Enhanced One-Loop Corrections to WIMP Annihilation and their Thermal
  Relic Density in the Coannihilation Region
  We consider quantum corrections to co-annihilation processes of Weakly
Interacting Massive Particles (WIMPs) due to the exchange of light bosons in
the initial state ("Sommerfeld corrections"). We work at one-loop level, i.e.
we assume that these corrections can be treated perturbatively. Co-annihilation
is important if there is at least one additional new particle with mass close
to the lightest WIMP, which is a Dark Matter candidate. In this case the
exchange of a (relatively light) boson in the initial state can change the
identity of the annihilating particles. The corrections we are interested in
factorize, as in the case of WIMP self-annihilation treated previously, but
they can mix different tree-level amplitudes. Moreover, even small mass
splittings between the external particles and those in the loop can change the
relevant loop functions significantly. We find exact analytical expressions for
these functions, and illustrate the effects by considering the cases of wino-
or higgsino-like neutralinos as examples.
