Koji Tsumura | Last modified date：2023.05.18 |

Associate Professor /
Department of Physics /
Faculty of Sciences

**Papers**

1. | Hajime Otsuka, Takashi Shimomura, Koji Tsumura, Yoshiki Uchida, Naoki Yamatsu, Pseudo-Nambu-Goldstone Dark Matter from Non-Abelian Gauge Symmetry, Physical Review D, https://doi.org/10.1103/PhysRevD.106.115033, 2022.12. |

2. | Kento Asai, Coh Miyao, Shohei Okawa, Koji Tsumura, Scalar Dark Matter with a μ-τ Flavored Mediator, Physical Review D, https://doi.org/10.1103/PhysRevD.106.035017, arXiv:2205.08998, 2022.08. |

3. | Takahiro Ohata, Kengo Takeuchi, Koji Tsumura, Baryon number non-conservation as Peccei-Quinn mechanism, Physical Review D, https://doi.org/10.1103/PhysRevD.104.035026, 2021.08, [URL], Baryon number is an accidental symmetry in the standard model, while Peccei-Quinn symmetry is hypothetical symmetry which is introduced to solve the strong CP problem. We study the possible connections between Peccei-Quinn symmetry and baryon number symmetry. In this framework, an axion is identified as the Nambu-Goldstone boson of baryon number violation. As a result, characteristic baryon number violating processes are predicted. We developed the general method to determine the baryon number and lepton number of new scalar in the axion model.. |

4. | Yoshihiko Abe, Takashi Toma, Koji Tsumura, Naoki Yamatsu, Pseudo-Nambu-Goldstone Dark Matter Model Inspired by Grand Unification, Physical Review D, https://doi.org/10.1103/PhysRevD.104.035011, 2021.08, [URL], A pseudo-Nambu-Goldstone boson (pNGB) is an attractive candidate for dark matter (DM) due to the simple evasion of the current severe limits of DM direct detection experiments. One of the pNGB DM models has been proposed based on a {\it gauged} U(1)B−L symmetry. The pNGB has long enough lifetime to be a DM and thermal relic abundance of pNGB DM can be fit with the observed value against the constraints on the DM decays from the cosmic-ray observations. The pNGB DM model can be embedded into an SO(10) pNGB DM model in the framework of an SO(10) grand unified theory, whose SO(10) is broken to the Pati-Salam gauge group at the unified scale, and further to the Standard Model gauge group at the intermediate scale. Unlike the previous pNGB DM model, the parameters such as the gauge coupling constants of U(1)B−L, the kinetic mixing parameter of between U(1)Y and U(1)B−L are determined by solving the renormalization group equations for gauge coupling constants with appropriate matching conditions. From the constraints of the DM lifetime and gamma-ray observations, the pNGB DM mass must be less than (100)GeV. We find that the thermal relic abundance can be consistent with all the constraints when the DM mass is close to half of the CP even Higg masses.. |

5. | Ryo Nagai, Masaharu Tanabashi, Koji Tsumura, Yoshiki Uchida, Scalar and fermion on-shell amplitudes in generalized Higgs effective field theory, Physical Review D, https://doi.org/10.1103/PhysRevD.104.015001, 2021.07, [URL], Beyond-the-standard-model (BSM) particles should be included in effective field theory in order to compute the scattering amplitudes involving these extra particles. We formulate an extension of Higgs effective field theory which contains an arbitrary number of scalar and fermion fields with arbitrary electric and chromoelectric charges. The BSM Higgs sector is described by using the nonlinear sigma model in a manner consistent with the spontaneous electroweak symmetry breaking. The chiral-order counting rule is arranged consistently with the loop expansion. The leading-order Lagrangian is organized in accord with the chiral-order counting rule. We use a geometrical language to describe the particle interactions. The parametrization redundancy in the effective Lagrangian is resolved by describing the on-shell scattering amplitudes only with the covariant quantities in the scalar/fermion field space. We introduce a useful coordinate (normal coordinate), which simplifies the computations of the on-shell amplitudes significantly. We show that the high-energy behaviors of the scattering amplitudes determine the “curvature tensors” in the scalar/fermion field space. The massive spinor–wave function formalism is shown to be useful in the computations of on-shell helicity amplitudes.. |

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7. | Yu Hamada, Koji Tsumura, Masatoshi Yamada, Scalegenesis and fermionic dark matters in the flatland scenario, European Physical Journal C, 10.1140/epjc/s10052-020-7929-3, 80, 5, 2020.05, We propose an extension of the standard model with Majorana-type fermionic dark matters based on the flatland scenario where all scalar coupling constants, including scalar mass terms, vanish at the Planck scale, i.e. the scalar potential is flat above the Planck scale. This scenario could be compatible with the asymptotic safety paradigm for quantum gravity. We search the parameter space so that the model reproduces the observed values such as the Higgs mass, the electroweak vacuum and the relic abundance of dark matter. We also investigate the spin-independent elastic cross section for the Majorana fermions and a nucleon. It is shown that the Majorana fermions as dark matter candidates could be tested by dark matter direct detection experiments such as XENON, LUX and PandaX-II. We demonstrate that within the minimal setup compatible with the flatland scenario at the Planck scale or asymptotically safe quantum gravity, the extended model could have a strong predictability.. |

8. | Yoshihiko Abe, Takashi Toma, Koji Tsumura, Pseudo-Nambu-Goldstone dark matter from gauged U(1)_{B-L} symmetry, Journal of High Energy Physics, 10.1007/JHEP05(2020)057, 2020, 5, 2020.05, A pseudo-Nambu-Goldstone boson (pNGB) is an attractive candidate for dark matter since the current severe limits of dark matter direct detection experiments are naturally evaded by its nature. We construct a model with pNGB dark matter based on a gauged U (1)_{B−L} symmetry, where no adhoc global symmetry is assumed. The model keeps natural suppression mechanism for the dark matter direct detection cross section. On the other hand, the pNGB can decay through the new high scale suppressed operators. We show that the pNGB has long enough lifetime to be a dark matter in the wide range of the parameter space of the model. The thermal relic abundance of pNGB dark matter can be fit with the observed value against the constraints on the dark matter decays from the cosmic-ray observations.. |

9. | Minoru Tanaka, Koji Tsumura, Noboru Sasao, Satoshi Uetake, Motohiko Yoshimura, QED background against atomic neutrino process with initial spatial phase, European Physical Journal Plus, 10.1140/epjp/s13360-020-00290-2, 135, 3, 2020.03, Atomic deexcitation emitting a neutrino pair and a photon is expected to provide a novel method of neutrino physics if it is enhanced by quantum coherence in a macroscopic target. However, the same enhancement mechanism may also lead to a serious problem of enhanced QED background process. We show that the QED background can be suppressed enough in the photonic crystal waveguide by using the spatial phase that is imprinted in the process of initial coherence generation in the target at excitation.. |

10. | Ryo Nagai, Masaharu Tanabashi, Koji Tsumura, Yoshiki Uchida, Symmetry and geometry in a generalized Higgs effective field theory Finiteness of oblique corrections versus perturbative unitarity, Physical Review D, 10.1103/PhysRevD.100.075020, 100, 7, 2019.10, We formulate a generalization of Higgs effective field theory (HEFT) including an arbitrary number of extra neutral and charged Higgs bosons - a generalized HEFT (GHEFT) - to describe nonminimal electroweak symmetry breaking models. Using the geometrical form of the GHEFT Lagrangian, which can be regarded as a nonlinear sigma model on a scalar manifold, it is shown that the scalar boson scattering amplitudes are described in terms of the Riemann curvature tensor (geometry) of the scalar manifold and the covariant derivatives of the potential. The coefficients of the one-loop divergent terms in the oblique correction parameters S and U can also be written in terms of the Killing vectors (symmetry) and the Riemann curvature tensor (geometry). It is found that the perturbative unitarity of the scattering amplitudes involving the Higgs bosons and the longitudinal gauge bosons demands that the scalar manifold be flat. The relationship between the finiteness of the electroweak oblique corrections and the perturbative unitarity of the scattering amplitudes is also clarified in this language: we verify that once the tree-level unitarity is ensured, the one-loop finiteness of the oblique correction parameters S and U is automatically guaranteed.. |

