New thermal relic targets for inelastic vector-portal dark matter

We examine the vector-portal inelastic dark matter (DM) model with DM mass m_ and dark photon mass m₀^{'}, in the ``forbidden dark matter'' regime where 1m₀^{'}/m_2, carefully tracking the dark sector temperature throughout freeze-out. The inelastic nature of the dark sector relaxes the stringent cosmic microwave background (CMB) and self-interaction constraints compared to symmetric DM models. We determine the CMB limits on both annihilations involving excited states and annihilation into e^+e^- through initial-state radiation of an A^', as well as limits on the DM self-scattering, which proceeds at the one-loop level. The unconstrained parameter space serves as an ideal target for accelerator A^' searches, and provides a DM self-interaction cross section that is large enough to observably impact small-scale structure.