Level Crossing between QCD Axion and Axion-Like Particle

TL;DR

This paper investigates the interaction between the QCD axion and axion-like particles, revealing a phenomenon called axion roulette that can lead to stable domain walls, with implications for cosmology and potential solutions to avoid cosmological disasters.

Contribution

It provides a detailed numerical analysis of the axion roulette phenomenon and explores conditions under which domain walls become unstable and decay, addressing cosmological stability issues.

Findings

Identification of parameter space for axion roulette occurrence

Demonstration of stable domain wall formation without cosmic strings

Proposal of energy bias to destabilize domain walls and prevent cosmological problems

Abstract

We study a level crossing between the QCD axion and an axion-like particle, focusing on the recently found phenomenon, the axion roulette, where the axion-like particle runs along the potential, passing through many crests and troughs, until it gets trapped in one of the potential minima. We perform detailed numerical calculations to determine the parameter space where the axion roulette takes place, and as a result domain walls are likely formed. The domain wall network without cosmic strings is practically stable, and it is nothing but a cosmological disaster. In a certain case, one can make domain walls unstable and decay quickly by introducing an energy bias without spoiling the Peccei-Quinn solution to the strong CP problem.