11. | Yoshihiko Abe, Takashi Toma, Koji Tsumura, A μ-τ-philic scalar doublet under Z_{n} flavor symmetry, Journal of High Energy Physics, 10.1007/JHEP06(2019)142, 2019, 6, 2019.06, We propose a minimal model which accommodates the long-standing anomaly of muon magnetic moment based on abelian discrete flavor symmetries. The standard model is extended by scalar doublets charged under a Z_{n} lepton flavor symmetry. In these models, a large contribution to the muon magnetic moment can be obtained by the chirality enhancement from new scalar mediated diagrams without conflicting with the flavor symmetry. Thanks to the lepton flavor symmetry, these models automatically forbid lepton flavor violation. The minimal model is based on Z_{4} symmetry with only one extra scalar doublet. In this model, we show that the parameter space favored by the muon g − 2 can easily be consistent with experimental constraints and theoretical bounds such as the electroweak precision tests, lepton universality, potential stability condition and triviality bound as well as the LHC direct search mass bound. The new contributions to the muon electric dipole moment and the Higgs decay into γγ can be indirect signals of the model.. |

12. | Kento Asai, Koichi Hamaguchi, Natsumi Nagata, Shih Yen Tseng, Koji Tsumura, Minimal Gauged U(1)_{Lα - Lβ} Models Driven into a Corner, Physical Review D, 10.1103/PhysRevD.99.055029, 99, 5, 2019.03, It is well known that the differences between the lepton numbers can be gauged with the standard model matter content. Such extended gauge theories, dubbed as the gauged U(1)Lα-Lβ models, have been widely discussed so far as potential candidates for physics beyond the Standard Model. In this work, we study the minimal versions of these gauge theories, where three right-handed neutrinos as well as a single U(1)Lα-Lβ symmetry breaking Higgs field - an SU(2)L singlet or doublet - are introduced. In these minimal models, the neutrino mass terms are constrained by the gauge symmetry, which result in the two-zero texture or two-zero minor structure of neutrino mass matrices. Such restrictive forms of neutrino mass matrices lead to nontrivial predictions for the neutrino oscillation parameters as well as the size of the mass eigenvalues. We find that due to this restriction the minimal gauged U(1)Lα-Lβ models are either incompatible with the observed values of the neutrino parameters or in strong tension with the Planck 2018 limit on the sum of the neutrino masses. Only the U(1)Lμ-Lτ model with an SU(2)L singlet U(1)Lμ-Lτ-breaking field barely evades the limit, which can be tested in the future neutrino experiments.. |

13. | Nabarun Chakrabarty, Cheng Wei Chiang, Takahiro Ohata, Koji Tsumura, Charged scalars confronting neutrino mass and muon g − 2 anomaly, Journal of High Energy Physics, 10.1007/JHEP12(2018)104, 2018, 12, 2018.12, The present work introduces two possible extensions of the Standard Model Higgs sector. In the first case, the Zee-Babu type model for the generation of neutrino mass is augmented with a scalar triplet and additional singly charged scalar singlets. The second scenario, on the other hand, generalizes the Type-II seesaw model by replicating the number of the scalar triplets. A ℤ_{3} symmetry is imposed in case of both the scenarios, but, allowed to be violated by terms of mass dimension two and three for generating neutrino masses and mixings. We examine how the models so introduced can explain the experimental observation on the muon anomalous magnetic moment. We estimate the two-loop contribution to neutrino mass induced by the scalar triplet, in addition to what comes from the doubly charged singlet in the usual Zee-Babu framework, in the first model. On the other hand, the neutrino mass arises in the usual Type-II fashion in the second model. In addition, the role of the ℤ_{3} symmetry in suppressing lepton flavor violation is also elucidated.. |

14. | Ernest Ma, Koji Tsumura, Syndetic extension of baryon and lepton numbers Proton decay and long-lived dark matter, Physical Review D, 10.1103/PhysRevD.98.035037, 98, 3, 2018.08, The well-known baryon and lepton numbers of the standard model of quarks and leptons are extended to include new fermions and bosons in a simple structure with several essential features. The usual heavy right-handed neutrino singlets (for neutrino mass and leptogenesis) are related to the axion which solves the strong CP problem. At the same time, baryon number is broken softly, allowing the proton to decay. Associated with this breaking, a long-lived dark-matter candidate (called the pseudosakharon) emerges. This new insight connects proton decay to a new component of dark matter.. |

15. | Cheng Wei Chiang, Koji Tsumura, Model with a gauged lepton flavor SU(2) symmetry, Journal of High Energy Physics, 10.1007/JHEP05(2018)069, 2018, 5, 2018.05, We propose a model having a gauged SU(2) symmetry associated with the second and third generations of leptons, dubbed SU(2)_{μτ}, of which U(1)Lμ−Lτ is an Abelian subgroup. In addition to the Standard Model fields, we introduce two types of scalar fields. One exotic scalar field is an SU(2)_{μτ}doublet and SM singlet that develops a nonzero vacuum expectation value at presumably multi-TeV scale to completely break the SU(2)_{μτ}symmetry, rendering three massive gauge bosons. At the same time, the other exotic scalar field, carrying electroweak as well as SU(2)_{μτ}charges, is induced to have a nonzero vacuum expectation value as well and breaks mass degeneracy between the muon and tau. We examine how the new particles in the model contribute to the muon anomalous magnetic moment in the parameter space compliant with the Michel decays of tau.. |

16. | Minoru Tanaka, Koji Tsumura, Noboru Sasao, Satoshi Uetake, Motohiko Yoshimura, Effects of initial spatial phase in radiative neutrino pair emission, Physical Review D, 10.1103/PhysRevD.96.113005, 96, 11, 2017.12, We study radiative neutrino pair emission in a deexcitation process of atoms taking into account the coherence effect in a macroscopic target system. In the course of preparing the coherent initial state to enhance the rate, a spatial phase factor is imprinted on the macroscopic target. It is shown that this initial spatial phase changes the kinematics of the radiative neutrino pair emission. We investigate effects of the initial spatial phase in the photon spectrum of the process. It turns out that the initial spatial phase provides us with significant improvements in exploring neutrino physics, such as the Dirac-Majorana distinction and the cosmic neutrino background.. |

17. | Ernest Ma, Takahiro Ohata, Koji Tsumura, Majoron as the QCD axion in a radiative seesaw model, Physical Review D, 10.1103/PhysRevD.96.075039, 96, 7, 2017.10, The smallness of neutrino mass, the strong CP problem, and the existence of dark matter are explained in an economical way. The neutrino mass is generated by the colored version of a radiative seesaw mechanism by using color adjoint mediators. The Majorana mass term of the adjoint fermion, which carries lepton number U(1)L, is induced by its spontaneous breaking, resulting in a Majoron which doubles as the quantum chromodynamics axion, thereby solving the strong CP problem. The breaking of U(1)L sets simultaneously the seesaw scale for neutrino mass and the Peccei-Quinn breaking scale. This axion is a good candidate for dark matter as usually assumed.. |

18. | Shu Yu Ho, Takashi Toma, Koji Tsumura, A radiative neutrino mass model with SIMP dark matter, Journal of High Energy Physics, 10.1007/JHEP07(2017)101, 2017, 7, 2017.07, We propose the first viable radiative seesaw model, in which the neutrino masses are induced radiatively via the two-loop Feynman diagram involving Strongly Interacting Massive Particles (SIMP). The stability of SIMP dark matter (DM) is ensured by a ℤ_{5} discrete symmetry, through which the DM annihilation rate is dominated by the 3 → 2 self-annihilating processes. The right amount of thermal relic abundance can be obtained with perturbative couplings in the resonant SIMP scenario, while the astrophysical bounds inferred from the Bullet cluster and spherical halo shapes can be satisfied. We show that SIMP DM is able to maintain kinetic equilibrium with thermal plasma until the freeze-out temperature via the Yukawa interactions associated with neutrino mass generation.. |

