Browsing by Author "Seto, Osamu"

Now showing 1 - 5 of 5
  • Thumbnail Image
    Item
    Galactic Center gamma ray excess from two Higgs doublet portal dark matter
    (American Physical Society, 2014-10-21) Okada, Nobuchika; Seto, Osamu; University of Alabama Tuscaloosa; Hokkai Gakuen University
    We consider a simple extension of the type-II two-Higgs-doublet model by introducing a real scalar as a candidate for dark matter in the present Universe. The main annihilation mode of the dark matter particle with a mass of around 31-40 GeV is into a b (b) over bar pair, and this annihilation mode suitably explains the observed excess of the gamma-ray flux from the Galactic Center. We identify the parameter region of the model that can fit the gamma-ray excess and satisfy phenomenological constraints, such as the observed dark matter relic density and the null results of direct dark matter search experiments. Most of the parameter region is found to be within the search reach of future direct dark matter detection experiments.
  • Thumbnail Image
    Item
    Gravitational waves from breaking of an extra U(1) in SO(10) grand unification
    (Oxford University Press, 2021) Okada, Nobuchika; Seto, Osamu; Uchida, Hikaru; University of Alabama Tuscaloosa; Hokkaido University
    Abstract In a class of gauged extended Standard Models (SMs), the breaking of the symmetry is not only a source for Majorana masses of right-handed (RH) neutrinos crucial for the seesaw mechanism, but also a source of stochastic gravitational wave (GW) background. Such extended models are well-motivated from the viewpoint of grand unification. In this paper, we discuss a successful ultraviolet completion of a extended SM by an grand unified model through an intermediate step of unification. With a parameter set that is compatible with the grand unification, we find that a first-order phase transition associated with the symmetry breaking can be strong enough to generate GWs with a detectable size of amplitude. We also find that the resultant GW amplitude reduces and its peak frequency becomes higher as the RH neutrino masses increase.
  • Thumbnail Image
    Item
    Gravitational waves from the minimal gauged U(1)(B-L) model
    (American Physical Society, 2019) Hasegawa, Taiki; Okada, Nobuchika; Seto, Osamu; Hokkaido University; University of Alabama Tuscaloosa
    An additional \(U\left(1\right)\) gauge interaction is one of the promising extensions of the standard model of particle physics. Among others, the \(U\left(1{\right)}_{B-L}\) gauge symmetry is particularly interesting because it addresses the origin of Majorana masses of right-handed neutrinos, which naturally leads to tiny light neutrino masses through the seesaw mechanism. We show that, based on the minimal \(U\left(1{\right)}_{B-L}\) model, the symmetry breaking of the extra \(U\left(1\right)\) gauge symmetry with its minimal Higgs sector in the early universe can exhibit the first-order phase transition and hence generate a large enough amplitude of stochastic gravitational wave radiation that is detectable in future experiments.
  • Thumbnail Image
    Item
    Inelastic extra U(1) charged scalar dark matter
    (American Physical Society, 2020) Okada, Nobuchika; Seto, Osamu; University of Alabama Tuscaloosa; Hokkaido University
    The null results in dark matter direct detection experiments imply the present scalar dark matter (DM) annihilation cross section to bottom quark pairs through the Higgs boson exchange is smaller than about \({10}^{-31}\text{}\text{}{\mathrm{cm}}^{3}/s\) for a wide DM mass range, which is much smaller than the required annihilation cross section for thermal relic DM. We propose models of a thermal relic DM with the present annihilation cross section being very suppressed. This property can be realized in an extra \(U\left(1\right)\) gauge interacting complex scalar DM, where the thermal DM abundance is determined by coannihilation through the gauge interaction while the present annihilation is governed by Higgs bosons exchange processes. An interaction between DM and the extra \(U\left(1\right)\) breaking Higgs field generates a small mass splitting between DM and its coannihilating partner so that coannihilation becomes possible and also the \({Z}^{\prime }\)-mediated scattering off with a nucleon in direct DM search becomes inelastic. We consider scalar dark matter in \(U\left(1{\right)}_{B-L}\), \(U\left(1{\right)}_{\left(B-L{\right)}_{3}}\) and \(U\left(1{\right)}_{{L}_{\mu }-{L}_{\tau }}\) extended models and identify viable parameter regions. We also discuss various implications to future DM detection experiments, the DM interpretation of the gamma-ray excess in the globular cluster 47 Tucanae, the muon anomalous magnetic moment, the Hubble tension and others.
  • Thumbnail Image
    Item
    Superheavy WIMP dark matter from incomplete thermalization
    (Elsevier, 2021) Okada, Nobuchika; Seto, Osamu; University of Alabama Tuscaloosa; Hokkaido University
    Although it is usually thought that a class of weakly interacting massive particle (WIMP) dark matters (DMs), which have the vector coupling with the Z boson, is denied by null results of the direct DM searches, such WIMP DMs are still viable if they are superheavy with the mass of \({m}_{DM}\gtrsim {10}^{9}\) GeV. In the future, the superheavy WIMP DMs can be searched up to \({m}_{DM}\simeq {10}^{12}\) GeV, which corresponds to the so-called neutrino floor limit. We show that the observed abundance of \({\Omega }_{\text{DM}}{h}^{2}\simeq 0.1\) for a superheavy WIMP DM can be reproduced by a suitable reheating temperature of \({T}_{R}\simeq {m}_{DM}/29\) after inflation, if the direct inflaton decay into DM is negligible or kinematically forbidden.