Fitting the Galactic Center Gamma-Ray Excess with Cascade Annihilations
  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.
