Hiroyuki Abe, Tatsuo Kobayashi, Keigo Sumita, Shohei Uemura  Phys. Rev. D 96, 026019 (2017)  2017/03/09  [Not refereed][Not invited]
     
    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, Akane Oikawa, Naoya Omoto, Hajime Otsuka, Ikumi Saga  PHYSICAL REVIEW D  95-  (6)  2017/03  [Not refereed][Not invited]
     
    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)  2017/01  [Not refereed][Not invited]
     
    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)  2016/12  [Not refereed][Not invited]
     
    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)  2016/10  [Not refereed][Not invited]
     
    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  2016/10  [Not refereed][Not invited]
     
    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  2016/09  [Not refereed][Not invited]
     
    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  2016/08/22  [Not refereed][Not invited]
     
    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)  2016/08  [Not refereed][Not invited]
     
    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)  2016/08  [Not refereed][Not invited]
     
    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)  2016/05  [Not refereed][Not invited]
     
    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)  2016/05  [Not refereed][Not invited]
     
    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)  2016/04  [Not refereed][Not invited]
     
    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)  2016/03  [Not refereed][Not invited]
     
    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)  2016/02  [Not refereed][Not invited]
     
    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)  2016/01  [Not refereed][Not invited]
     
    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)  2015/11  [Not refereed][Not invited]
     
    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)  2015/09  [Not refereed][Not invited]
     
    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  2015/08  [Not refereed][Not invited]
     
    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)  2015/07  [Not refereed][Not invited]
     
    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)  2015/07  [Not refereed][Not invited]
     
    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)  2015/07  [Not refereed][Not invited]
     
    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)  2015/06  [Not refereed][Not invited]
     
    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  2015/05/28  [Not refereed][Not invited]
     
    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  2015/05  [Not refereed][Not invited]
     
    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.
  • Tatsuo Kobayashi  2015/04/22  [Not refereed][Not invited]
     
    We study the next-to-minimal supersymmetric standard model with the TeV scale
    mirage mediation. The 125 GeV Higgs boson mass is realized with O(10)% tuning
    for 1.5 TeV gluino and 1TeV stop masses.
  • Hiroyuki Abe, Tatsuo Kobayashi, Hajime Otsuka  JOURNAL OF HIGH ENERGY PHYSICS  (4)  2015/04  [Not refereed][Not invited]
     
    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)  2015/03  [Not refereed][Not invited]
     
    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  2015/02  [Not refereed][Not invited]
     
    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  2015/01  [Not refereed][Not invited]
     
    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)  2014/11  [Not refereed][Not invited]
     
    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.
  • Tetsutaro Higaki, Tatsuo Kobayashi, Osamu Seto, Yuya Yamaguchi  JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS  (10)  2014/10  [Not refereed][Not invited]
     
    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)  2014/10  [Not refereed][Not invited]
     
    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  2014/09  [Not refereed][Not invited]
     
    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.
  • Hiroyuki Abe, Tatsuo Kobayashi, Hiroshi Ohki, Keigo Sumita, Yoshiyuki Tatsuta  JOURNAL OF HIGH ENERGY PHYSICS  (6)  2014/06  [Not refereed][Not invited]
     
    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, Atsushi Ogasahara, Yuji Omura, Fumihiro Takayama, Daiki Yasuhara  JHEP10(2014)183  2014/05/15  [Not refereed][Not invited]
     
    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.
  • Yuta Hamada, Tatsuo Kobayashi, Shohei Uemura  JOURNAL OF HIGH ENERGY PHYSICS  (5)  2014/05  [Not refereed][Not invited]
     
    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)  2014/05  [Not refereed][Not invited]
     
    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)  2014/04  [Not refereed][Not invited]
     
    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.
  • ABE HIROYUKI, KOBAYASHI TATSUO, OKI HIROSHI, SUMITA KEIGO, TATSUTA YOSHIYUKI  日本物理学会講演概要集  69-  (1)  13  2014/03/05  [Not refereed][Not invited]
  • ABE TOMOHIRO, FUJIMOTO NORIHIRO, KOBAYASHI TATSUO, MIURA TAKASHI, NISHIWAKI KENJI, SAKAMOTO MAKOTO  日本物理学会講演概要集  69-  (1)  12  2014/03/05  [Not refereed][Not invited]
  • 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  2014/03/05  [Not refereed][Not invited]
  • 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  2014/03/05  [Not refereed][Not invited]
  • Nana G. Cabo Bizet, Tatsuo Kobayashi, Damian K. Mayorga Pena, Susha L. Parameswaran, Matthias Schmitz, Ivonne Zavala  JOURNAL OF HIGH ENERGY PHYSICS  (2)  2014/02  [Not refereed][Not invited]
     
    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)  2014/01  [Not refereed][Not invited]
     
    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)  2014/01  [Not refereed][Not invited]
     
    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)  2014/01  [Not refereed][Not invited]
     
    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)  2013/11  [Not refereed][Not invited]
     
    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  2013/10  [Not refereed][Not invited]
     
    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  Meeting abstracts of the Physical Society of Japan  68-  (2)  2013/08/26  [Not refereed][Not invited]
  • ABE HIROYUKI, KOBAYASHI TATSUO, OKI HIROSHI, TSUNODA KEIGO, TATSUTA YOSHIYUKI  日本物理学会講演概要集  68-  (2)  1  2013/08/26  [Not refereed][Not invited]
  • 小林 達夫  数理科学  51-  (7)  51  -56  2013/07  [Not refereed][Not invited]
  • 小林達夫  数理科学  51-  (7)  51  -56  2013/07/01  [Not refereed][Not invited]
  • Nana G. Cabo Bizet, Tatsuo Kobayashi, Damian K. Mayorga Pena, Susha L. Parameswaran, Matthias Schmitz, Ivonne Zavala  JOURNAL OF HIGH ENERGY PHYSICS  (5)  2013/05  [Not refereed][Not invited]
     
    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  2013/05  [Not refereed][Not invited]
     
    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)  2013/05  [Not refereed][Not invited]
     
    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)  2013/04/08  [Not refereed][Not invited]
     
