A post-inflationary kinetic axion

TL;DR

This paper introduces a new axion dark matter production mechanism via kinetic misalignment triggered by a Hubble-induced phase transition during a post-inflationary kination era, leading to observable consequences for future axion searches.

Contribution

It proposes a novel axion production scenario involving a Hubble-induced phase transition and domain formation, expanding the understanding of axion dark matter origins beyond standard misalignment.

Findings

Identifies two thermal histories affecting axion abundance.

Demonstrates axion production can occur even with zero net global charge.

Suggests the mechanism is testable with next-generation axion experiments.

Abstract

We present a novel realization of axion kinetic misalignment, triggered by a Hubble-induced phase transition during a post-inflationary stiff (kination) era. A negative Ricci scalar flips the sign of a non-minimally coupled mass term for a non-minimally coupled complex field $\Phi$, driving its radial mode to large amplitudes via a tachyonic instability. At large $|\Phi|$, higher-dimensional $U(1)$-breaking operators become relevant and impart a kick in the angular direction, generating a conserved $U(1)$ charge that sustains rotation as the symmetry is approximately restored. Because phases randomize across causally disconnected regions, multiple domains with distinct charges form. The subsequent axion potential converts the domain charges into an axion abundance, yielding dark matter even when the net global charge vanishes. We analyze the dynamics through a linear, domain-averaged treatment and identify two thermal histories: (i) Ricci reheating via saxion decays to Higgs bosons; (ii) external reheating with efficient damping of saxion energy by Higgs/fermion scatterings. The mechanism populates regions underabundant in standard misalignment, which are accessible to next generation axion searches.