Radiative corrections to bino-stop co-annihilation

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, the other particle does not freeze out before the WIMP 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.