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

TL;DR

This paper analyzes how Majorana dark matter annihilates predominantly into fermion pairs plus W/Z bosons due to helicity suppression in direct fermion pair production, impacting indirect detection signals.

Contribution

It provides a detailed explanation of the conditions under which s-wave suppression occurs or is lifted in Majorana DM annihilation, emphasizing the dominance of gauge boson radiation modes.

Findings

Electroweak and electromagnetic bremsstrahlung dominate Majorana DM annihilation.

Suppression of direct fermion pair production leads to enhanced gauge boson radiation.

Space-based observations challenge models with increased W/Z decay products.

Abstract

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.