Thermal abundance of non-relativistic relics with Sommerfeld enhancement
  We propose an analytic treatment for computing the relic abundances of
non-relativistic particles whose annihilation rate at chemical decoupling is
increased by Sommerfeld enhancement. We find approximate rational functions
that closely fit the thermal average of Sommerfeld-enhanced cross sections in
the massless limit of force carriers for s- and p-wave annihilations. We
demonstrate that, with the approximate thermally-averaged cross sections
implemented, the standard analytic method for the final relic abundances
provides accuracy to within 1% even for the case of Sommerfeld enhancement.
