Heavy QCD axion dark matter from avoided level crossing

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 $100\,\mu{\rm eV}$. We point out a natural production mechanism for QCD axion dark matter at masses up to existing astrophysical bounds (${\scr O}(20 \, \mathrm{meV})$ for the most minimal models and ${\scr O}(1 \, \mathrm{eV})$ 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.