Damping of an oscillating scalar field indirectly coupled to a thermal bath

TL;DR

This paper investigates how an oscillating scalar field loses energy through indirect interactions with a thermal bath, considering different mediator field behaviors, and calculates damping rates across various parameters.

Contribution

It introduces a non-local damping description for stable mediator fields and provides comprehensive damping rate calculations for both decay and stable cases.

Findings

Damping rates are computed for all parameter regions.

The non-local damping approach is validated for stable mediators.

Estimated damping times match previous results in decay scenarios.

Abstract

The damping process of a homogeneous oscillating scalar field that indirectly interacts with a thermal bath through a mediator field is investigated over a wide range of model parameters. We consider two types of mediator fields, those that can decay to the thermal bath and those that are individually stable but pair annihilate. The former case has been extensively studied in the literature by treating the damping as a local effect after integrating out the assumed close-to-equilibrium mediator field. The same approach does not apply if the mediator field is stable and freezes out of equilibrium. To account for the latter case, we adopt a non-local description of damping that is only meaningful when we consider full half-oscillations of the field being damped. The damping rates of the oscillating scalar field and the corresponding heating rate of the thermal bath in all bulk parameter regions are calculated in both cases, corroborating previous results in the direct decay case. Using the obtained results, the time it takes for the amplitude of the scalar field to be substantially damped is estimated.