Dark matter freeze-in via a kinetically mixed dark photon

We study a model in which dark matter is a Dirac fermion with sub-GeV mass that interacts with the Standard Model via a dark photon, which kinetically mixes with the photon. We use the Boltzmann equation to calculate the relic abundance predicted by freeze-in and find the coupling strength required to match the experimental value $\Omega h^2=0.11$. Finally, we compute direct detection cross sections for various dark photon masses. Our results motivate work to increase the sensitivity of current and future direct detection experiments, in order to probe regions in parameter space which are currently inaccessible.