New Thermal Relic Targets for Inelastic Vector-Portal Dark Matter

We examine the vector-portal inelastic dark matter (DM) model with DM mass $m_{\chi }$ and dark photon mass $m_{A^{\mathrm{\prime }}}$, in the `forbidden dark matter’ regime where $1\lesssim m_{A^{\mathrm{\prime }}}\mathrm{/}{m}_{\chi }\lesssim 2$, carefully tracking the dark sector temperature throughout freezeout. 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 $A^{\mathrm{\prime }}$, 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 $A^{\mathrm{\prime }}$ searches, and provides a DM self-interaction cross section that is large enough to observably impact small-scale structure.