    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  2013/04/02  [Not refereed][Not invited]
     
    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.
  • Fujimoto Yukihiro, Miura Takashi, Sakamoto Makoto, Kobayashi Tatsuo, Nishiwaki Kenji  Meeting abstracts of the Physical Society of Japan  68-  (1)  2013/03/26  [Not refereed][Not invited]
  • 安倍 博之, 小林 達夫, 角田 慶吾, 龍田 佳幸  Meeting abstracts of the Physical Society of Japan  68-  (1)  2013/03/26  [Not refereed][Not invited]
  • ABE HIROYUKI, KOBAYASHI TATSUO, SUMITA KEIGO, TATSUTA YOSHIYUKI  日本物理学会講演概要集  68-  (1)  6  2013/03/05  [Not refereed][Not invited]
  • FUJIMOTO YUKIHIRO, MIURA TAKASHI, SAKAMOTO MAKOTO, KOBAYASHI TATSUO, NISHIWAKI KENJI  日本物理学会講演概要集  68-  (1)  7  2013/03/05  [Not refereed][Not invited]
  • Minoru Eto, Yuta Hamada, Kohei Kamada, Tatsuo Kobayashi, Keisuke Ohashi, Yutaka Ookouchi  JOURNAL OF HIGH ENERGY PHYSICS  (3)  2013/03  [Not refereed][Not invited]
     
    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.
  • Yuta Hamada, Kohei Kamada, Tatsuo Kobayashi, Yutaka Ookouchi  JCAP 1304 (2013) 043  2012/11/24  [Not refereed][Not invited]
     
    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)  2012/11  [Not refereed][Not invited]
     
    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  2012/11  [Not refereed][Not invited]
     
    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  2012/10  [Not refereed][Not invited]
     
    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)  2012/07  [Not refereed][Not invited]
     
    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.
  • Tatsuo Kobayashi, Susha L. Parameswaran, Saul Ramos-Sanchez, Ivonne Zavala  JOURNAL OF HIGH ENERGY PHYSICS  (5)  2012/05  [Not refereed][Not invited]
     
    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.
  • Tatsuo Kobayashi, Hiroki Makino, Ken-ichi Okumura, Takashi Shimomura, Tsubasa Takahashi  2012/04/17  [Not refereed][Not invited]
     
    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
    (\lambda/\kappa)/tan\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.
  • Abe Hiroyuki, Oikawa Akane, Oki Hiroshi, Kobayasi Tatsuo, Sumita Keigo  Meeting abstracts of the Physical Society of Japan  67-  (1)  2012/03/05  [Not refereed][Not invited]
  • Abe Hiroyuki, Oikawa Akane, Ohki Hiroshi, Kobayashi Tatsuo, Sumita Keigo, Nomoto Yuki  Meeting abstracts of the Physical Society of Japan  67-  (1)  2012/03/05  [Not refereed][Not invited]
  • ABE HIROYUKI, OIKAWA AKANE, OKI HIROSHI, KOBAYASHI TATSUO, SUMITA KEIGO  日本物理学会講演概要集  67-  (1)  9  2012/03/05  [Not refereed][Not invited]
  • 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  2012/03/05  [Not refereed][Not invited]
  • Hajime Ishimori, Tatsuo Kobayashi  2012/01/17  [Not refereed][Not invited]
     
    We study the lepton flavor models with the flavor symmetry (Z_N \times Z_N
    \times Z_N)\rtimes Z_3. Our models predict non-vanishing discrete values of
    \theta_{13} as well as \theta_{12} and \theta_{23} depending on N. For certain
    values, our models realize the tri-bimaximal mixing angles with \theta_{13}=0.
    For other values, our models provide with discrete deviation from the
    tri-bimaximal mixing angles.
  • Yoshimi Kanehata, Tatsuo Kobayashi, Yasufumi Konishi, Osamu Seto, Takashi Shimomura  Progress of Theoretical Physics  126-  (6)  1051  -1076  2011/12  [Not refereed][Not invited]
     
    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)  2011/10  [Not refereed][Not invited]
     
    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  2011/09  [Not refereed][Not invited]
     
    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)  2011/08  [Not refereed][Not invited]
     
    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.
  • Kane Gordon, 小林 達夫  Parity.  26-  (8)  22  -29  2011/08  [Not refereed][Not invited]
  • Tatsuo Azeyanagi, Tatsuo Kobayashi, Atsushi Ogasahara, Koichi Yoshioka  2011/06/15  [Not refereed][Not invited]
     
    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)  2011/06  [Not refereed][Not invited]
     
    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.
  • Tatsuo Kobayashi, Manabu Sakai  2010/12/10  [Not refereed][Not invited]
     
    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.
  • Yoshimi Kanehata, Tatsuo Kobayashi, Yasufumi Konishi, Takashi Shimomura  PHYSICAL REVIEW D  82-  (7)  2010/10  [Not refereed][Not invited]
     
    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  2010/10  [Not refereed][Not invited]
     
    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)  2010/09  [Not refereed][Not invited]
     
    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  2010/08/18  [Not refereed][Not invited]
  • Tatsuo Kobayashi, Takashi Shimomura  Phys.Rev.D82:035008,2010  2010/06/01  [Not refereed][Not invited]
     
    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  2010/05  [Not refereed][Not invited]
     
    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, Hiroshi Okada, Yusuke Shimizu, Morimitsu Tanimoto  Prog.Theor.Phys.Suppl.183:1-163,2010  2010/03/18  [Not refereed][Not invited]
     
    We review pedagogically non-Abelian discrete groups, which play an important
    role in the particle physics. We show group-theoretical aspects for many
    concrete groups, such as representations, their tensor products. We explain how
    to derive, conjugacy classes, characters, representations, and tensor products
    for these groups (with a finite number). We discussed them explicitly for
    $S_N$, $A_N$, $T'$, $D_N$, $Q_N$, $\Sigma(2N^2)$, $\Delta(3N^2)$, $T_7$,
    $\Sigma(3N^3)$ and $\Delta(6N^2)$, which have been applied for model building
    in the 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 about anomalies of non-Abelian discrete symmetries by using the path
    integral approach.
  • 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  2010/03/01  [Not refereed][Not invited]
  • Tatsuo Kobayashi, Ryosuke Maruyama, Masaki Murata, Hiroshi Ohki, Manabu Sakai  2010/02/15  [Not refereed][Not invited]
     
