Thermal Damping of Mass-Modulating Scalars

Abstract

The cosmological evolution of a scalar field is shaped by Hubble damping. Any non-gravitational couplings of the scalar with the primordial thermal bath generically contribute additional damping. Although rarely considered, such thermal damping could be the dominant dissipative effect. We derive approximate but highly general thermal-damping rates of scalar fields that modulate the masses of thermally populated particles. We extend previous results to cover cases of particular phenomenological interest where the scalar background oscillates sinusoidally but not necessarily slowly compared to the thermalization rates of the primordial bath. Based on these results, we estimate the thermal damping of scalars coupled to neutrinos linearly, to gluons quadratically, and to WIMPs linearly, and demonstrate its importance in certain parameter space of these models. We also estimate the thermal damping rates in models of QCD axion.