Sommerfeld Enhancements for Thermal Relic Dark Matter
  The annihilation cross section of thermal relic dark matter determines both
its relic density and indirect detection signals. We determine how large
indirect signals may be in scenarios with Sommerfeld-enhanced annihilation,
subject to the constraint that the dark matter has the correct relic density.
This work refines our previous analysis through detailed treatments of resonant
Sommerfeld enhancement and the effect of Sommerfeld enhancement on freeze out.
Sommerfeld enhancements raise many interesting issues in the freeze out
calculation, and we find that the cutoff of resonant enhancement, the
equilibration of force carriers, the temperature of kinetic decoupling, and the
efficiency of self-interactions for preserving thermal velocity distributions
all play a role. These effects may have striking consequences; for example, for
resonantly-enhanced Sommerfeld annihilation, dark matter freezes out but may
then chemically recouple, implying highly suppressed indirect signals, in
contrast to naive expectations. In the minimal scenario with standard
astrophysical assumptions, and tuning all parameters to maximize the signal, we
find that, for force-carrier mass m_phi = 250 MeV and dark matter masses m_X =
0.1, 0.3, and 1 TeV, the maximal Sommerfeld enhancement factors are S_eff = 7,
30, and 90, respectively. Such boosts are too small to explain both the PAMELA
and Fermi excesses. Non-minimal models may require smaller boosts, but the
bounds on S_eff could also be more stringent, and dedicated freeze out analyses
are required. For concreteness, we focus on 4 mu final states, but we also
discuss 4 e and other modes, deviations from standard astrophysical assumptions
and non-minimal particle physics models, and we outline the steps required to
determine if such considerations may lead to a self-consistent explanation of
the PAMELA or Fermi excesses.