    We study 10D super Yang-Mills E8 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.
  • Hiroyuki Abe, Kang-Sin Choi, Tatsuo Kobayashi, Hiroshi Ohki  2010/01/12  [Not refereed][Not invited]
     
    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 $\Delta(54)$.
  • Tatsuo Kobayashi, Yuichiro Nakai, Ryo Takahashi  JOURNAL OF HIGH ENERGY PHYSICS  (1)  2010/01  [Not refereed][Not invited]
     
    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)  2009/12  [Not refereed][Not invited]
     
    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)  2009/12  [Not refereed][Not invited]
     
    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.
  • 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  2009/08/18  [Not refereed][Not invited]
  • Hiroyuki Abe, Kang-Sin Choi, Tatsuo Kobayashi, Hiroshi Ohki  JOURNAL OF HIGH ENERGY PHYSICS  (6)  2009/06  [Not refereed][Not invited]
     
    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  2009/06  [Not refereed][Not invited]
     
    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 Ohki  Nucl.Phys.B820:317-333,2009  2009/04/17  [Not refereed][Not invited]
     
    We study non-abelian discrete flavor symmetries, which can appear in
    magnetized brane models. For example, $D_4$, $\Delta(27)$ and $\Delta(54)$ can
    appear and matter fields with several representations can appear. We also study
    the orbifold background, where non-abelian flavor symmetries are broken in a
    certain way.
  • Hajime Ishimori, Tatsuo Kobayashi, Hiroshi Okada, Yusuke Shimizu, Morimitsu Tanimoto  JOURNAL OF HIGH ENERGY PHYSICS  (4)  2009/04  [Not refereed][Not invited]
     
    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.
  • ISHIMORI HAJIME, KOBAYASHI TATSUO, OKADA HIROSHI, SHIMIZU YUSUKE, TANIMOTO MORIMITSU  日本物理学会講演概要集  64-  (1)  9  2009/03/03  [Not refereed][Not invited]
  • ISHIMORI HAJIME, KOBAYASHI TATSUO, OMURA YUJI, TANIMOTO MORIMITSU  日本物理学会講演概要集  64-  (1)  18  2009/03/03  [Not refereed][Not invited]
  • Hiroyuki Abe, Tetsutaro Higaki, Tatsuo Kobayashi, Kazutoshi Ohta, Yuji Omura, Haruhiko Terao  PHYSICAL REVIEW D  79-  (4)  2009/02  [Not refereed][Not invited]
     
    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  2008/12  [Not refereed][Not invited]
     
    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)  2008/12  [Not refereed][Not invited]
     
    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)  2008/09/01  [Not refereed][Not invited]
     
    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.
  • Kobayashi Tatsuo, Okumura Ken-ichi  Butsuri  63-  (8)  603  -611  2008/08/05  [Not refereed][Not invited]
     
    Moduli play an important role in low-energy effective theory derived from superstring theory. Their vacuum expectation values provide strength of gauge and Yukawa couplings. Furthermore, their F-components contribute to supersymmetry breaking. Recently, Kachru, Kallosh, Linde and Trivedi prosoed a new type of moduli stabilization mechanism, where all of moduli are stabilized at certain values and the vacuum energy almost vanishes. Furthermore, supersymmetry breaking terms have been studied by Choi, et al. That leads to a unique pattern of soft supersymmetry breaking terms, that is called the mirage mediation. Here we review on the mirage mediation of supersymmetry breaking. In particular, we explain its implication on the fine-tunig problem in the Higgs sector of the minimal supersymmetric standard model.
  • KOBAYASHI TATSUO, OKUMURA KEN'ICHI  日本物理学会誌  63-  (8)  603  -611  2008/08/05  [Not refereed][Not invited]
  • Hajime Ishimori, Tatsuo Kobayashi, Yuji Omura, Morimitsu Tanimoto  JHEP 0812:082,2008  2008/07/29  [Not refereed][Not invited]
     
    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)  2008/07  [Not refereed][Not invited]
     
    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  2008/07  [Not refereed][Not invited]
     
    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)  2008/06  [Not refereed][Not invited]
     
    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  2008/04  [Not refereed][Not invited]
     
    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)  2008/04/01  [Not refereed][Not invited]
     
    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.
  • ISHIMORI HAJIME, KOBAYASHI TATSUO, OKI YO, OMURA YUJI, TAKAHASHI RYO, TANIMOTO MORIMITSU  日本物理学会講演概要集  63-  (1)  10  2008/02/29  [Not refereed][Not invited]
  • Hiroyuki Abe, Tatsuo Kobayashi, Yuji Omura  Phys.Rev.D77:065001,2008  2007/12/16  [Not refereed][Not invited]
     
    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)  2007/11  [Not refereed][Not invited]
     
    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)  2007/09  [Not refereed][Not invited]
     
    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)  2007/09/01  [Not refereed][Not invited]
     
    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)  2007/09  [Not refereed][Not invited]
     
    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  2007/08/23  [Not refereed][Not invited]
     
    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)  2007/07  [Not refereed][Not invited]
     
    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.
  • Tatsuo Kobayashi, Hans Peter Nilles, Felix Ploeger, Stuart Raby, Michael Ratz  NUCLEAR PHYSICS B  768-  (1-2)  135  -156  2007/04  [Not refereed][Not invited]
     
    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.
  • Takeshi Chiba, Tatsuo Kobayashi, Masahide Yamaguchi, Jun'ichi Yokoyama  PHYSICAL REVIEW D  75-  (4)  2007/02  [Not refereed][Not invited]
     
    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)  2007/01  [Not refereed][Not invited]
     
    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.
  • Kiwoon Choi, Kwang Sik Jeong, Tatsuo Kobayashi, Ken-ichi Okumura  Phys.Rev.D75:095012,2007  2006/12/21  [Not refereed][Not invited]
     
