QCD Axion Dark Matter from level crossing with refined adiabatic condition

TL;DR

This paper explores how level crossing in two-axion systems affects axion abundance and dark matter, introducing a basis-independent adiabatic condition and analyzing phenomenological implications for axion detection.

Contribution

It provides a new basis-independent adiabatic condition for axion systems and analyzes the impact of level crossing on axion dark matter abundance.

Findings

Derived an improved, basis-independent adiabatic condition.

Identified viable axion mass and decay constant regions for dark matter.

Revealed potential experimental signatures through axion-photon coupling relations.

Abstract

We investigate the level-crossing phenomenon in two-axion systems, where the mass eigenvalues intersect as the mass of one axion increases with the cooling of the universe. This phenomenon can significantly alter the abundance of axions in the early universe. Our study focuses on its impact on the QCD axion and an axion-like particle, identifying viable regions of axion mass and decay constant that explain the observed dark matter. We demonstrate the equivalence of two different bases for describing the axion system in the existing literature. Furthermore, we derive an improved expression for the adiabatic condition that overcomes limitations in earlier formulations. This new formulation is basis-independent, and we numerically validate its effectiveness. Our analysis reveals specific relations between axion masses and axion-photon couplings within the viable region. These relations could potentially serve as a smoking gun signal for this scenario if confirmed experimentally. We also find that, using the chiral perturbation model, the thermal friction on the QCD axion might be significantly larger than previously estimated. Additionally, we show that a simple model with axion mixing can naturally realize either a heavier or lighter QCD axion.