19. | Minoru Tanaka, Koji Tsumura, Noboru Sasao, Motohiko Yoshimura, Toward background-free RENP using a photonic crystal waveguide, Progress of Theoretical and Experimental Physics, 10.1093/ptep/ptx035, 2017, 4, 2017.04, We study how to suppress multiphoton emission background in QED against radiative emission of a neutrino pair (RENP) from atoms. We pursue the possibility of background suppression using the photonic band structure of periodic dielectric media, called photonic crystals. The modification of photon emission rate in photonic crystal waveguides is quantitatively examined to clarify the condition of background-free RENP.. |

20. | Koji Tsumura, Masatoshi Yamada, Yuya Yamaguchi, Gravitational wave from dark sector with dark pion, Journal of Cosmology and Astroparticle Physics, 10.1088/1475-7516/2017/07/044, 2017, 7, 2017.01, In this work, we investigate the spectra of gravitational waves produced by chiral symmetry breaking in dark quantum chromodynamics (dQCD) sector. The dark pion (π) can be a dark matter candidate as weakly interacting massive particle (WIMP) or strongly interacting massive particle (SIMP). For a WIMP scenario, we introduce the dQCD sector coupled to the standard model (SM) sector with classical scale invariance and investigate the annihilation process of the dark pion via the 2π 2 SM process. For a SIMP scenario, we investigate the 3π 2π annihilation process of the dark pion as a SIMP using chiral perturbation theory. We find that in the WIMP scenario the gravitational wave background spectra can be observed by future space gravitational wave antennas. On the other hand, when the dark pion is the SIMP dark matter with the constraints for the chiral perturbative limit and pion-pion scattering cross section, the chiral phase transition becomes crossover and then the gravitational waves are not produced.. |

21. | Yuta Hamada, Koji Tsumura, Daiki Yasuhara, Reheating era leptogenesis in models with a seesaw mechanism, Physical Review D, 10.1103/PhysRevD.95.103505, 95, 10, 2017.01, Observed baryon asymmetry can be achieved not only by the decay of right-handed neutrinos but also by the scattering processes in the reheating era. In the latter scenario, new physics in high energy scale does not need to be specified, but only two types of the higher dimensional operator of the standard model particles are assumed in the previous work. In this paper, we examine the origin of the higher dimensional operators assuming models with a certain seesaw mechanism at the high energy scale. The seesaw mechanism seems to be a simple realization of the reheating era leptogenesis because the lepton number violating interaction is included. We show that the effective interaction giving CP violating phases is provided in the several types of models and also the reheating era leptogenesis actually works in such models. Additionally, we discuss a possibility for lowering the reheating temperature in the radiative seesaw models, where the large Yukawa coupling is naturally realized.. |

22. | Shu Yu Ho, Takashi Toma, Koji Tsumura, Systematic U(1)_{B-L} Extensions of Loop-Induced Neutrino Mass Models with Dark Matter, Physical Review D, 10.1103/PhysRevD.94.033007, 94, 3, 2016.08, We study the gauged U(1)B-L extensions of the models for neutrino masses and dark matter. In this class of models, tiny neutrino masses are radiatively induced through the loop diagrams, while the origin of the dark matter stability is guaranteed by the remnant of the gauge symmetry. Depending on how the lepton number conservation is violated, these models are systematically classified. We present complete lists for the one-loop Z2 and the two-loop Z3 radiative seesaw models as examples of the classification. The anomaly cancellation conditions in these models are also discussed.. |

23. | Yuta Hamada, Hikaru Kawai, Kiyoharu Kawana, Koji Tsumura, Models of the LHC diphoton excesses valid up to the Planck scale, Physical Review D, 10.1103/PhysRevD.94.014007, 94, 1, 2016.07, We discuss a possibility to explain the LHC diphoton excesses at 750 GeV by the new scalar X that couples to the gauge bosons through the loop of new massive particles with standard model charges. We assume that the new particles decay into the standard model particles at the tree level. We systematically examine the models that preserve the vacuum stability and the perturbativity up to the Planck scale. When we take scalars for the new particles, we find that only a few diquark and dilepton models can explain the observed diphoton cross section without conflicting the experimental mass bounds. When we take vectorlike fermions for the new particles, we find rather different situations depending on whether their couplings to X are scalar or pseudoscalar type. In the former case, a few models are allowed if we introduce only one species of fermions. The more fermions we introduce, the more models are allowed. In the latter case, most of the models are allowed because of the large coupling between X and photon. It is interesting that the allowed mass regions of the scalar particles might be reached by the next lepton colliders.. |

24. | Yuji Omura, Kazuhiro Tobe, Koji Tsumura, Survey of Higgs interpretations of the diboson excesses, Physical Review D - Particles, Fields, Gravitation and Cosmology, 10.1103/PhysRevD.92.055015, 92, 5, 2015.09, We investigate diboson signals in the standard model (SM) with an extended Higgs sector, motivated by the excesses in the diboson channels at the LHC. We begin with the unitarity sum rules of the weak gauge boson scattering assuming the Higgs sector is extended. According to the sum rules, we discuss the Higgs interpretations of the diboson signals and the consistency with the ATLAS diboson anomaly and other experimental constraints. As a concrete example, we propose a two-Higgs-doublet model where the Yukawa coupling of an extra CP-even scalar with up-type quark is relatively large. The diboson (WW and ZZ) signals can be explained by 2 TeV CP-even Higgs boson, while the partners, the CP-odd and the charged Higgs bosons in the extra doublet, are severely constrained by both the LHC direct search and the indirect search via flavor observables. Especially, in order to avoid the diboson resonance search in the Vh channel, further extensions of the model are required. The diboson excess is correlated with the SM Higgs signals in our framework, so that the precise measurement of the SM Higgs boson is also important to test the Higgs interpretation.. |

25. | Yuta Hamada, Kiyoharu Kawana, Koji Tsumura, Landau pole in the Standard Model with weakly interacting scalar fields, Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics, 10.1016/j.physletb.2015.05.072, 747, 238-244, 2015.07, We consider the Standard Model with a new scalar field X which is an n_{X} representation of the SU(2)_{L} with a hypercharge Y_{X}. The renormalization group running effects on the new scalar quartic coupling constants are evaluated. Even if we set the scalar quartic coupling constants to be zero at the scale of the new scalar field, the coupling constants are induced by the one-loop effect of the weak gauge bosons. Once non-vanishing couplings are generated, the couplings rapidly increase by renormalization group effect of the quartic coupling constant itself. As a result, the Landau pole appears below Planck scale if n_{X}≥4. We find that the scale of the obtained Landau pole is much lower than that evaluated by solving the one-loop beta function of the gauge coupling constants.. |

26. | Cheng Wei Chiang, Koji Tsumura, Properties and searches of the exotic neutral Higgs bosons in the Georgi-Machacek model, Journal of High Energy Physics, 10.1007/JHEP04(2015)113, 2015, 4, 2015.04, Abstract: The Georgi-Machacek model predicts the existence of four neutral Higgs bosons, one of which can be identified as the 125-GeV Higgs boson. The latest Higgs data favor the parameter space of small mixing angle α between the two custodial singlets of the model. The other two neutral Higgs bosons belong respectively to the custodial triplet and quintet. We study the general decay and production properties of these particles in the small-α scenario. Constraints on the SU(2)_{L} triplet vacuum expectation value are obtained as a function of the exotic Higgs boson masses using latest ATLAS data of various search channels for additional neutral Higgs bosons.. |

