Nonequilibrium perturbation theory for complex scalar fields

TL;DR

This paper develops a real-time perturbation theory for complex scalar fields out of equilibrium, incorporating dissipative effects into propagators, leading to a Boltzmann-like evolution of occupation numbers and highlighting differences due to chemical potential.

Contribution

It introduces a novel perturbation framework that accounts for dissipation and quasiparticle evolution in non-equilibrium scalar fields, extending standard methods.

Findings

Occupation numbers evolve according to a Boltzmann-like equation.

Thermal masses and decay widths differ for particles and antiparticles.

Non-zero chemical potential affects quasiparticle properties.

Abstract

Real-time perturbation theory is formulated for complex scalar fields away from thermal equilibrium in such a way that dissipative effects arising from the absorptive parts of loop diagrams are approximately resummed into the unperturbed propagators. Low order calculations of physical quantities then involve quasiparticle occupation numbers which evolve with the changing state of the field system, in contrast to standard perturbation theory, where these occupation numbers are frozen at their initial values. The evolution equation of the occupation numbers can be cast approximately in the form of a Boltzmann equation. Particular attention is given to the effects of a non-zero chemical potential, and it is found that the thermal masses and decay widths of quasiparticle modes are different for particles and antiparticles.