Mass Mixing between QCD Axions

TL;DR

This paper explores a new level crossing phenomenon in the mass mixing of QCD axions, affecting their cosmological evolution and relic density, with potential implications for dark matter and early universe cosmology.

Contribution

It introduces a novel level crossing mechanism in QCD axion mass mixing and analyzes its impact on axion relic density and cosmological evolution.

Findings

Level crossing can occur at or before the QCD phase transition.

The transition affects axion energy density evolution.

Estimated relic density of QCD axion dark matter in this scenario.

Abstract

We introduce a novel level crossing phenomenon in the mass mixing between the QCD axions, one canonical QCD axion and one $Z_{\mathcal N}$ axion. The level crossing can take place at or slightly before the QCD phase transition critical temperature, depending on the ratio of the axion decay constants $\sim1.69$. The cosmological evolution of the mass eigenvalues in these two scenarios is similar; however, the transition of axion energy density differs significantly. Finally, we estimate the relic density of the QCD axion dark matter in this context. Additionally, this level crossing may have some interesting cosmological implications.