    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 set-up for TeV scale mirage mediation which allows an
    extra-dimensional interpretation for the origin of supersymmetry breaking and
    naturally gives an weak-scale size of the Higgs B-parameter. The set-up
    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 FCNC processes.
  • Stefan Forste, Tatsuo Kobayashi, Hiroshi Ohki, Kei-jiro Takahashi  JHEP 0703:011,2007  2006/12/06  [Not refereed][Not invited]
     
    We classify compactification lattices for supersymmetric Z2 times Z2
    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.
  • Tatsuo Kobayashi, Yuji Omura, Haruhiko Terao  PHYSICAL REVIEW D  74-  (5)  2006/09  [Not refereed][Not invited]
     
    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)  2006/07  [Not refereed][Not invited]
     
    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  2006/05  [Not refereed][Not invited]
     
    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 Okumura  PHYSICS LETTERS B  633-  (2-3)  355  -361  2006/02  [Not refereed][Not invited]
     
    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.
  • Hiroyuki Abe, Tetsutaro Higaki, Tatsuo Kobayashi  Physical Review D - Particles, Fields, Gravitation and Cosmology  74-  (4)  2006  [Not refereed][Not invited]
     
    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.
  • Hiroyuki Abe, Tetsutaro Higaki, Tatsuo Kobayashi  Phys.Rev.D73:046005,2006  2005/11/16  [Not refereed][Not invited]
     
    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 K\"ahler 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 non-perturbative 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.
  • T Higaki, N Kitazawa, T Kobayashi, K Takahashi  PHYSICAL REVIEW D  72-  (8)  2005/10  [Not refereed][Not invited]
     
    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.
  • Tatsuo Kobayashi, Carlos Munoz  JHEP 0601 (2006) 044  2005/08/29  [Not refereed][Not invited]
     
    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.
  • T Kobayashi, H Nakano, H Terao  PHYSICAL REVIEW D  71-  (11)  2005/06  [Not refereed][Not invited]
     
    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  2005/05  [Not refereed][Not invited]
     
    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)  2005/05  [Not refereed][Not invited]
     
    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  2005/04  [Not refereed][Not invited]
     
    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  2005/02  [Not refereed][Not invited]
     
    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.
  • 寺尾治彦, 小林達夫, 白野宏昌  素粒子論研究  110-  (4)  D7-D8  2005/01/20  [Not refereed][Not invited]
  • T Kobayashi, S Raby, RJ Zhang  NUCLEAR PHYSICS B  704-  (1-2)  3  -55  2005/01  [Not refereed][Not invited]
     
    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  2004/09/30  [Not refereed][Not invited]
     
    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  2004/09  [Not refereed][Not invited]
     
    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  2004/07  [Not refereed][Not invited]
     
    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  2004/06  [Not refereed][Not invited]
     
    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)  2004/04  [Not refereed][Not invited]
     
    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  2004/03/31  [Not refereed][Not invited]
     
    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  2004/03/04  [Not refereed][Not invited]
     
    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  2004/02/19  [Not refereed][Not invited]
     
    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)  2004/02  [Not refereed][Not invited]
     
    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 Seto  PHYSICAL REVIEW D  69-  (2)  2004/01  [Not refereed][Not invited]
     
    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  2003/08/21  [Not refereed][Not invited]
     
    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  2003/08  [Not refereed][Not invited]
     
    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  2003/08  [Not refereed][Not invited]
     
    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, O Lebedev  PHYSICS LETTERS B  565-  (1-4)  193  -200  2003/07  [Not refereed][Not invited]
     
    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  2003/07  [Not refereed][Not invited]
     
    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.
  • Tatsuo Kobayashi, Oleg Lebedev  Phys.Lett. B565 (2003) 193-200  2003/04/25  [Not refereed][Not invited]
     
    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.
  • Tatsuo Kobayashi, Oleg Lebedev  Phys.Lett. B566 (2003) 164-170  2003/03/03  [Not refereed][Not invited]
     
    Realistic heterotic string models require the presence of the Wilson lines.
    We study the effect of continuous Wilson lines on the heterotic Yukawa
    couplings and find that the presence of continuous Wilson lines affects the
    magnitude of the twisted Yukawa couplings resulting in their stronger
    hierarchy.
  • H Abe, T Kobayashi, N Maru, K Yoshioka  PHYSICAL REVIEW D  67-  (4)  045019  2003/02  [Not refereed][Not invited]
     
    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)  2003/02  [Not refereed][Not invited]
     
    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  2003/01/18  [Not refereed][Not invited]
     
    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.
  • Prog. Theor. Phys.  110-  (2)  247  -267  2003  [Not refereed][Not invited]
  • Hiroyuki Abe, Tetsutaro Higaki, Tatsuo Kobayashi  Progress of Theoretical Physics  109-  (5)  809  -830  2003  [Not refereed][Not invited]
     
    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.
  • Yukawa Hierarchy Transfer Based on Superconformal Dynamics and Geometrical Realization in String Models
    JHEP  0302-  022  2003  [Not refereed][Not invited]
  • Tatsuo Kobayashi, Hiroaki Nakano, Haruhiko Terao, Koichi Yoshioka  Progress of Theoretical Physics  110-  (2)  247  -267  2003  [Not refereed][Not invited]
     
    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  [Not refereed][Not invited]
     
    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  [Not refereed][Not invited]
     
    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.
  • K. S. Babu, Tatsuo Kobayashi, Jisuke Kubo  Phys.Rev. D67 (2003) 075018  D67-  075018  2002/12/24  [Not refereed][Not invited]
     
    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  PHYSICAL REVIEW D  66-  (9)  095011  2002/11  [Not refereed][Not invited]
     
    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)  2002/11  [Not refereed][Not invited]
     
    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  2002/05/08  [Not refereed][Not invited]
     
    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  2002/04  [Not refereed][Not invited]
     
    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.
  • Phys. Rev.  D65-  015006  2002  [Not refereed][Not invited]
  • Tatsuo Kobayashi, Nobuhito Maru, Koichi Yoshioka  Eur. Phys. J.  C29-  277  2001/10/09  [Not refereed][Not invited]
     
