Heavy QCD axion dark matter from avoided level crossing

Abstract

The QCD axion offers a natural resolution to the strong CP problem and provides a compelling dark matter candidate. If the QCD axion constitutes all the dark matter, the simplest models pick out a narrow range of masses around $100\mu \mathrm{eV}$. We point out a natural production mechanism for QCD axion dark matter at masses up to existing astrophysical bounds ($\text{scr}O(20\mathrm{meV})$ for the most minimal models and $\text{scr}O(1\mathrm{eV})$ for nucleophobic models). If the QCD axion mixes with a sterile axion, the relative temperature dependence of their potentials can lead to an avoided level crossing of their mass eigenstates. This leads to a near-total transfer of energy density from the sterile axion to the QCD axion, resulting in a late-time QCD axion abundance sufficient to make up all of present-day dark matter. Our result provides additional theoretical motivation for several direct detection experiments that will probe this part of parameter space in the near future.

Comments: 6 pages, 2 figures, matches published version