Dark Matter's secret liaisons: phenomenology of a dark U(1) sector with bound states

Dark matter charged under a dark U(1) force appears in many extensions of the Standard Model, and has been invoked to explain anomalies in cosmic-ray data, as well as a self-interacting dark matter candidate. In this paper, we perform a comprehensive phenomenological analysis of such a model, assuming that the dark matter abundance arises from the thermal freeze-out of the dark interactions. We include, for the first time, bound-state effects both in the dark matter production and in the indirect detection signals, and quantify their importance for Fermi, AMS-02, and CMB experiments. We find that dark photons in the MeV to GeV range are in conflict with observations, for any dark matter mass in the range 1 GeV to 100 TeV. Instead, heavier dark photons are viable. We point out that their late decays can produce significant entropy and thus dilute the dark matter density. This can lower considerably the dark coupling needed to obtain the dark matter abundance, and in turn relax the existing constraints.