    In this letter, we discuss a four-dimensional model with modulus fields which
    are responsible for supersymmetry breaking. Given non-trivial moduli dependence
    of the action, the model is found to give a proper description of
    higher-dimensional supersymmetry breaking. We explicitly calculate 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.
  • Kobayashi Tatsuo  Soryushiron Kenkyu  103-  (6)  F43  -F46  2001/09/20  [Not refereed][Not invited]
  • Tatsuo Kobayashi, Hiroaki Nakano, Haruhiko Terao  Phys.Rev. D65 (2002) 015006  2001/07/04  [Not refereed][Not invited]
     
    We propose a new type of supersymmetric models coupled to superconformal
    field theories (SCFT's), 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 SM sector and the SC sector. Interestingly, this hierarchy is
    inversely transferred to the Yukawa couplings in the SM sector. In this type of
    models, 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  Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics  506-  (1-2)  187  -192  2001/05/03  [Not refereed][Not invited]
     
    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.
  • Tatsuo Kobayashi, Haruhiko Terao  Phys.Rev. D64 (2001) 075003  2001/03/03  [Not refereed][Not invited]
     
    We study soft SUSY breaking parameters in the Nelson-Strassler type of
    models: SUSY standard models coupled with SCFTs. In this type of models, soft
    SUSY breaking parameters including sfermion masses can be suppressed around the
    decoupling scale of SCFTs. We clarify the condition to derive exponential
    suppression of sfermion masses within the framework of pure SCFTs. 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 SM 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 Electro-Weak scale by a singlet field coupled with the SCFT.
  • Sfermion masses in Nelson-Strassler-Type models : SUSY standard models coupled with SCFTs(??)
    Phys. Rev.  D64-  075003  2001  [Not refereed][Not invited]
  • Phys. Lett.  506-  (3/4)  323  -330  2001  [Not refereed][Not invited]
  • Phys. Rev.  D63-  115009  2001  [Not refereed][Not invited]
  • Phys. Rev.  D63-  113017  2001  [Not refereed][Not invited]
  • Manuel Drees, Yeong Gyun Kirn, Tatsuo Kobayashi, Mihoko M. Nojiri  Physical Review D  63-  (11)  2001  [Not refereed][Not invited]
     
    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  [Not refereed][Not invited]
     
    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  Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics  489-  (1-2)  233  -242  2000/09/14  [Not refereed][Not invited]
     
    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  2000/08/08  [Not refereed][Not invited]
     
    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  2000/07/27  [Not refereed][Not invited]
     
    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.
  • Tatsuo Kobayashi, Haruhiko Terao  Phys.Lett. B489 (2000) 233-242  2000/05/26  [Not refereed][Not invited]
     
    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.
  • KOBAYASHI TATSUO  Soryushiron Kenkyu  101-  (2)  B25  -B26  2000/05/20  [Not refereed][Not invited]
  • Masako Bando, Tatsuo Kobayashi, Tatsuya Noguchi, Koichi Yoshioka  Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics  480-  (1-2)  187  -192  2000/05/04  [Not refereed][Not invited]
     
    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.
  • Tatsuo Kobayashi, Koichi Yoshioka  Phys.Rev. D62 (2000) 115003  2000/05/01  [Not refereed][Not invited]
     
    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 soft masses. We also discuss
    possible realizations of this scenario and the characteristic sparticle
    spectrum in the models.
  • Marcela Carena, Katri Huitu, Tatsuo Kobayashi  Nucl.Phys.B592:164-182,2001  2000/03/20  [Not refereed][Not invited]
     
    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  2000/03/10  [Not refereed][Not invited]
     
    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.
  • Phys. Rev. Lett.  85-  (26, Pt.1)  5527  -5530  2000  [Not refereed][Not invited]
  • "RG-invariant Sum Rule in a Generalization of Anomaly-Mediated SUSY Breaking Models"(jointly worked)
    Nucl. Phys.  B592-  164  -182  2000  [Not refereed][Not invited]
  • "Infrared alignment of SUSY flavor structures"(jointly worked)
    Phys. Rev.  D62-  115003  2000  [Not refereed][Not invited]
  • "EDM-free supersymmetric CP violation with non-universal soft terms"(jointly worked)
    Nucl. Phys.  B580-  275  -288  2000  [Not refereed][Not invited]
  • Phys. Lett.  480-  (1/2)  187  -192  2000  [Not refereed][Not invited]
  • "Electroweak symmetry breaking and s-spectrum in M-theory"(jointly worked)
    Nucl. Phys.  B580-  3  -28  2000  [Not refereed][Not invited]
  • "Phenomenological Constraints on SUSY SU(5) GUTs with Non-universal Gaugino Masses"(jointly worked)
    Phys. Rev.  D61-  035001  2000  [Not refereed][Not invited]
  • "Bottom-up Approach to B-parameter in Minimal Supersymmetric Standard Model"(jointly worked)
    Int. J. Mod. Phys.  A15-  81  -104  2000  [Not refereed][Not invited]
  • Katri Huitu, Yoshiharu Kawamura, Tatsuo Kobayashi, Kai Puolamaki  Phys.Lett. B468 (1999) 111-117  1999/09/02  [Not refereed][Not invited]
     
    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  1999/06/21  [Not refereed][Not invited]
     
    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  1999/06/16  [Not refereed][Not invited]
     
    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  1999/06/14  [Not refereed][Not invited]
     
    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.
  • Tatsuo Kobayashi, Jisuke Kubo, Hitoshi Shimabukuro  Nucl.Phys. B580 (2000) 3-28  1999/04/01  [Not refereed][Not invited]
     
    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  1999/04/01  [Not refereed][Not invited]
     
    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.
  • Katri Huitu, Yoshiharu Kawamura, Tatsuo Kobayashi, Kai Puolamaki  Phys.Rev. D61 (2000) 035001  1999/03/30  [Not refereed][Not invited]
     
    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 to 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.
  • Masud Chaichian, Tatsuo Kobayashi  Phys.Lett. B481 (2000) 26-32  481-  (1)  26  -32  1999/03/12  [Not refereed][Not invited]
     
