Likely Dominance of WIMP Annihilation to Fermion Pair + W/Z

Abstract

Arguably, the most popular candidate for Dark Matter (DM) is a massive, stable, Majorana fermion. However, annihilation of Majorana DM to two fermions features a helicity-suppressed s-wave rate. The process radiating a gauge boson via electroweak (EW) and electromagnetic (EM) bremsstrahlung removes this s-wave suppression, and is likely to be the dominant modes of gauge-singlet Majorana DM annihilation. Given their enhanced annihilation rate with radiatedW and Z gauge bosons, and the subsequent dominant W/Z decays via hadronic channels, Majorana DM tends to produce more antiprotons than positrons. This result contrasts with observations, thereby presenting a challenge to model building with Majorana DM.