GeV-scale thermal dark matter from dark photons: tightly constrained, yet allowed

Abstract: GeV-scale thermal dark matter (DM) is highly constrained by the null results of both direct and indirect detection experiments, especially in the context of simplified models. In this work, we study the interplay of collider, direct and indirect detection constraints on an extension of the dark Abelian Higgs model that includes a Dirac fermionic DM candidate, $\chi$. We take into account in a consistent fashion the dilution of the indirect and direct detection signals when the relic abundance of $\chi$ is smaller than the total observed DM density (assuming that it is a subdominant component in those cases). As a consequence, we show that indirect detection constraints cannot probe regions with large kinetic mixing, and direct detection experiments provide the leading constraints in most of the parameter space. Collider searches for the (invisibly decaying) vector mediator provide complementary bounds in areas with large kinetic mixing. We find that the only way to avoid both indirect and direct detection limits is in narrow windows of parameter space close to $m_\chi\lesssim m_{Z_D}/2$, when $\chi$ is produced resonantly in the early universe, and it can constitute all of the DM. For this to happen, a small dark sector coupling is required: $\alpha_D~\lesssim10^{-3}$ for DM masses below $6$ GeV, or $\alpha_D~\lesssim10^{-5}$ for DM masses larger than $10$ GeV. The remaining areas of the parameter space can be probed in a complementary way by future direct detection experiments (which will narrow down the allowed area around the resonant region) and collider searches (which will set limits for smaller values of the kinetic mixing).