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