    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.
  • Phys. Lett.  468-  (1/2)  111  -117  1999  [Not refereed][Not invited]
  • Phys. Lett.  466-  (2/4)  95  -99  1999  [Not refereed][Not invited]
  • "CP violation in supersymmetric model with non-degenerate A-terms"(jointly worked)
    Phys. Rev.  D60-  075003  1999  [Not refereed][Not invited]
  • "Running of Soft Parameters in Extra Space-Time Dimensions"(jointly worked)
    Nucl. Phys.  B550-  99  -122  1999  [Not refereed][Not invited]
  • 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  [Not refereed][Not invited]
     
    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.
  • KUBO JISUKE, KOBAYASHI TATSUO, SHIMABUKURO HITOSHI  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  1998/12  [Not refereed][Not invited]
  • Andreas Karch, Tatsuo Kobayashi, Jisuke Kubo, George Zoupanos  Phys.Lett. B441 (1998) 235-242  1998/08/29  [Not refereed][Not invited]
     
    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  1998/08/24  [Not refereed][Not invited]
     
    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  1998/08/19  [Not refereed][Not invited]
     
    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.
  • KOBAYASHI TATSUO  Soryushiron Kenkyu  97-  (4)  D31  -D33  1998/07/20  [Not refereed][Not invited]
  • Yoshiharu Kawamura, Tatsuo Kobayashi, Hitoshi Shimabukuro  Phys.Lett.B436:108-116,1998  1998/05/15  [Not refereed][Not invited]
     
    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.
  • Masud Chaichian, Wen-Feng Chen, Tatsuo Kobayashi  Phys.Lett. B432 (1998) 120-127  1998/03/18  [Not refereed][Not invited]
     
    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  1998/02/16  [Not refereed][Not invited]
     
    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  1998/02/11  [Not refereed][Not invited]
     
    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  1998/02/07  [Not refereed][Not invited]
     
    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.
  • Phys. Lett.  442-  (1/4)  192  -198  1998  [Not refereed][Not invited]
  • Phys. Lett.  441-  (1/4)  235  -242  1998  [Not refereed][Not invited]
  • Phya. Lett.  436-  (1/2)  108  -116  1998  [Not refereed][Not invited]
  • Phys. Lett.  432-  (1/2)  120  -127  1998  [Not refereed][Not invited]
  • Phys. Lett.  432-  (1/2)  108  -113  1998  [Not refereed][Not invited]
  • Phys. Lett.  427-  (3/4)  291  -299  1998  [Not refereed][Not invited]
  • "Yukawa Unification in Moduli-dominant SUSY Breaking"(jointly worked)
    Nucl. Phys.  B526-  99  -114  1998  [Not refereed][Not invited]
  • "Constraints on Finite Soft Supersymmetry-Breaking Terms"(jointly worked)
    Nucl. Phys.  B511-  45  -68  1998  [Not refereed][Not invited]
  • Int. J. Mod. Phys.  13-  (13)  2201  -2215  1998  [Not refereed][Not invited]
  • KUBO HARUSUKE, KOBAYASHI TATSUO, SHIMABUKURO HITOSHI  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  1997/12  [Not refereed][Not invited]
  • Yoshiharu Kawamura, Tatsuo Kobayashi, Jisuke Kubo  Phys.Lett. B432 (1998) 108-113  1997/10/24  [Not refereed][Not invited]
     
    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  1997/09/29  [Not refereed][Not invited]
     
    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.
  • Tatsuo Kobayashi, Jisuke Kubo, Myriam Mondragon, George Zoupanos  Nucl.Phys. B511 (1998) 45-68  1997/07/22  [Not refereed][Not invited]
     
    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 the
    non-universality of the SSB terms and requiring that the lightest superparticle
    particleis neutral, we constrain the parameter space of the SSB sector in each
    model.
  • Tatsuo Kobayashi, Hiroaki Nakano  Nuclear Physics B  496-  (1-2)  103  -131  1997/07/07  [Not refereed][Not invited]
     
    "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.
  • Naruhiko Aizawa, Tatsuo Kobayashi, Haru-Tada Sato  Int.J.Mod.Phys. A12 (1997) 5867-5883  1997/06/25  [Not refereed][Not invited]
     
    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.
  • KAWAMURA YOSHIHARU, KOBAYASHI TATSUO  素粒子論研究  95-  (1)  A60-A61  1997/04  [Not refereed][Not invited]
  • Yoshiharu Kawamura, Tatsuo Kobayashi, Jisuke Kubo  Phys.Lett. B405 (1997) 64-70  1997/03/14  [Not refereed][Not invited]
     
    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.
  • "Phenomenological Implications of Moduli-dominant SUSY Breaking"(jointly worked)
    Nucl. Phys.  B502-  37  -58  1997  [Not refereed][Not invited]
  • Phys. Lett.  405-  (1/2)  64  -70  1997  [Not refereed][Not invited]
  • "Anomalous U(1) Symmetry in Orbifold String Models"(jointly worked)
    Nucl. Phys.  B496-  103  -131  1997  [Not refereed][Not invited]
  • Int. J. Mod. Phys.  12-  (32)  5867  -5883  1997  [Not refereed][Not invited]
  • Mod. Phys. Lett.  12-  (8)  561  -571  1997  [Not refereed][Not invited]
  • Tatsuo Kobayashi, Zhi-zhong Xing  Mod.Phys.Lett. A12 (1997) 561-572  1996/09/26  [Not refereed][Not invited]
     
    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  Phys.Lett. B400 (1997) 284-294  400-  (3/4)  284  -294  1996/09/25  [Not refereed][Not invited]
     
    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  Phys.Rev. D56 (1997) 3844-3859  56-  (7)  3844  -3859  1996/08/06  [Not refereed][Not invited]
     
    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  Nucl.Phys. B481 (1996) 539-576  1996/07/01  [Not refereed][Not invited]
     
    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.
  • Tatsuo Kobayashi, Yoshio Yamagishi  Phys.Lett. B381 (1996) 169-176  1996/01/31  [Not refereed][Not invited]
     
