W/Z bremsstrahlung as the dominant annihilation channel for dark matter, revisited

Abstract

We revisit the calculation of electroweak bremsstrahlung contributions to dark matter annihilation. Dark matter annihilation to leptons is necessarily accompanied by electroweak radiative corrections, in which a W or Z boson is also radiated. Significantly, while many dark matter models feature a helicity suppressed annihilation rate to fermions, bremsstrahlung process can remove this helicity suppression such that the branching ratios Br(ℓνW), Br(ℓ+ℓ-Z), and Br(ν̄νZ) dominate over Br(ℓ+ℓ-) and Br(ν̄ν). We find this is most significant in the limit where the dark matter mass is nearly degenerate with the mass of the boson which mediates the annihilation process. Electroweak bremsstrahlung ha..