27. | Ryo Nagai, Masaharu Tanabashi, Koji Tsumura, Does unitarity imply finiteness of electroweak oblique corrections at one loop? Constraining extra neutral Higgs bosons, Physical Review D - Particles, Fields, Gravitation and Cosmology, 10.1103/PhysRevD.91.034030, 91, 3, 2015.02, Introducing an arbitrary number of neutral Higgs bosons in the electroweak symmetry breaking sector, we derive a set of conditions among Higgs couplings which need to be satisfied to maintain the unitarity of the high energy scattering amplitudes of weak gauge bosons at the tree level (unitarity sum rules). It is shown that the unitarity sum rules require the tree level ρ parameter to be 1, without explicitly invoking the custodial symmetry arguments. The one loop finiteness of the electroweak oblique corrections is automatically guaranteed once these unitarity sum rules are imposed among Higgs couplings. Severe constraints on the lightest Higgs coupling (125 GeV Higgs coupling) and the mass of the second lightest Higgs boson are obtained from the unitarity and the results of the electroweak precision tests (oblique parameter measurements). These results are compared with the effective theory of the light Higgs boson, and we find simple relationships between the mass of the second lightest Higgs boson in our framework and the ultraviolet cutoff in the effective theory framework.. |

28. | Koji Tsumura, Discrimination of models including doubly charged scalar bosons using tau decays, Nuclear Physics B - Proceedings Supplements, 10.1016/j.nuclphysbps.2014.09.063, 253-255, 230-231, 2014.10, The doubly charged scalar boson (H^{±±}) is introduced in several models of the new physics beyond the standard model. The H^{±±} has Yukawa interactions with two left-handed charged leptons or two right-handed charged leptons depending on the models. We study kinematical properties of H^{±±} decay products through tau leptons in order to discriminate the chiral structures of the new Yukawa interaction. The chirality of tau leptons can be measured by the energy distributions of its decay products, and thus the chiral structure of the new Yukawa interaction can be traced in the invariant-mass distributions of the H^{±±} decay products.. |

29. | Shinya Kanemura, Koji Tsumura, Kei Yagyu, Hiroshi Yokoya, Fingerprinting nonminimal Higgs sectors, Physical Review D - Particles, Fields, Gravitation and Cosmology, 10.1103/PhysRevD.90.075001, 90, 7, 2014.10, After the discovery of the standard model-like Higgs boson at the LHC, the structure of the Higgs sector remains unknown. We discuss how it can be determined by the combination of direct and indirect searches for additional Higgs bosons at future collider experiments. First of all, we evaluate expected excluded regions for the mass of additional neutral Higgs bosons from direct searches at the LHC with the 14 TeV collision energy in the two Higgs doublet models with a softly broken Z2 symmetry. Second, precision measurements of the Higgs boson couplings at future experiments can be used for the indirect search of extended Higgs sectors if the measured coupling constant with the gauge boson slightly deviates from the standard model value. In particular, in the two Higgs doublet model with the softly broken discrete symmetry, there are four types of Yukawa interactions, so that they can be discriminated by measuring the pattern of deviations in Yukawa coupling constants. Furthermore, we can fingerprint various extended Higgs sectors with future precision data by detecting the pattern of deviations in the coupling constants of the standard model-like Higgs boson. We demonstrate how the pattern of deviations can be different among various Higgs sectors that predict the electroweak rho parameter to be unity, such as models with additional an isospin singlet, a doublet, triplets, or a septet. We conclude that, as long as the gauge coupling constant of the Higgs boson slightly differs from the standard model prediction but is enough to be detected at the LHC and its high-luminosity run or at the International Linear Collider, we can identify the nonminimal Higgs sector even without direct discovery of additional Higgs bosons at the LHC.. |

30. | Basudeb Dasgupta, Ernest Ma, Koji Tsumura, Weakly interacting massive particle dark matter and radiative neutrino mass from Peccei-Quinn symmetry, Physical Review D - Particles, Fields, Gravitation and Cosmology, 10.1103/PhysRevD.89.041702, 89, 4, 2014.02, The Peccei-Quinn anomalous global U(1)PQ symmetry is important for solving the strong CP problem with a cosmologically relevant axion. We add to this the simple (but hitherto unexplored) observation that it also has a residual Z2 symmetry which may be responsible for a second component of dark matter, i.e., an absolutely stable weakly interacting singlet scalar. This new insight provides a theoretical justification for this simplest of all possible dark-matter models. It also connects with the notion of generating radiative neutrino mass through dark matter. Two such specific realizations are proposed. In our general scenario, dark-matter detection is guaranteed at existing direct-detection experiments and/or axion searches. Observable signals at the Large Hadron Collider are discussed.. |

31. | Kaori Fuyuto, Junji Hisano, Natsumi Nagata, Koji Tsumura, QCD corrections to quark (chromo)electric dipole moments in high-scale supersymmetry, Journal of High Energy Physics, 10.1007/JHEP12(2013)010, 2013, 12, 2013.12, Recent results from the LHC experiments, both for the Higgs mass measurement and the direct search for supersymmetric (SUSY) particles, might indicate that the SUSY breaking scale is much higher than the electroweak scale. Although it is difficult to investigate such a scenario at collider experiments, the measurement of the hadronic electric dipole moments is one of promising ways to detect the effects of the SUSY particles. These effects are expressed in terms of the CP-violating effective operators defined at the SUSY breaking scale, which involve quarks, gluons, photons, and gluinos. In this paper, we discuss the QCD corrections to the effective operators in the high-scale SUSY scenario. To appropriately evaluate the radiative corrections in the presence of large mass hierarchy among the SUSY particles, we exploit an effective theoretical approach based on the renormalization-group equations. As a result, it is found that the low-energy quark electric and chromoelectric dipole moments may differ from those evaluated in previous works by O (100) % and O (10) %, respectively.. |

32. | Shinya Kanemura, Koji Tsumura, Hiroshi Yokoya, Determination of tanâ¡β from the Higgs boson decay at linear colliders, Physical Review D - Particles, Fields, Gravitation and Cosmology, 10.1103/PhysRevD.88.055010, 88, 5, 2013.09, In the two Higgs doublet models, tanâ¡β is an important parameter, which is defined as the ratio of the vacuum expectation values of the doublets. We study how accurately tanâ¡β can be determined at linear colliders via the precision measurement of the decay branching fraction of the standard model (SM)-like Higgs boson. Since the effective coupling constants of the Higgs boson with the weak gauge bosons are expected to be measured accurately, the branching ratios can be precisely determined. Consequently, tanâ¡β can be determined with a certain amount of accuracy. Comparing the method to those using direct production of the additional Higgs bosons, we find that, depending on the type of Yukawa interactions, the precision measurement of the decay of the SM-like Higgs boson can be the best way to determine tanâ¡β, when the deviations in the coupling constants with the gauge boson from the SM prediction are observed at linear colliders.. |

33. | Koji Tsumura, Discrimination of models including H ^{±±} by using tau decay, 23rd Hadron Collider Physics Symposium 2012, HCP 2012
EPJ Web of Conferences, 10.1051/epjconf/20134918004, 49, 2013.09, The doubly charged scalar boson (H^{±±}) is introduced in several models of the new physics beyond the standard model. The H^{±±} can have Yukawa interactions with two left-handed charged leptons or two righthanded charged leptons. We study kinematical properties of H^{±±} decay products through tau leptons in order to discriminate the chiral structures of the new Yukawa interaction. The chirality of tau leptons can be measured by the energy distributions of the tau decay products, and thus the chiral structure of the new Yukawa interaction can be traced in the invariant-mass distributions of the H ^{±±} decay products [1].. |

