Gauge Field Production and Schwinger Reheating in Runaway Axion Inflation

TL;DR

This paper investigates a novel reheating mechanism in a specific inflationary scenario where the Universe's energy is dominated by the inflaton's kinetic energy, using gauge field production and the Schwinger effect to achieve thermalization.

Contribution

It introduces a new reheating process called 'Schwinger reheating' in runaway axion inflation models, utilizing gauge field amplification and pair production during the kination phase.

Findings

Successful reheating can occur via gauge field amplification and Schwinger pair production.

Parameter sets exist where the produced particles dominate the energy, leading to thermalization.

Constraints from cosmological observations are considered.

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 inflaton particle 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 pseudoscalar inflaton and the gauge field during the kination era and 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 field and subsequent thermalization of the pair-produced particles, a successful reheating of the Universe can be achieved by 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.