Meso-inflationary Peccei–Quinn Symmetry Breaking with Nonminimal Coupling

Abstract

We study a realization of the inflationary scenario where the Peccei–Quinn (PQ) symmetry is spontaneously broken during inflation, facilitated by its nonminimal coupling to gravity. This results in, effectively, a two-field inflation: the early stage is driven by an inflaton field with the PQ symmetry intact, and the later stage is driven by the PQ scalar after its effective mass becomes tachyonic, causing destabilization from the origin. The nonminimal coupling serves the dual purpose of restoring the PQ symmetry during early inflation and flattening the PQ potential posttachyonic shift, allowing for continued slow-roll. We analyze the inflationary background solutions and scalar perturbations, which are amplified at small scales via significant isocurvature perturbations generated near the symmetry-breaking epoch. These perturbations lead to second-order gravitational waves, detectable by next-generation space-based experiments.