arXiv subjects

Keiko I. Nagao

Publications and source records attributed to Keiko I. Nagao.

31 records · Page 2Linked to original sources

arXiv2019

A new mechanism for generating particle number asymmetry through interactions

A new mechanism for generating particle number asymmetry (PNA) has been developed. This mechanism is realized with a Lagrangian including a complex scalar field and a neutral scalar field. The complex scalar carries U(1) charge which is associated with the PNA. It is written in terms of the condensation and Green's function, which is obtained with two-particle irreducible (2PI) closed time path (CTP) effective action (EA). In the spatially flat universe with a time-dependent scale factor, the time evolution of the PNA is computed. We start with an initial condition where only the condensation of the neutral scalar is non-zero. The initial condition for the fields is specified by a density operator parameterized by the temperature of the universe. With the above initial conditions, the PNA vanishes at the initial time and later it is generated through the interaction between the complex scalar and the condensation of the neutral scalar. We investigate the case that both the interaction and the expansion rate of the universe are small and include their effects up to the first order of the perturbation. The expanding universe causes the effects of the dilution of the PNA, freezing interaction and the redshift of the particle energy. As for the time dependence of the PNA, we found that PNA oscillates at the early time and it begins to dump at the later time. The period and the amplitude of the oscillation depend on the mass spectrum of the model, the temperature and the expansion rate of the universe.

hep-th↗

arXiv2018

Anisotropy of dark matter velocity distribution

Direct detection of dark matter with directional sensitivity has the potential to discriminate the dark matter velocity distribution. Especially, it will be suitable to discriminate isotropic distribution from anisotropic one. Analyzing data produced with Monte-Carlo simulation, required conditions for the discrimination is estimated. If energy threshold of detector is optimized, $O(10^3-10^4)$ event number is required to discriminate the anisotropy.

astro-ph.IM↗

arXiv2017

Measurement of anisotropy of dark matter velocity distribution using directional detection

Although the velocity distribution of dark matter is assumed to be generally isotropic, some studies have found that $\sim\hspace{-0.1cm}25$\% of the distribution can have anisotropic components. As the directional detection of dark matter is sensitive to both the recoil energy and direction of nuclear recoil, directional information can prove useful in measuring the distribution of dark matter. Using a Monte Carlo simulation based on the modeled directional detection of dark matter, we analyze the differences between isotropic and anisotropic distributions and show that the isotropic case can be rejected at a 90\% confidence level if $O(10^4)$ events can be obtained.

astro-ph.CO↗

arXiv2016

Creation and evolution of particle numberasymmetry in an expanding universe

We introduce a model which may generate particle number asymmetry in an expanding Universe. The model includes CP violating and particle number violating interactions. The model consists of a real scalar field and a complex scalar field. Starting with an initial condition specified by a density matrix, we show how the asymmetry is created through the interaction and how it evolves at later time. We compute the asymmetry using non-equilibrium quantum field theory and as a first test of the model, we study how the asymmetry evolves in the flat limit.

hep-ph↗

arXiv2014

Directional Dark Matter Search and Velocity Distribution

Directional detection of dark matter is the next generation experiment, which is expected to have better back ground rejection efficiency than conventional direct search. Another intriguing possibility of the experiment by means of the directional information is measurement the velocity distribution of dark matter. Especially, it will be potent to figure out whether the velocity distribution is anisotropic. Supposing three distribution models, we discuss the possibility in one of the directional dark matter searches, nuclear emulsion detector.

hep-ph↗

arXiv2013

Simplified Models for Dark Matter Interacting with Quarks

We investigate simplified models in which dark matter particles, taken to be either Dirac or Majorana fermions, couple to quarks via colored mediators. We determine bounds from colliders and direct detection experiments, and show how the interplay of the two leads to a complementary view of this class of dark matter models. Forecasts for future searches in light of the current constraints are presented.

