Heavy QCD axion dark matter from avoided level crossing
David Cyncynates, Jedidiah O. Thompson
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 $100\text{ }\text{ }\mathrm{\ensuremath{\mu}}\mathrm{eV}$. We point out a natural production mechanism for QCD axion dark matter at masses up to existing astrophysical bounds [$\mathcal{O}(20\text{ }\text{ }\mathrm{meV})$ for the most minimal models and $\mathcal{O}(1\text{ }\text{ }\mathrm{eV})$ 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.