M. Yoshimura

Particle production and dissipative cosmic field

Large amplitude oscillation of cosmic field that may occur right after inflation and in the decay process of weakly interacting fields gives rise to violent particle production via the parametric resonance. In the large amplitude limit the problem of back reaction against the field oscillation is solved and the energy spectrum of created particles is determined in a semi-classical approximation. For large enough coupling or large enough amplitude the resulting energy spectrum is broadly distributed, implying larger production of high energy particles than what a simple estimate of the reheating temperature due to the Born formula would suggest.

Particle production and gravitino abundance after inflation

Thermal history after inflation is studied in a chaotic inflation model with supersymmetric couplings of the inflaton to matter fields. Time evolution equation is solved in a formalism that incorporates both the back reaction of particle production and the cosmological expansion. The effect of the parametric resonance gives rise to a rapid initial phase of the inflaton decay followed by a slow stage of the Born term decay. Thermalization takes place immediately after the first explosive stage for a medium strength of the coupling among created particles. As an application we calculate time evolution of the gravitino abundance that is produced by ordinary particles directly created from the inflaton decay, which typically results in much more enhanced yield than what a naive estimate based on the Born term would suggest.

Quantum dissipation and decay in a medium

Quantum dissipation in a thermal environment is investigated, using the path-integral approach. The reduced density matrix of the harmonic oscillator system coupled to a thermal bath of oscillators is derived for an arbitrary spectrum of bath oscillators. Time evolution and the end point of the two-body decay of unstable particles are then elucidated: After early transient times unstable particles undergo the exponential decay, followed by the power-law decay and finally ending in a mixed state of residual particles containing contributions from both on and off the mass shell, whose abundance does not suffer from the Boltzmann suppression.

Time evolution of unstable particle decay seen with finite resolution

Time evolution of the decay process of unstable particles is investigated in field theory models. We first formulate how to renormalize the non-decay amplitude beyond perturbation theory and then discuss short-time behavior of very long-lived particles. Two different formalisms, one that does and one that does not, assume existence of the asymptotic field of unstable particles are considered. The non-decay amplitude is then calculated by introducing a finite time resolution of measurement, which makes it possible to discuss both renormalizable and non-renormalizable decay interaction including the nucleon decay. In ordinary circumstances the onset of the exponential decay law starts at times as early as at roughly the resolution time, but with an enhanced amplitude which may be measurable. It is confirmed that the short-time formula

Prolonged decay and CP asymmetry

1 - \Gamma t

Dynamics of barrier penetration in a thermal medium: Exact result for the inverted harmonic oscillator

of the exponential decay law may be used to set limits on the nucleon decay rate in underground experiments. On the other hand, an exceptional example of S-wave decay of very small Q-value is found, which does not have the exponential period at all.

Quantum system under periodic perturbation: Effect of environment

Time evolution of unstable particles that occur in the expanding universe is investigated. The off-shell effect not included in the Boltzmann-like equation is important for the decay process when the temperature becomes much below the mass of unstable particle. When the off-shell effect is taken into account, the thermal abundance of unstable particles at low temperatures has a power law behavior of temperature

New kinetic equation for pair-annihilating particles: Generalization of the Boltzmann equation

T

Quantum kinetic equation and cosmic pair annihilation

,

Resonance enhanced tunneling

\frac{\Gamma}{M}(\frac{T}{M})^{\alpha + 1}