Gauge field production and Schwinger reheating in runaway axion inflation

Abstract

In a class of (pseudoscalar) inflation, inflationary phase is followed by a kination phase,where the Universe is dominated by the kinetic energy of the inflaton that runs away in a vanishing scalar potential.In this class of postinflationary evolution of the Universe, reheating of the Universe cannot be achieved by the inflatonparticle decay, which requires its coherent oscillation in a quadratic potential.In this study, we explore the U(1) gauge field production through the Chern-Simons coupling between the pseudoscalarinflaton and the gauge field during the kination eraand examine the subsequent pair-particle production induced by the amplified gauge field known as the Schwinger effect,which can lead to reheating of the Universe.We find that with a rough estimate of the Schwinger effect for the Standard Model hyper U(1) gauge fieldand subsequent thermalization of the pair-produced particles,a successful reheating of the Universe can be achievedby their eventual domination over the kinetic energy of the inflaton,with some reasonable parameter sets.This can be understood as a concrete realization of the “Schwinger reheating”.Constraints from the later-time cosmology are also discussed.