34. | Junji Hisano, Koji Tsumura, Higgs boson mixes with an SU(2) septet representation, Physical Review D - Particles, Fields, Gravitation and Cosmology, 10.1103/PhysRevD.87.053004, 87, 5, 2013.03, We study the possibility of the Higgs boson, which consists of an SU(2) doublet and a septet. The vacuum expectation value of a septet with hypercharge Y=2 is known to preserve the electroweak rho parameter unity at the tree level. Therefore, the septet can give a significant contribution to the electroweak symmetry breaking. Because of the mixing with the septet, the gauge coupling of the standard-model-like Higgs boson is larger than that in the standard model. We show the sizable vacuum expectation value of the Higgs septet can be allowed under the constraint from the electroweak precision data. The signal strengths of the Higgs boson for the diphoton and a pair of weak gauge boson decay channels at the LHC are enhanced, while those for the fermionic decay modes are suppressed. The mass of additional neutral Higgs boson is also bounded by the current LHC data for the standard model Higgs boson. We discuss the phenomenology of the multiply charged Higgs bosons, which come from the septet.. |

35. | Masaya Kohda, Hiroaki Sugiyama, Koji Tsumura, Lepton number violation at the LHC with leptoquark and diquark, Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics, 10.1016/j.physletb.2012.12.048, 718, 4-5, 1436-1440, 2013.01, We investigate a model in which tiny neutrino masses are generated at the two-loop level by using scalar leptoquark and diquark multiplets. The diquark can be singly produced at the LHC, and it can decay into a pair of leptoquarks through the lepton number violating interaction. Subsequent decays of the two leptoquarks can provide a clear signature of the lepton number violation, namely two QCD jets and a pair of same-signed charged leptons without missing energy. We show that the signal process is not suppressed while neutrino masses are appropriately suppressed.. |

36. | Hiroaki Sugiyama, Koji Tsumura, Hiroshi Yokoya, Discrimination of models including doubly charged scalar bosons by using tau lepton decay distributions, Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics, 10.1016/j.physletb.2012.09.044, 717, 1-3, 229-234, 2012.10, The doubly charged scalar boson (H ^{±±}) is introduced in several models of the new physics beyond the standard model. The H ^{±±} has Yukawa interactions with two left-handed charged leptons or two right-handed charged leptons depending on the models. We study kinematical properties of H ^{±±} decay products through tau leptons in order to discriminate the chiral structures of the new Yukawa interaction. The chirality of tau leptons can be measured by the energy distributions of the tau decay products, and thus the chiral structure of the new Yukawa interaction can be traced in the invariant-mass distributions of the H ^{±±} decay products. We perform simulation studies for the typical decay patterns of the H ^{±±} with simple event selections and tau-tagging procedures, and show that the chiral structure of the Yukawa interactions of H ^{±±} can be distinguished by measuring the invariant-mass distributions.. |

37. | Junji Hisano, Koji Tsumura, Masaki J.S. Yang, QCD corrections to neutron electric dipole moment from dimension-six four-quark operators, Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics, 10.1016/j.physletb.2012.06.038, 713, 4-5, 473-480, 2012.07, In this Letter, the renormalization-group equations for the (flavor-conserving) CP-violating interaction are derived up to the dimension six, including all the four-quark operators, at one-loop level. We apply them to the models with the neutral scalar boson or the color-octet scalar boson which have CP-violating Yukawa interactions with quarks, and discuss the neutron electric dipole moment in these models.. |

38. | Shin Ichi Kawada, Nozomi Maeda, Tohru Takahashi, Katsumasa Ikematsu, Keisuke Fujii, Yoshimasa Kurihara, Koji Tsumura, Daisuke Harada, Shinya Kanemura, Feasibility study of the measurement of Higgs pair creation at a photon linear collider, Physical Review D - Particles, Fields, Gravitation and Cosmology, 10.1103/PhysRevD.85.113009, 85, 11, 2012.06, We studied the feasibility of the measurement of Higgs pair creation at a photon linear collider. From the sensitivity to the anomalous self-coupling of the Higgs boson, the optimum γγ collision energy was found to be around 270 GeV for a Higgs mass of 120GeV/c2. We found that large backgrounds such as γγ→W ^{+}W ^{-}, ZZ, and bb̄bb̄ can be suppressed if correct assignment of tracks to parent partons is achieved and Higgs pair events can be observed with a statistical significance of ∼5σ by operating the photon linear collider for 5 years.. |

39. | Shinya Kanemura, Koji Tsumura, Hiroshi Yokoya, Multi-τ signatures at the LHC in the two Higgs doublet model, Physical Review D - Particles, Fields, Gravitation and Cosmology, 10.1103/PhysRevD.85.095001, 85, 9, 2012.05, A detailed simulation study is performed for multi-τ signatures at the Large Hadron Collider, which can be used to probe additional Higgs bosons with lepton-specific Yukawa interactions. We here consider the two Higgs doublet model with the Type-X Yukawa interaction, where nonstandard Higgs bosons predominantly decay into tau leptons. These extra Higgs bosons can be pair produced at hadron colliders. Consequently, multi-τ originated signals appear in the final state as a promising signature of such a model. We find that the main background can be considerably reduced by requiring the high multiplicity of leptons and tau-jets with appropriate kinematical cuts in the final state. Thus, assuming the integrated luminosity of a hundred of inverse fb, the excess can be seen in various three- and four-lepton channels. With the integrated luminosity of thousands of inverse fb, the determination of the mass as well as ratios of leptonic decay branching ratios of these Higgs bosons would also be possible.. |

40. | Cheng Wei Chiang, Takaaki Nomura, Koji Tsumura, Search for doubly charged Higgs bosons using the same-sign diboson mode at the LHC, Physical Review D - Particles, Fields, Gravitation and Cosmology, 10.1103/PhysRevD.85.095023, 85, 9, 2012.05, Doubly charged Higgs bosons are predicted in many new physics models with an extended Higgs sector that contains a Higgs triplet field. Current experimental searches have been focusing mainly on the scenario in which the same-sign dilepton decay modes are the dominant ones. We study the scenario where the vacuum expectation value of the triplet field is sufficiently large so that the associated charged Higgs bosons decay dominantly to a pair of weak gauge bosons instead. A detailed simulation of the signal and the backgrounds is performed for the CERN Large Hadron Collider (LHC) at the collision energy of 8 TeV and 14 TeV. We find that different cuts should be imposed for the events, depending on whether the doubly charged Higgs boson mass is greater than about 200 GeV. In the higher mass region, the forward jet tagging proves to be useful in enhancing the signal significance. We show the discovery reach of the LHC running at 8 and 14 TeV, with two benchmark triplet vacuum expectation values. With an integrated luminosity of 10fb ^{-}1 at 8 TeV, the doubly charged Higgs boson with a mass of ∼180GeV can be tested at 5σ level in such a scenario.. |

41. | Bo Ren, Koji Tsumura, Xiao Gang He, Higgs quadruplet for the type III seesaw model and implications for μ→eγ and μ-e conversion, Physical Review D - Particles, Fields, Gravitation and Cosmology, 10.1103/PhysRevD.84.073004, 84, 7, 2011.10, In the type III seesaw model, the heavy neutrinos are contained in leptonic triplet representations. The Yukawa couplings of the triplet fermion and the left-handed neutrinos with the doublet Higgs field produce the Dirac mass terms. Together with the Majorana masses for the leptonic triplets, the light neutrinos obtain nonzero seesaw masses. We point out that it is also possible to have a quadruplet Higgs field to produce the Dirac mass terms to facilitate the seesaw mechanism. The vacuum expectation value of the quadruplet Higgs is constrained to be small by electroweak precision data. Therefore, the Yukawa couplings of a quadruplet can be much larger than those for a doublet. We also find that unlike the usual type III seesaw model where at least two copies of leptonic triplets are needed, with both doublet and quadruplet Higgs representations, just one leptonic triplet is possible to have a phenomenologically acceptable model because light neutrino masses can receive sizable contributions at both tree and one-loop levels. Large Yukawa couplings of the quadruplet can induce observable effects for lepton flavor violating processes μ→eγ and μ-e conversion. Implications of the recent μ→eγ limit from MEG and the limit on μ-e conversion on Au are also given. Some interesting collider signatures for the doubly charged Higgs boson in the quadruplet are discussed.. |

