Light QCD Axion Dark Matter from Double Level Crossings

TL;DR

This paper explores how level crossings in mass mixing between the $Z_{ mathcal N}$ axion and axionlike particles can influence dark matter relic density, offering new insights into axion cosmology and potential detection.

Contribution

It introduces the concept of single and double level crossings in $Z_{ mathcal N}$ axion and ALP mixing, revealing their effects on relic density and cosmological evolution.

Findings

Single level crossing can suppress axion relic density.

Double level crossings can either enhance or suppress relic density.

Level crossings occur at different cosmological epochs, affecting axion abundance.

Abstract

The even light QCD axion called the $Z_{\mathcal N}$ axion can both solve the strong CP problem and account for the dark matter (DM). We point out that the single and double level crossings can naturally take place in the mass mixing between the $Z_{\mathcal N}$ axion and axionlike particle (ALP). The first level crossing occurs much earlier than the QCD phase transition, while the second level crossing occurs exactly during the QCD phase transition if it exists. We also find that the single level crossing can transform into the double level crossings, depending on the ALP mass $m_A$ versus the zero-temperature $Z_{\mathcal N}$ axion mass $m_{a,0}$. Compared with the no level crossing case, the $Z_{\mathcal N}$ axion relic density can be suppressed in the single level crossing, and enhanced or suppressed in the double level crossings.