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.