  In co-annihilation scenarios, the weakly interacting dark matter particle
(WIMP) is close in mass to another particle that can decay into the WIMP. As a
result, both particles freeze out at roughly the same time in the early
universe, and both contribute to the effective dark matter annihilation
cross-section. Since the heavier particle does not need to be weakly
interacting, the co-annihilation processes are in general subject to sizeable
radiative corrections. Here this is analyzed for the example of neutralino-stop
co-annihilation in supersymmetry. The leading QCD corrections are calculated
and it is found that they have a large effect on the effective annihilation
cross-section, reaching more than 50% in some regions of parameter space.
