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Sapporo, Japan

Hokkai Gakuen University is a private university in Sapporo, Hokkaidō, Japan. The precursor of the school was founded in 1885, and it was chartered as a university in 1952. Wikipedia.


Seto O.,Hokkai Gakuen University
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2015

We show that the standard model (SM)-like Higgs boson may decay into neutrinos with a sizable decay branching ratio in one well-known two Higgs doublet model, so-called neutrinophilic Higgs model. This could happen if the mass of the lighter extra neutral Higgs boson is smaller than one half of the SM-like Higgs boson mass. The definite prediction of this scenario is that the rate of the SM-like Higgs boson decay into diphoton normalized by the SM value is about 0.9. In the case that a neutrino is Majorana particle, a displaced vertex of right-handed neutrino decay would be additionally observed. This example indicates that a large invisible Higgs boson decay could be irrelevant to dark matter. © 2015 American Physical Society. Source


Choi K.-Y.,Korea Astronomy and Space Science Institute | Seto O.,Hokkai Gakuen University
Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics | Year: 2014

We consider axino warm dark matter in a supersymmetric axion model with R-parity violation. In this scenario, axino with the mass ma~≃7keV can decay into photon and neutrino resulting in the X-ray line signal at 3.5 keV, which might be the origin of unidentified X-ray emissions from galaxy clusters and Andromeda galaxy detected by the XMM-Newton X-ray observatory. © 2014 Elsevier B.V. Source


Okada N.,University of Alabama | Seto O.,Hokkai Gakuen University
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2012

We propose a scenario with a fermion dark matter, where the dark matter particle used to be the Dirac fermion, but it takes the form of the Majorana fermion at a late time. The relic number density of the dark matter is determined by the dark matter asymmetry generated through the same mechanism as leptogenesis when the dark matter was the Dirac fermion. After efficient dark matter annihilation processes have frozen-out, a phase transition of a scalar field takes place and generates Majorana mass terms to turn the dark matter particle into the Majorana fermion. In order to address this scenario in detail, we propose two simple models. The first one is based on the Standard Model (SM) gauge group and the dark matter originates the SU(2) L doublet Dirac fermion, analogous to the Higgsino-like neutralino in supersymmetric models. We estimate the spin-independent/dependent elastic scattering cross sections of this late-time Majorana dark matter with a proton and find the possibility to discover it by the direct and/or indirect dark matter search experiments in the near future. The second model is based on the B-L gauged extension of the SM, where the dark matter is a SM singlet. Although this model is similar to the so-called Higgs portal dark matter scenario, the spin-independent elastic scattering cross section can be large enough to detect this dark matter in future experiments. © 2012 American Physical Society. Source


Choi K.-Y.,Asia Pacific Center for Theoretical Physics | Choi K.-Y.,Pohang University of Science and Technology | Seto O.,Hokkai Gakuen University
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2012

We show that a Dirac right-handed scalar neutrino can be dark matter (DM) as a weakly interacting massive particle in the neutrinophilic Higgs model. When the additional Higgs fields couple only to the leptonic sector through neutrino Yukawa couplings, the right number of relic density of DM can be obtained from thermal freeze-out of the DM annihilation into charged leptons and neutrinos. At present epoch, this tree-level annihilation into fermions is suppressed by the velocity of DM, and the one-loop annihilation cross section into γγ can be dominant because relevant coupling constants are different. Hence, the recently observed (tentative) gamma-ray line signal in the Fermi-Large Area Telescope can be naturally explained by the annihilation of right-handed sneutrino DM. © 2012 American Physical Society. Source


Okada N.,University of Alabama | Seto O.,Hokkai Gakuen University
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2014

It has been recently pointed out that the excess of the gamma ray spectrum in the Fermi bubbles at low latitude can be well explained by the annihilation of dark matter particles. The best-fit candidate corresponds to the annihilation of a dark matter with mass of around 62 GeV into bb̄ with the cross section, σvâ‰3.3×10-26 cm3/s, or the annihilation of a dark matter with mass of around 10 GeV into a tau lepton pair with the cross section, σvâ‰5.6×10-27 cm3/s. We point out that the Higgs portal dark matter models are perfectly compatible with this interpretation of the dark matter annihilation, satisfying other phenomenological constraints. We also show that the parameter region which reproduces the best-fit values can be partly explored by the future direct dark matter search at the XENON1T. © 2014 American Physical Society. Source

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