42. | Shinya Kanemura, Yasuhiro Okada, Hiroyuki Taniguchi, Koji Tsumura, Indirect bounds on heavy scalar masses of the two-Higgs-doublet model in light of recent Higgs boson searches, Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics, 10.1016/j.physletb.2011.09.035, 704, 4, 303-307, 2011.10, We study an upper bound on masses of additional scalar bosons from the electroweak precision data and theoretical constraints such as perturbative unitarity and vacuum stability in the two-Higgs-doublet model taking account of recent Higgs boson search results. If the mass of the Standard-Model-like Higgs boson is rather heavy and is outside the allowed region by the electroweak precision data, such a discrepancy should be compensated by contributions from the additional scalar bosons. We show the upper bound on masses of the additional scalar bosons to be about 2 (1) TeV for the mass of the Standard-Model-like Higgs boson to be 240 (500) GeV.. |

43. | N. G. Deshpande, T. Enkhbat, T. Fukuyama, X. G. He, L. H. Tsai, K. Tsumura, μ-e conversion with four generations, Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics, 10.1016/j.physletb.2011.08.045, 703, 5, 562-566, 2011.09, We study μ-e conversion with sequential four generations. A large mass for the fourth generation neutrino can enhance the conversion rate by orders of magnitude. We compare constraints obtained from μ-e conversion using experimental bounds on various nuclei with those from μ→eγ and μ→eēe. We find that the current bound from μ-e conversion with Au puts the most stringent constraint in this model. The relevant flavor changing parameter λ_{;μe}=V^{*}4_{μ}Ve4 is constrained to be less than 1.6×10^{-}5 for the fourth generation neutrino mass larger than 100 GeV. Implications for future μ-e conversion, μ→eγ and μ→eēe experiments are discussed.. |

44. | Takeshi Fukuyama, Hiroaki Sugiyama, Koji Tsumura, Phenomenology in the Zee model with the A_{4} symmetry, Physical Review D - Particles, Fields, Gravitation and Cosmology, 10.1103/PhysRevD.83.056016, 83, 5, 2011.03, The Zee model generates neutrino masses at the one-loop level by adding charged SU(2)_{L}-singlet and extra SU(2)_{L}-doublet scalars to the standard model of particle physics. We introduce the softly broken A _{4} symmetry to the Zee model as the origin of the nontrivial structure of the lepton flavor mixing. This model is compatible with the tribimaximal mixing which agrees well with neutrino oscillation measurements. Then, a sum rule m_{1}eiα_{12}+2m_{2}+3m_{3}eiα _{32}=0 is obtained and it results in Δm312<0 and m_{3} ≥1.8×10^{-}2eV. The effective mass |(M_{ν}) _{ee}| for the neutrinoless double beta decay is predicted as |(M _{ν})_{ee}|≥1.7×10^{-}2eV. The characteristic particles in this model are SU(2)_{L}-singlet charged Higgs bosons sα+(α=ξ,η,ζ) which are made from a 3 representation of A_{4}. Contributions of sα± to the lepton flavor violating decays of charged leptons are almost forbidden by an approximately remaining Z_{3} symmetry; only BR(τ→ēμ μ) can be sizable by the flavor changing neutral current interaction with SU(2)_{L}-doublet scalars. Therefore, sα± can easily be light enough to be discovered at the LHC with satisfying current constraints. The flavor structures of BR(sα-→ℓν) are also discussed.. |

45. | K. Tsumura, Higgs boson pair production in new physics models at hadron, lepton, and photon colliders, Nuovo Cimento della Societa Italiana di Fisica C, 10.1393/ncc/i2011-11007-8, 34, SUPP1, 77-84, 2011.01, We discuss Higgs boson pair production processes at future hadron and lepton colliders including the photon collision option in several new physics models, i.e. the two-Higgs-doublet model, the scalar leptoquark model, the sequential fourth-generation fermion model and the vectorlike quark model. Cross sections for these processes can differ significantly from the standard model predictions, due to the nondecoupling effect in the triple Higgs boson coupling constant. For the one-loop induced processes such as gg → hh and γγ → hh, new physics particles can mediate in a loop diagram, which can affect cross sections. Measurements of the Higgs boson pair production processes can be useful not only to test the standard model but also to explore new physics models.. |

46. | Naoyuki Haba, Koji Tsumura, ν-Two Higgs doublet model and its collider phenomenology, Journal of High Energy Physics, 10.1007/JHEP06(2011)068, 2011, 6, 2011.01, Smallness of neutrino masses can be explained by introducing a tiny vacuum expectation value of an extra-Higgs doublet which couples to right-handed neutrinos (NR). This situation is naturally realized in ν-Two Higgs Doublet Model (νTHDM), where a TeV-scale seesaw mechanism can work well. We investigate observable phenomenology of νTHDM at LHC and ILC experiments. Charged Higgs boson (H^{±}) in νTHDM is almost originated from the extra-Higgs doublet and its coupling strength to neutrinos are not small. Then this model induces rich phenomenology at the LHC, for example, when m _{H±} > MN, observable charged tracks can be induced from long lived charged Higgs. On the other hand, when mH ^{±} < MN, right-handed neutrinos can be long-lived, and secondary vertices may be tagged at the LHC. The νTHDM also predicts observable lepton number violating process at the ILC.. |

47. | Eri Asakawa, Daisuke Harada, Shinya Kanemura, Yasuhiro Okada, Koji Tsumura, Higgs boson pair production in new physics models at hadron, lepton, and photon colliders, Physical Review D - Particles, Fields, Gravitation and Cosmology, 10.1103/PhysRevD.82.115002, 82, 11, 2010.12, We study Higgs boson pair production processes at future hadron and lepton colliders including the photon collision option in several new physics models; i.e., the two-Higgs-doublet model, the scalar leptoquark model, the sequential fourth generation fermion model and the vectorlike quark model. Cross sections for these processes can deviate significantly from the standard model predictions due to the one-loop correction to the triple Higgs boson coupling constant. For the one-loop induced processes such as gg→hh and γγ→hh, where h is the (lightest) Higgs boson and g and γ respectively represent a gluon and a photon, the cross sections can also be affected by new physics particles via additional one-loop diagrams. In the two-Higgs-doublet model and scalar leptoquark models, cross sections of e ^{+}e^{-}→hhZ and γγ→hh can be enhanced due to the nondecoupling effect in the one-loop corrections to the triple Higgs boson coupling constant. In the sequential fourth generation fermion model, the cross section for gg→hh becomes very large because of the loop effect of the fermions. In the vectorlike quark model, effects are small because the theory has decoupling property. Measurements of the Higgs boson pair production processes can be useful to explore new physics through the determination of the Higgs potential.. |

48. | Takeshi Fukuyama, Hiroaki Sugiyama, Koji Tsumura, Phenomenology in the Higgs triplet model with the A4 symmetry, Physical Review D - Particles, Fields, Gravitation and Cosmology, 10.1103/PhysRevD.82.036004, 82, 3, 2010.08, We discuss the phenomenology of doubly and singly charged Higgs bosons [of SU(2)L-triplet fields] in the simplest A4-symmetric version of the Higgs triplet model. Mass eigenstates of these Higgs bosons are obtained explicitly from the Higgs potential. It is shown that their decays into a pair of leptons have unique flavor structures which can be tested at the LHC if some of their masses are below the TeV scale. Sizable decay rates for τ→μ̄ee and τ→ēμμ can be obtained naturally, while other τ→̄^{′′′}, μ→ēee, and →^{′}γ, are almost forbidden in this model. Contributions of these Higgs bosons to the nonstandard interactions of neutrinos are also considered.. |

