Scalar field damping at high temperatures

TL;DR

This paper investigates the damping of a scalar field in a hot plasma at high temperatures, deriving a relation between damping and bulk viscosity, with implications for early universe reheating processes.

Contribution

It introduces a method to compute scalar field damping via an effective Boltzmann equation, linking it to bulk viscosity, and provides a numerical estimate of the damping coefficient.

Findings

Damping coefficient is approximately 10^4.

Damping can be expressed through bulk viscosity.

Effective Boltzmann equation approach simplifies calculations.

Abstract

The motion of a scalar field that interacts with a hot plasma, like the inflaton during reheating, is damped, which is a dissipative process. At high temperatures the damping can be described by a local term in the effective equation of motion. The damping coefficient is sensitive to multiple scattering. In the loop expansion its computation would require an all-order resummation. Instead we solve an effective Boltzmann equation, similarly to the computation of transport coefficients. For an interaction with another scalar field we obtain a simple relation between the damping coefficient and the bulk viscosity, so that one can make use of known results for the latter. The numerical prefactor of the damping coefficient turns out to be rather large, of order $ 10 ^ 4 $.