QCD Axion Domain Walls from Super-Cooling First Order Phase Transition

TL;DR

This paper investigates how a supercooled first-order phase transition at MeV-GeV scales could influence the evolution of the QCD axion, potentially leading to domain wall formation and affecting dark matter abundance.

Contribution

It introduces a novel connection between supercooled phase transitions and QCD axion dynamics, including domain wall formation and mini kinetic misalignment effects.

Findings

Supercooled FOPT can induce mini kinetic misalignment of the axion.

QCD axion domain walls may form under certain conditions.

Implications for dark matter and NANOGrav gravitational wave signals.

Abstract

The QCD axion is a well-motivated hypothetical particle beyond the Standard Model (SM) and a compelling dark matter candidate. Its relic abundance is highly sensitive to the thermal history of the universe when the temperature is around the QCD confinement scale. Meanwhile, the NANOGrav Collaboration has reported evidence for a stochastic gravitational wave background, which could originate from a supercooled first-order phase transition (FOPT) with a nucleation temperature around the O(MeV-GeV) scale. We explore how such an FOPT might alter the evolution of the QCD axion. Our findings suggest that it could induce the axion to go through a short stage of mini kinetic misalignment. Moreover, in some parameter regime, the formation of QCD axion domain walls becomes generically expected. This has intriguing implications for both the existence of the QCD axion and the FOPT interpretation of the NANOGrav signal.