49. | Koji Tsumura, Liliana Velasco-Sevilla, Phenomenology of flavon fields at the LHC, Physical Review D - Particles, Fields, Gravitation and Cosmology, 10.1103/PhysRevD.81.036012, 81, 3, 2010.02, We study low energy constraints from flavor violating processes, production, and decay at the LHC of a scalar field φ (flavon) associated to the breaking of a nonsupersymmetric Abelian family symmetry at the TeV scale. This symmetry is constrained to reproduce fermion masses and mixing, up to O(1) coefficients. The nonsupersymmetric gauged U(1) models considered are severely restricted by cancellation of anomalies and LEP bounds on contact interactions; consequently its phenomenology is out of the LHC reach. We therefore introduce an effective U(1) which is not gauged and is broken explicitly by a CP-odd term at the TeV scale. This helps us to explore flavor violating processes, production, and decay at the LHC for these kind of light scalars. In this context we first study the constraints on the flavon mass and its vacuum expectation value from low energy flavor changing processes such as μ→eγ. We find that a flavon of about mφ 150GeV could be experimentally allowed. These kinds of flavons could be significantly generated at the LHC via the gluon fusion mechanism and the single top production channel gu→tφ. The produced flavons can have characteristic decay modes such as tc̄ for mφ mt, and τμ̄ for mφ mt, which could be effectively useful to detect flavons.. |

50. | Takeshi Fukuyama, Hiroaki Sugiyama, Koji Tsumura, Constraints from LFV processes in the Higgs Triplet Model, Journal of High Energy Physics, 10.1007/JHEP03(2010)044, 2010, 3, 2010.01, Constraints from lepton flavor violating processes are translated into lower bounds on vΔmH±± in the Higgs Triplet Model by considering correlations through the neutrino mass matrix. It is shown that μ → eγ, rare τ decays (especially, τ → μ̄ee), and the muonium conversion can give a more stringent bound on vΔlmH± ± in some parameter regions than the bound from μ → ēee which is expected naively to give the most stringent one. We consider the cases of suppressed μ → ēee not only for CP-conserving sets of phases but also for arbitrary values.. |

51. | K. Tsumura, Lepton flavour violation at an electron-photon collider, Nuovo Cimento della Societa Italiana di Fisica C, 10.1393/ncc/i2010-10597-9, 33, 2, 103-108, 2010.01, We study the LFV Higgs production processes e^{-}γ → ℓ^{-} φ (ℓ = μ, τ; φ = H, A) as a probe of Higgs-mediated LFV couplings at an electron-photon collider, where H and A are extra CP even and odd Higgs bosons, respectively, in the two Higgs doublet model. Under the constraints from the current data of muon and tau rare decay, the cross-section can be significantly large. It would improve the experimental upper bounds on the effective LFV coupling constants. In addition, the chirality nature of the LFV Higgs coupling constants can be measured by selecting electron beam polarizations. PACS 12.15. Ji - Applications of electroweak models to specific processes. PACS 12.60.Fr - Extensions of electroweak Higgs sector. PACS 14.60.-z - Leptons. PACS 14.60.Fg-Taus.. |

52. | Shinya Kanemura, Koji Tsumura, Effects of the anomalous Higgs couplings on the Higgs boson production at the Large Hadron Collider, European Physical Journal C, 10.1140/epjc/s10052-009-1077-0, 63, 1, 11-21, 2009.09, We study the impact of dimension-six operators on single- and double-Higgs production rates via gluon fusion at the Large Hadron Collider (LHC). If the top-Yukawa coupling is modified by some new physics whose scale is of the TeV scale, its effect changes the cross sections of single-Higgs production gg→H and double-Higgs production gg→HH through the top-loop diagram. In particular, double-Higgs production can receive significant enhancement from the effective top-Yukawa coupling and the new dimension-five coupling which are induced by the dimension-six operator. Comparing these results to the forthcoming data at the LHC, one can extract information of the dimension-six operators relevant to the top quark and the Higgs boson.. |

53. | Mayumi Aoki, Shinya Kanemura, Koji Tsumura, Kei Yagyu, Models of Yukawa interaction in the two Higgs doublet model, and their collider phenomenology, Physical Review D - Particles, Fields, Gravitation and Cosmology, 10.1103/PhysRevD.80.015017, 80, 1, 2009.08, Possible models of Yukawa interaction are discussed in the two Higgs doublet model (THDM) under the discrete symmetry imposed to avoid the flavor changing neutral current at the leading order. It is known that there are four types of such models corresponding to the possible different assignment of charges for the discrete symmetry on quarks and leptons. We first examine the decay properties of Higgs bosons in each type model, and summarize constraints on the models from current experimental data. We then shed light on the differences among these models in collider phenomenology. In particular, we mainly discuss the so-called type-II THDM and type-X THDM. The type-II THDM corresponds to the model with the same Yukawa interaction as the minimal supersymmetric standard model. On the other hand, in the type-X THDM, additional Higgs bosons can predominantly decay into leptons. This scenario may be interesting because of the motivation for a light charged Higgs boson scenario such as in the TeV-scale model of neutrinos, dark matter, and baryogenesis. We study how we can distinguish the type-X THDM from the minimal supersymmetric standard model at the Large Hadron Collider and the International Linear Collider.. |

54. | Shinya Kanemura, Koji Tsumura, A powerful tool for measuring Higgs boson associated lepton flavour violation, Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics, 10.1016/j.physletb.2009.03.041, 674, 4-5, 295-298, 2009.04, In models with extended Higgs sectors, Higgs-boson-mediated Lepton Flavour Violation (LFV) can naturally appear. We study the physics potential of an electron-photon collider on searching LFV processes e^{-} γ → ℓ^{-} φ (ℓ = μ, τ; φ = H, A) where H and A are extra CP even and odd Higgs bosons, respectively, in the minimal supersymmetric standard model and the effective two Higgs doublet model. The production cross section can be significantly large for the maximal allowed values of the LFV coupling constants under the current experimental data. Present experimental upper bounds on the effective LFV coupling constants would be considerably improved by searching these processes, which would be better than MEG and COMET experiments and also those at LHCb and SuperKEKB. Moreover, one can separately measure chirality of effective LFV coupling constants via these processes by selecting electron polarizations.. |

55. | Eri Asakawa, Daisuke Harada, Shinya Kanemura, Yasuhiro Okada, Koji Tsumura, Higgs boson pair production at a photon-photon collision in the two Higgs doublet model, Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics, 10.1016/j.physletb.2009.01.050, 672, 4-5, 354-360, 2009.03, We calculate the cross section of Higgs boson pair production at a photon collider in the two Higgs doublet model. We focus on the scenario in which the lightest CP even Higgs boson (h) has the Standard Model like couplings to the gauge bosons. We take into account the one-loop correction to the hhh coupling as well as additional one-loop diagrams due to charged Higgs bosons to the γ γ → h h helicity amplitudes. It is found that the full cross section can be enhanced by both these effects to a considerable level. We discuss the impact of these corrections on the hhh coupling measurement at the photon collider.. |

56. | Shinya Kanemura, Koichi Matsuda, Toshihiko Ota, Serguey Petcov, Tetsuo Shindou, Eiichi Takasugi, Koji Tsumura, CP violation due to multi-Froggatt-Nielsen fields, European Physical Journal C, 10.1140/epjc/s10052-007-0343-2, 51, 4, 927-931, 2007.08, We study how to incorporate CP violation in the Froggatt-Nielsen (FN) mechanism. To this end, we introduce non-renormalizable interactions with a flavor democratic structure to the fermion mass generation sector. It is found that at least two iso-singlet scalar fields with a discrete symmetry imposed are necessary to generate CP violation due to the appearance of the relative phase between their vacuum expectation values. In the simplest model, the ratios of quark masses and the Cabibbo-Kobayashi-Maskawa (CKM) matrix including the CP violating phase are determined by the CKM element |V_{us}| and the ratio of two vacuum expectation values of FN fields, R=|R|e^{iα} (a magnitude and a phase). It is demonstrated how the angles φ_{i} (i=1,...,3) of the unitarity triangle and the CKM off-diagonal elements |V _{ub}| and |V_{cb}| are predicted as a function of |V _{us}|, |R| and α. Although the predicted value of the CP violating phase does not agree with the experimental data within the simplest model, the basic idea of our scenario would be promising if one wants to construct a more realistic model of flavor and CP violation.. |