    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  Phys.Lett.B375:141-148,1996; ERRATUM-ibid.B388:867,1996  1996/01/30  [Not refereed][Not invited]
     
    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  [Not refereed][Not invited]
  • Tatsuo Kobayashi, Zhi-Zhong Xing  Modern Physics Letters A  11-  (31)  2531  -2537  1996  [Not refereed][Not invited]
     
    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.
  • Phys. Lett.  381-  (1/3)  169  -176  1996  [Not refereed][Not invited]
  • Phys. Lett.  375-  (1/4)  141  -148  1996  [Not refereed][Not invited]
  • Int. J. Mod. Phys.  11-  (5)  903  -920  1996  [Not refereed][Not invited]
  • Mod. Lett.  11-  (7)  535  -543  1996  [Not refereed][Not invited]
  • KOBAYASHI TATSUO, YAMAGISHI YOSHIO  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  1995/12  [Not refereed][Not invited]
  • Kobayashi Tatsuo, Yamagishi Yoshio  Abstracts of the meeting of the Physical Society of Japan. Sectional meeting  1995-  (1)  1995/09/12  [Not refereed][Not invited]
  • KOBAYASHI TATSUO, YAMAGISHI YOSHIO  日本物理学会講演概要集(分科会)  1995-  (Autumn Pt 1)  14  1995/09  [Not refereed][Not invited]
  • Kobayashi Tatsuo, Suematsu Daijiro, Yamada Kiyonori, Yamagishi Yoshio  Abstracts of the meeting of the Physical Society of Japan. Annual meeting  50-  (1)  1995/03/16  [Not refereed][Not invited]
  • KOBAYASHI TATSUO, SUEMATSU DAIJIRO, YAMADA KIYONORI, YAMAGISHI YOSHIO  日本物理学会講演概要集(年会)  50th-  (1)  17  1995/03  [Not refereed][Not invited]
  • Phys. Lett.  358-  (3/4)  253  -258  1995  [Not refereed][Not invited]
  • Phys. Lett.  354-  (3/4)  264  -270  1995  [Not refereed][Not invited]
  • Tatsuo KOBAYASHI, Masahiko KONMURA, Daijiro SUEMATSU, Kiyonori YAMADA, Yoshio YAMAGISHI  Prog. Theor. Phys.  94-  (3)  417  -433  1995  [Not refereed][Not invited]
  • Phys. Lett.  348-  (3/4)  402  -410  1995  [Not refereed][Not invited]
  • Phys. Lett.  343-  (1/4)  122  -127  1995  [Not refereed][Not invited]
  • Int. J. Mod. Phys.  10-  (10)  1393  -1411  1995  [Not refereed][Not invited]
  • "Minimal String Unification and Constraint on Hidden Sector", (jointly worked)
    Nucl. Phys.  B434-  210  -230  1995  [Not refereed][Not invited]
  • Kobayashi Tatsuo  Soryushiron Kenkyu  90-  (3)  C81  -C85  1994/12/20  [Not refereed][Not invited]
  • 小林 達夫  素粒子論研究  90-  (2)  1994/11/20  [Not refereed][Not invited]
  • 小林達夫  素粒子論研究  90-  (2)  B51  1994/11  [Not refereed][Not invited]
  • KOBAYASHI TATSUO, KONMURA MASAHIKO, SUEMATSU DAIJIRO, YAMADA KIYONORI, YAMAGISHI YOSHIO  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  1994/11  [Not refereed][Not invited]
  • Kobayashi Tatsuo  Soryushiron Kenkyu  89-  (4)  D92  -D94  1994/07/20  [Not refereed][Not invited]
  • T KOBAYASHI  PHYSICS LETTERS B  326-  (3-4)  231  -237  1994/05  [Not refereed][Not invited]
     
    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.
  • Phys. Lett.  329-  (1)  27  -32  1994  [Not refereed][Not invited]
  • Phys. Lett.  325-  (1/2)  77  -84  1994  [Not refereed][Not invited]
  • Phys. Lett.  322-  (4)  331  -339  1994  [Not refereed][Not invited]
  • Int. J. Mod. Phys.  9-  (1)  87  -125  1994  [Not refereed][Not invited]
  • Tatsuo Kobayashi  Mod.Phys.Lett.A8:3741-3748,1993  8-  (39)  3741  -3748  1993/08/10  [Not refereed][Not invited]
     
    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.
  • Tatsuo Kobayashi  Zeitschrift für Physik C Particles and Fields  60-  (1)  101  -109  1993/03  [Not refereed][Not invited]
     
    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  1993/02/15  [Not refereed][Not invited]
     
    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.
  • Int. J. Mod. Phys.  8-  (20)  3553  -3563  1993  [Not refereed][Not invited]
  • Phys. Lett.  306-  (1/2)  27  -33  1993  [Not refereed][Not invited]
  • Z. Phys.  58-  (4)  559  -565  1993  [Not refereed][Not invited]
  • Tatsuo Kobayashi, Tsuneo Uematsu  Z.Phys. C58 (1993) 559-566  1992/10/30  [Not refereed][Not invited]
     
    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.
  • Kobayashi T  秋の分科会講演予稿集  1992-  (1)  1992/09/14  [Not refereed][Not invited]
  • 小林達夫  日本物理学会講演概要集(分科会)  1992-  (Autumn Pt 1)  2  1992/09  [Not refereed][Not invited]
  • 小林 達夫  Soryushiron Kenkyu  85-  (4)  D3  -D5  1992/07/20  [Not refereed][Not invited]
  • KOBAYASHI TATSUO  素粒子論研究  85-  (4)  D3-D5  1992/07  [Not refereed][Not invited]
  • Tatsuo Kobayashi, Tsuneo Uematsu  Zeitschrift für Physik C Particles and Fields  56-  (2)  193  -199  1992/06  [Not refereed][Not invited]
     
