Kyohei Mukaida

Dynamics of oscillating scalar field in thermal environment

There often appear coherently oscillating scalar fields in particle physics motivated cosmological scenarios, which may have rich phenomenological consequences. Scalar fields should somehow interact with background thermal bath in order to decay into radiation at an appropriate epoch, but introducing some couplings to the scalar field makes the dynamics complicated. We investigate in detail the dynamics of a coherently oscillating scalar field, which has renormalizable couplings to another field interacting with thermal background. The scalar field dynamics and its resultant abundance are significantly modified by taking account of following effects: (1) thermal correction to the effective potential, (2) dissipation effect on the scalar field in thermal bath, (3) non-perturbative particle production events and (4) formation of non-topological solitons. There appear many time scales depending on the scalar mass, amplitude, couplings and the background temperature, which make the efficiencies of these effects non-trivial.

Dark Matter Production in Late Time Reheating

We estimate dark matter density for the Universe with a reheating temperature smaller than the mass of dark matter, assuming dark matter to be a weakly interacting massive particle. During the reheating process, an inaton decays and releases high energy particles, which are scattered inelastically by the thermal plasma and emit many particles. Dark matters are produced through these inelastic scattering processes and pair creation processes by high energy particles. We properly take account of the Landau-Pomeranchuk-Migdal eect on inelastic processes and show that the resultant energy density of dark matter is much larger than that estimated in the literature and can be consistent with that observed when the mass of dark matter is larger than O(100) GeV.

Dissipative Effects on Reheating after Inflation

The inflaton must convert its energy into radiation after inflation, which, in a conventional scenario, is caused by the perturbative inflaton decay. This reheating process would be much more complicated in some cases: the decay products obtain masses from an oscillating inflaton and thermal environment, and hence the conventional reheating scenario can be modified. We study in detail processes of particle production from the inflaton, their subsequent thermalization and evolution of inflaton/plasma system by taking dissipation of the inflaton in a hot plasma into account. It is shown that the reheating temperature is significantly affected by these effects.

Thermalization after/during reheating

A bstractIf reheating of the Universe takes place via Planck-suppressed decay, it seems that the thermalization of produced particles might be delayed, since they have large energy/small number densities and number violating large angle scatterings which decrease the momentum of particles by large amount are inefficient correspondingly. In this paper, we study the thermalization of such “under occupied” decay products in detail, following recent developments in understanding the thermalization of non-abelian plasma. Contrary to the above naive expectation, it is shown that in most cases thermalization after/during reheating occurs instantaneously by properly taking account of scatterings with small angles and of particles with small momenta. In particular, the condition for instantaneous thermalization before the completion of reheating is found to be

Inflationary primordial black holes for the LIGO gravitational wave events and pulsar timing array experiments

{\alpha^{{{8 \left/ {5} \right.}}}}\gg \left( {{{{{m_{\phi }}}} \left/ {{{M_{\mathrm{pl}}}}} \right.}} \right){{\left( {{{{M_{\mathrm{pl}}^2{\varGamma_{\phi }}}} \left/ {{m_{\phi}^3}} \right.}} \right)}^{{{1 \left/ {5} \right.}}}}

WIMP Dark Matter in a Well-Tempered Regime: A case study on Singlet-Doublets Fermionic WIMP

, which is much milder than that obtained in previous works with small angle scatterings taken into account.

Fate of Electroweak Vacuum during Preheating

Primordial black holes (PBHs) are one of the candidates to explain the gravitational wave (GW) signals observed by the LIGO detectors. Among several phenomena in the early Universe, cosmic inflation is a major example to generate PBHs from large primordial density perturbations. In this paper, we discuss the possibility to interpret the observed GW events as mergers of PBHs which are produced by cosmic inflation. The primordial curvature perturbation should be large enough to produce a sizable amount of PBHs and thus we have several other probes to test this scenario. We point out that the current pulsar timing array (PTA) experiments already put severe constraints on GWs generated via the second-order effects, and that the observation of the cosmic microwave background (CMB) puts severe restriction on its

The universe dominated by oscillating scalar with non-minimal derivative coupling to gravity

\mu

Fate of Z 2 symmetric scalar field

distortion. In particular, it is found that the scalar power spectrum should have a very sharp peak at

Particle production after inflation with non-minimal derivative coupling to gravity

k \sim 10^{6}