57. | Shinya Kanemura, Daisuke Nomura, Koji Tsumura, Dimension-six top-quark Higgs-boson interaction and its effect in collider phenomenology, Physical Review D - Particles, Fields, Gravitation and Cosmology, 10.1103/PhysRevD.74.076007, 74, 7, 2006.11, Measurement of the Yukawa interaction between the top quark and the Higgs boson should be useful to clarify the mechanism of fermion mass generation. We discuss the impact of nonstandard interactions characterized by dimension-six operators on the effective top Yukawa coupling. The cross section of the process e-e+→W-W+νν̄→tt̄νν̄ is calculated including these operators, and possible deviation from the standard model prediction is evaluated under the constraint from perturbative unitarity and current experimental data. We find that if the new physics scale is in a TeV region, the cross section can be significantly enhanced due to the nonstandard interactions. Such a large effect should be detectable at the International Linear Collider.. |

58. | Shinya Kanemura, Toshihiko Ota, Koji Tsumura, Lepton flavor violation in Higgs boson decays under the rare tau decay results, Physical Review D - Particles, Fields, Gravitation and Cosmology, 10.1103/PhysRevD.73.016006, 73, 1, 2006.01, We study lepton flavor violation (LFV) associated with tau leptons in the framework of the two Higgs doublet model, in which LFV couplings are introduced as a deviation from Model II Yukawa interaction. Parameters of the model are constrained from experimental results and also from requirements of theoretical consistencies such as vacuum stability and perturbative unitarity. Current data for rare tau decays provide substantial upper limits on the LFV Yukawa couplings in the large tanβ region (tanβ30), which are comparable with predictions in fundamental theories. Here tanβ is the ratio of vacuum expectation values of the two Higgs doublets. We show that a search for the LFV decays 0→τ±μ (τ±e) of neutral Higgs bosons (0=h,H and A) at future collider experiments can be useful to further constrain the LFV couplings especially in the relatively small tanβ region (tanβ30), where rare tau decay data cannot give any strong limit.. |

59. | S. Kanemura, K. Matsuda, H. Nakano, T. Ota, T. Shindou, E. Takasugi, K. Tsumura, Bi-maximal mixing at GUT, the low energy data and the leptogenesis, Nuclear Physics B - Proceedings Supplements, 10.1016/j.nuclphysbps.2005.05.067, 149, 357-359, 2005.12, In the framework of the minimum supersymmetric model with right-handed neutrinos, we consider the Bi-maximal mixing which is realized at the GUT scale and discuss a question that this model can reproduce the low energy phenomena and the leptogenesis.. |

60. | S. Kanemura, K. Matsuda, T. Ota, T. Shindou, E. Takasugi, K. Tsumura, Yukawa matrix for the neutrino and Lepton Flavour Violation, Nuclear Physics B - Proceedings Supplements, 10.1016/j.nuclphysbps.2005.05.068, 149, 360-362, 2005.12, We estimate the magnitude of Lepton Flavour Violation (LFV) from the phase of the neutrino Yukawa matrix. In the minimal supersymmetric standard model with right-handed neutrinos, the LFV processes l_{i} → l_{jγ} can appear through the slepton mixing, which comes from the renormalization group effect on the right-handed neutrino Yukawa interaction between the Grand Unified Theory scale and the heavy right-handed neutrino mass scale. Two types of phases exist in the neutrino Yukawa matrix. One is the Majorana phase, which can change the magnitude of the LFV branching ratios by a few factor. The other phases relate for the size of the Yukawa hierarchy and its phase effect can change the LFV branching ratios by several orders of magnitude.. |

61. | Shinya Kanemura, Koichi Matsuda, Toshihiko Ota, Tetsuo Shindou, Eiichi Takasugi, Koji Tsumura, Enhancement of lepton flavor violation in a model with bimaximal mixing at the grand unification scale, Physical Review D - Particles, Fields, Gravitation and Cosmology, 10.1103/PhysRevD.72.093004, 72, 9, 2005.11, We study phenomenological predictions in the scenario with the quasidegenerate relation among neutrino Dirac masses, mD1mD2 |

62. | Shinya Kanemura, Koichi Matsuda, Toshihiko Ota, Tetsuo Shindou, Eiichi Takasugi, Koji Tsumura, Erratum Phase effects from the general neutrino Yukawa matrix on lepton flavor violation (Physical Review D - Particles, Fields, Gravitation and Cosmology (2005) 72 (055012)), Physical Review D - Particles, Fields, Gravitation and Cosmology, 10.1103/PhysRevD.72.059904, 72, 5, 2005.09. |

63. | Shinya Kanemura, Koichi Matsuda, Toshihiko Ota, Tetsuo Shindou, Eiichi Takasugi, Koji Tsumura, Phase effects from the general neutrino Yukawa matrix on lepton flavor violation, Physical Review D - Particles, Fields, Gravitation and Cosmology, 10.1103/PhysRevD.72.055012, 72, 5, 2005.09, We examine contributions from Majorana phases to lepton flavor violating processes in the framework of the minimal supersymmetric standard model with heavy right-handed neutrinos. All phases in the complex neutrino Yukawa matrix are taken into account in our study. We find that in the scenario with universal soft-breaking terms sizable phase effects can appear on the lepton flavor violating processes such as μ→eγ, τ→eγ, and τ→μγ. In particular, the branching ratio of μ→eγ can be considerably enhanced due to the Majorana phases, so that it can be much greater than that of τ→μγ.. |

64. | S. Kanemura, K. Matsuda, T. Ota, T. Shindou, E. Takasugi, K. Tsumura, Phase contribution to LFV, Nuclear Physics B - Proceedings Supplements, 10.1016/j.nuclphysbps.2005.01.189, 143, 1-3 SPEC. ISS., 2005.06. |

65. | S. Kanemura, K. Matsuda, T. Ota, T. Shindou, E. Takasugi, K. Tsumura, Radiative corrections of neutrino mass matrix and lepton flavor violation processes, Nuclear Physics B - Proceedings Supplements, 10.1016/j.nuclphysbps.2005.01.186, 143, 1-3 SPEC. ISS., 2005.06. |

66. | Shinya Kanemura, Koichi Matsuda, Toshihiko Ota, Tetsuo Shindou, Eiichi Takasugi, Koji Tsumura, Search for lepton flavor violation in the Higgs boson decay at a linear collider, Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics, 10.1016/j.physletb.2004.08.024, 599, 1-2, 83-91, 2004.10, We discuss possibility of direct search for lepton flavor violation (LFV) in Yukawa interaction by measuring the branching ratio for the decay of the lightest Higgs boson (h ^{0}) into a τ-μ pair at a linear collider (LC). We study the significance of the signal process, e ^{+}e ^{-}→Z*→Zh ^{0}→Zτ ^{±}μ ^{∓}, against the backgrounds such as e ^{+}e ^{-}→Zτ ^{+}τ ^{-}→Zτ ^{±}μ ^{∓}+ missings. After taking appropriate kinematic cuts, the number of the background event is considerably reduced, so that the signal can be visible when the branching ratio of h ^{0}→τ ^{±}μ ^{∓} is larger than about 10 ^{-4}. In a minimal supersymmetric Standard Model scenario, the effective coupling of h ^{0}τ ^{±}μ ^{∓} can be generated at the loop level due to the slepton mixing. When supersymmetric mass parameters are larger than TeV scales, the branching ratio can be as large as several times 10 ^{-4}. Therefore, the signal can be marginally visible at a LC. In the general two-Higgs-doublet model, the possible maximal value for the branching ratio of h ^{0}→τ ^{±}μ ^{∓} can reach to a few times 10 ^{-3} within the available experimental bound, so that we can obtain larger significance.. |

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