Scalar field damping at high temperatures

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 $10^4$.

arXiv comment18 pages, explanations and some details added, references added, (non-)role of scale-invariance violation clarified, results unchanged. Closely resembles published version

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• Measuring the Inflaton Coupling in the CMB
• Evolution of coupled scalar perturbations through smooth reheating. I. Dissipative regime
• Fate of homogeneous ${\mathbb{Z}}_2$-symmetric scalar condensates
• Evolution of coupled scalar perturbations through smooth reheating. II. Thermal fluctuation regime
• Energy and momentum dependence of the soft-axion interaction rate