Consistent Scenarios for Cosmic-Ray Excesses from Sommerfeld-Enhanced
  Dark Matter Annihilation
  Anomalies in direct and indirect detection have motivated models of dark
matter consisting of a multiplet of nearly-degenerate states, coupled by a new
GeV-scale interaction. We perform a careful analysis of the thermal freezeout
of dark matter annihilation in such a scenario. We compute the range of "boost
factors" arising from Sommerfeld enhancement in the local halo for models which
produce the correct relic density, and show the effect of including constraints
on the saturated enhancement from the cosmic microwave background (CMB). We
find that boost factors from Sommerfeld enhancement of up to ~800 are possible
in the local halo. When the CMB bounds on the saturated enhancement are
applied, the maximal boost factor is reduced to ~400 for 1-2 TeV dark matter
and sub-GeV force carriers, but remains large enough to explain the observed
Fermi and PAMELA electronic signals. We describe regions in the DM mass-boost
factor plane where the cosmic ray data is well fit for a range of final states,
and show that Sommerfeld enhancement alone is enough to provide the large
annihilation cross sections required to fit the data, although for light
mediator masses (less than ~200 MeV) there is tension with the CMB constraints
in the absence of astrophysical boost factors from substructure. Additionally,
we consider the circumstances under which WIMPonium formation is relevant and
find for heavy WIMPs (greater than ~2 TeV) and soft-spectrum annihilation
channels it can be an important consideration; we find regions with dark matter
mass greater than 2.8 TeV that are consistent with the CMB bounds and have
~600-700 present-day boost factors.
