On the likely dominance of WIMP annihilation to fermion pair+W/Z (and implication for indirect detection)

Arguably, the most popular candidate for Dark Matter (DM) is a massive, stable, Majorana fermion. However, annihilation of Majorana DM to two fermions often features a helicity-suppressed s-wave rate. Radiating a gauge boson via electroweak (EW) and electromagnetic (EM) bremsstrahlung removes this s-wave suppression. The main purpose of this talk is to explain in some detail why the branching ratio to a fermion pair is likely suppressed while the decay to the pair plus a W/Z is not. In doing so, we investigate the general conditions for s-wave suppression and un-suppression using Fierz transformations and partial wave expansions. Suppression for the 2-to-2 process is sufficiently severe that the EW and EM bremsstrahlung are likely to be the dominant modes of gauge-singlet Majorana DM annihilation. We end this talk with a discussion of the challenge presented by space-based data for Majorana DM models, given that the enhanced rate to radiated W and Z gauge bosons and their dominant decay via hadronic channels tends to produce more anti-protons than are observed.