hep-ph↗

arXiv2013

Measuring Dark Matter Distribution in Directional Direct Detection

Direct detection of dark matter with directional sensitivity offers not only measurement of both recoil energy and direction of dark matter, but also a way to understand dark matter distribution in the Galaxy. Maxwell distribution is usually supposed as the distribution near the Earth, however, deviation from that, caused by tidal streams in the Galaxy, has been suggested. We explore the possibility of distinguishing the distribution by direct detection using nuclear emulsions.

hep-ph↗

arXiv2012

Isospin Violating Dark Matter Search by Nuclear Emulsion Detector

Dark matter signal and its annual modulation of event number are observed by some direct searches in small mass region. However, the regions have been excluded by others. The isospin-violating dark matter is a hopeful candidate to explain the discrepancy. We study the possibility that a future project of dark matter search using nuclear emulsion can reach favored region by the isospin-violating dark matter. Since the detector has the directional sensitivity, it is expected to examine the region including the modulation property.

hep-ph↗

arXiv2012

Directional Search for Isospin-Violating Dark Matter with Nuclear Emulsion

Some of direct dark matter searches reported not only positive signals but also annual modulation of the signal event. However, the parameter spaces have been excluded by other experiments. Isospin violating dark matter solves the contradiction by supposing different coupling to proton and neutron. We study the possibility to test the favored parameter region by isospin violating dark matter model with the future detector of dark matter using the nuclear emulsion. Since the nuclear emulsion detector has directional sensitivity, the detector is expected to examine whether the annual modulations observed other experiments is caused by dark matter or background signals.

hep-ph↗

arXiv2011

Signatures of Dark Matter in Inert Triplet Models

In this talk, we will review the signatures of dark matter in two inert triplet models. For the first model with the hypercharge Y=0, the dark matter mass around 5.5 TeV is favored by both the WMAP data and the direct detection. In contrast, for the second model of Y=2, it is excluded by the direct detection experiments although dark matter with its mass around 2.8 TeV is allowed by WMAP.

hep-ph↗

arXiv2011

Dark Matter in Inert Triplet Models

We study the inert triplet models, in which the standard model (SM) is extended to have a new SU(2)L triplet scalar (Y=0 or 2) with an Z2 symmetry. We show that the neutral component of the triplet can be a good dark matter candidate. In particular, for the hypercharge Y=0 triplet model, the WMAP data favors the region where the dark matter mass is around 5.5 TeV, which is also consistent with the direct detection experiments. In contrast, for the Y=2 model, although dark matter with its mass around 2.8 TeV is allowed by WMAP, it is excluded by the direct detection experiments because the spin-independent cross section is enhanced by the Z mediated tree-level scattering process.

hep-ph↗

arXiv2010

Fate of the False Vacuum Revisited

We find the novel effect on the decay of a false vacuum in view of quantum field theory, which is induced by a field coupling to the scalar field related to a first-order phase transition. This effect of the environment can never be included in the traditional method using the effective potential, and, in fact, acts as dissipative and fluctuation effects on tunneling phenomena. We show that the decay of the false vacuum is drastically either enhanced or suppressed. It is also clarified what kind of interaction enhance or suppress the tunneling probability.

hep-ph↗

arXiv2009

LHC signature of supersymmetric models with non-universal sfermion masses

We study the LHC signature of the minimal supersymmetric standard model with non-universal sfermion masses. In the model, soft masses of gauginos and the 3rd generation of 10 of SU(5) are around the weak scale, while other sfermion soft mass is universal and around a few TeV. Such sfermion mass spectrum is motivated not only from flavor, CP and naturalness constraints but also from E_6 grand unified model with non-Abelian horizontal (flavor) symmetry. The characteristic signature of the model at the LHC is the dominance of the events with 4 b partons in the final state together with high rate of mildly boosted top quark arising from gluino decay. The prominent high p_T jet also arises from squark decay. We show it is possible to find the characteristic signature in the early stage of the LHC. The discrimination of our scenario from some CMSSM model points with similar signature may be possible with large integrated luminosity. The result of sparticle mass measurement using exclusive channel with the help of hemisphere analysis, and inclusive measurement of gluino and squark masses using M_{T2} and M_{T2}^{min} in some representative model points are presented.

hep-ph↗