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

Dark matter (DM) charged under a dark U(1) force appears in many extensionsof the Standard Model, and has been invoked to explain anomalies in cosmic-raydata, as well as a self-interacting DM candidate. In this paper, we perform acomprehensive phenomenological analysis of such a model, assuming that the DMabundance arises from the thermal freeze-out of the dark interactions. Weinclude, for the first time, bound-state effects both in the DM production andin the indirect detection signals, and quantify their importance for Fermi,AMS, and CMB experiments. We find that DM in the mass range 1 GeV to 100 TeV,annihilating into dark photons of MeV to GeV mass, is in conflict withobservations. Instead, DM annihilation into heavier dark photons is viable. Wepoint out that the late decays of multi-GeV dark photons can producesignificant entropy and thus dilute the DM density. This can lower considerablythe dark coupling needed to obtain the DM abundance, and in turn relax theexisting constraints.