Pseudo-Nambu-Goldstone Boson Production from Inflaton Coupling during Reheating

TL;DR

This paper studies how pseudo-Nambu-Goldstone bosons (pNGBs) are produced during the reheating phase after inflation, focusing on their coupling to the inflaton and implications for dark radiation and cosmological constraints.

Contribution

It provides a general calculation of inflaton decay into pNGBs during reheating, including specific analysis of axion production with a non-minimal gravitational coupling.

Findings

pNGBs can be significantly produced during reheating

Production of axions as dark radiation is possible

New constraints on inflationary models from pNGB relics

Abstract

The existence of pseudo-Nambu-Goldstone boson (pNGB) fields is a common feature in many models beyond the Standard Model, characterized by their exclusive derivative couplings. This paper investigates a scenario where a pNGB is coupled to the inflaton field during the reheating phase of the early universe. We calculate the perturbative decay rate of a coherently oscillating inflaton into pNGBs on a general basis, considering both constant and field-dependent couplings with monomial potentials at the minimum. As a concrete application, we explore the production of axions when the radial mode of the Peccei-Quinn (PQ) scalar serves as the inflaton, particularly in the presence of a large gravitational non-minimal coupling. Our findings suggest that the presence of pNGBs during reheating can lead to significant non-thermal relics, offering new constraints on inflationary reheating models and providing potential observational signatures in the form of dark radiation.