Cosmology and direct detection of the Dark Axion Portal
  The Dark Axion Portal provides a model for Dark Matter (DM) in which both
Dark Photons $\gamma^\prime$ and Axions $a$ can contribute to the present day
abundance of DM. We study the parameter space of the Dark Axion Portal to
pinpoint regions of the parameter space where $\gamma^\prime$ and $a$ can be
produced with sufficient abundance to account for the cosmic DM density, while
still being detectable in planned direct detection and axion haloscope
experiments. In particular, we explore the production of eV-scale Dark Photons
in the Dark Axion Portal, taking into account a possible kinetic mixing between
the dark and visible photons, which is essential for the detection of dark
photons through absorption in direct searches. We show that a non-zero kinetic
mixing does not generally spoil the phenomenology of the model, leaving both
the axion and dark photon stable. Viable production mechanisms point to a
sub-dominant population of dark photons making up $\lesssim 10\%$ of the DM,
with the remainder consisting of axion DM. Dark photons in the mass range
$m_{\gamma^\prime} \sim 20-200\,\mathrm{eV}$ and axions in the mass range $m_a
\sim 30 - 400\,\mu\mathrm{eV}$ may be produced with these abundances
self-consistently in the Dark Axion Portal and are within the reach of future
direct searches.