    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  [Not refereed][Not invited]
  • "Correspondence of Z4×Z4 Orbifold Model and 26 Model"
    Suppl. Prog. Theor. Phys.  110-  277  -290  1992  [Not refereed][Not invited]
  • Phys. Lett.  262-  (4)  425  -431  1991  [Not refereed][Not invited]
  • "Analyses on the Wilson Lines of ZN Orbifold Models", (jointly worked)
    Phys. Lett.  B257-  56  -62  1991  [Not refereed][Not invited]
  • "Yukawa Coupling Condition of ZN Orbifold Models", (jointly worked)
    Phys. Lett.  B245-  441  -446  1990  [Not refereed][Not invited]
  • "ZN Orbifold Models", (jointly worked)
    Nucl. Phys.  B341-  611  -640  1990  [Not refereed][Not invited]
  • Phys. Lett.  227-  (3/4)  381  -386  1989  [Not refereed][Not invited]
  • Prog. Theor. Phys.  82-  (1)  171  -182  1989  [Not refereed][Not invited]
  • Phys. Lett.  218-  (2)  169  -175  1989  [Not refereed][Not invited]
  • Phys. Lett.  212-  (3)  339  -342  1988  [Not refereed][Not invited]
  • Localication of Heterotic Anomalies on Various Hyper Surfaces of T^6/Z_4
    Phys. Rev.  D-  [Not refereed][Not invited]

Research Grants & Projects

  • 文部科学省:科学研究費補助金(基盤研究(C))
    Date (from‐to) : 2013 -2015 
    Author : 小林 達夫
  • Ministry of Education, Culture, Sports, Science and Technology:Grants-in-Aid for Scientific Research(基盤研究(C))
    Date (from‐to) : 2008 -2011 
    Author : Tatsuo KOBAYASHI
     
    We have studied four-dimensional low-energy effective field theory, derived from superstring theory, in particular heterotic string theory on the orbifolds and magnetized/intersecting brane models. We have studied Abelian and non-Abelian discrete symmetries, which appear in these string models. Also we have studied their anomalies and found that the universal relations among certain anomalies. Furthermore, by explicit string computations we have shown stringy coupling selection rules, which seem to be difficult to understand from the symmetries of low-energy effective field theories.
  • Ministry of Education, Culture, Sports, Science and Technology:Grants-in-Aid for Scientific Research(基盤研究(C))
    Date (from‐to) : 2005 -2007 
    Author : Tatsuo KOBAYASHI
     
    The purpose of this research is to clarify which flavor structures can be derived from superstring theory and to study their phenomenological implications.Recently, several discrete non-Abelian flavor symmetries were proposed to derive realistic mass matrices for quarks and leptons. However, their origins are not clear in 4D field theories. In 2006, we clarified that geometrical structure of compact space can provide their origins in superstring theories and typical flavor symmetries, which can appear from heterotic orbifold models, are D4 and Delta(54). In 2007, using these results, we studied which types of quark/lepton mass matrices can be realized from D4 flavor symmetries. Furthermore, when we consider supersymmetric theories, flavor symmetries constrain not only fermion mass matrices, but also mass matrices of their superpartners, i.e. supersymmetry breaking terms. We also studied specific patterns of supersymmetric breaking terms, which are predicted from the above D4 flavor symmetries.In addition, we computed higher order couplings, i.e. their moduli-dependence. We also studied moduli stabilization, and supersymmetry breaking and their phenomenological implications.
  • 文部科学省:科学研究費補助金(特定領域研究)
    Date (from‐to) : 2004 -2005 
    Author : 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。
  • Ministry of Education, Culture, Sports, Science and Technology:Grants-in-Aid for Scientific Research(基盤研究(C))
    Date (from‐to) : 2002 -2003 
    Author : Tatsuo KOBAYASHI
     
    Understanding the origin of fermion masses and mixing angles is one of important issues in particle physics. Within the framework of supersymmetric models, a mechanism to lead to realistic Yukawa matrices, in general, affects sfermion masses as well as the so-called A-terms. There are stringent experimental bounds on degeneracies of these SUSY breaking terms through FCNC processes. Taking into account these points, we have studied several types of flavor mechanisms.One type of flavor mechanisms which we have studied is supersymmetric standard models coupled with superconformal sectors, which have superconformal fixed points. In this type of models, superconformal dynamics generates large anomalous dimensions leading to hierarchically suppressed Yukawa couplings and at the same time controls sfermion masses so as to make them degenerate. Also we have studied flavor mechanism with 5D AdS background, which leads to the same result as the superconformal approach.We have also studied the model, which has S_3 symmetry as a flavor symmetry. Such symmetry has been studied in the literature, but it is broken at high energy. However, what is new in our model is that we extend the Higgs sector and impose the S_3 symmetry to the Higgs sector, too. The S_3 symmetry is broken around the electroweak scale. Such symmetry play a role in controlling SUSY breaking terms, so as to be consistent with experimental bounds on FCNC processes.Furthermore, we have also calculated Yukawa couplings in heterotic orbifold models including several moduli, e.g. continuous Wilson line moduli. One of our important results is that only the imaginary part of Kahler moduli contributes the physical CP phase in the Yukawa sector, while the other moduli, e.g. continuous and discrete Wilson lines do not contribute.
  • 超対称性の破れと超弦理論の現象論的性質
  • Supersymmetry Breaking and Superstring Phenomenology

Educational Activities

Teaching Experience

  • 現代物理学
    開講年度 : 2018
    課程区分 : 学士課程
    開講学部 : 総合教育部
  • The World of Science and Technology
    開講年度 : 2018
    課程区分 : 学士課程
    開講学部 : 全学教育
    キーワード : アインシュタイン、相対性理論、光速、同時性、ローレンツ変換、ローレンツ収縮、時間の遅れ、双子のパラドックス、質量とエネルギーの等価性、重力、等価原理、膨張宇宙
  • Theory of Fields
    開講年度 : 2018
    課程区分 : 学士課程
    開講学部 : 理学部
    キーワード : 相対論的量子力学、Dirac 方程式、場の理論、量子電磁気学
  • Relativity
    開講年度 : 2018
    課程区分 : 学士課程
    開講学部 : 理学部
    キーワード : 相対性理論、重力、宇宙論、ブラックホール
  • Mathematical Methods of Physics I
    開講年度 : 2018
    課程区分 : 学士課程
    開講学部 : 理学部
    キーワード : 常微分方程式、ベクトル解析


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