Particle production from oscillating scalar backgrounds in an FLRW universe

We study particle production from oscillating scalar backgrounds in a spatially flat Friedmann-Lema\^itre-Robertson-Walker universe using non-equilibrium quantum field theory which fully captures the thermal effects and backreaction effects. To be concrete, we consider a $Z_2$-symmetric two-scalar model with quartic interactions. For quasi-harmonic oscillations, we adopt the multi-scale analysis to obtain analytical approximate expressions for the self-consistent evolution of the scalar background and the energy density of the produced particles in terms of the retarded self-energy and retarded proper four-vertex function, whose imaginary parts characterize different condensate decay channels and lead to dissipation. We find that reheating in this model can be complete if the imaginary part of the retarded self-energy is not negligible, which causes dissipation only at finite temperature through Landau damping. Our results can be generalized to more complicated models and thus could provide a general framework for studying the perturbative reheating process in the early Universe or perturbative production of Dark Matter from an oscillating inflaton.Comment: 33 pages, 14 figures; v2: minor typos corrected, refs added; v3: clarifying comments added, refs adde