Fitting the Galactic Center gamma-ray excess with cascade annihilations

Abstract

The apparent excess of gamma rays in an extended region in the direction of the Galactic Center has a spatial distribution and amplitude that are suggestive of dark matter annihilations. If this excess is indeed due to dark matter annihilations, it would indicate the presence of both dark matter and an additional particle beyond the Standard Model that mediates the interactions between the dark matter and Standard Model states. We introduce reference models describing dark matter annihilation to pairs of these new mediators, which decouples the SM-mediator coupling from the thermal annihilation cross section and easily explains the lack of direct detection signals. We determine the parameter regions that give good descriptions of the gamma-ray excess for several motivated choices of mediator couplings to the SM. We find fermion dark matter with mass 7--26 GeV and a dark vector mediator, or scalar dark matter in the 10--50 GeV range (Higgs portal mediator) or 10--65 GeV range (gluophilic mediator) can provide a comparable or improved fit, compared to the case of direct annihilation. We demonstrate that these models can easily satisfy all constraints from collider experiments, direct detection, and cosmology.