Dark Matter Targets for Axion-like Particle Searches
  Many existing and proposed experiments targeting QCD axion dark matter (DM)
can also search for a broad class of axion-like particles (ALPs). We analyze
the experimental sensitivities to electromagnetically-coupled ALP DM in
different cosmological scenarios with the relic abundance set by the
misalignment mechanism. We obtain benchmark DM targets for the standard thermal
cosmology, a pre-nucleosynthesis period of early matter domination, and a
period of kination. These targets are theoretically simple and assume
$\mathcal{O}(1)$ misalignment angles, avoiding fine-tuning of the initial
conditions. We find that some experiments will have sensitivity to these ALP DM
targets before they are sensitive to the QCD axion, and others can potentially
reach interesting targets below the QCD band. The ALP DM abundance also depends
on the origin of the ALP mass. Temperature-dependent masses that are generated
by strong dynamics (as for the QCD axion) correspond to DM candidates with
smaller decay constants, resulting in even better detection prospects.
