Limits on Threshold and "Sommerfeld" Enhancements in Dark Matter
  Annihilation
  We find model-independent upper limits on rates of dark matter annihilation
in galactic halos. The Born approximation generally fails, while exotic
threshold enhancements akin to "Sommerfeld factors" also turn out to be
baseless The most efficient annihilation mechanism involves perturbatively
small decay widths that have largely been ignored. Widths that are very small
compared to TeV mass scales suffice to effectively saturate unitarity bounds on
annihilation rates. Bound state formation in weakly coupled theories produces
small effects due to wave function normalizations. Unitarity shows the
Sommerfeld factor cannot produce large changes in cross sections, and serves to
identify where those approximations break down.
