Monochromatic Gamma Rays from Dark Matter Annihilation to Leptons

TL;DR

This paper explores how dark matter annihilations into leptons can produce monochromatic gamma rays, analyzing the conditions under which gamma-ray lines are significant and their implications for dark matter detection.

Contribution

It provides a detailed calculation of the ratio of gamma-ray line production to lepton pair production in dark matter annihilation, highlighting scenarios with large gamma-ray line branching fractions.

Findings

Loop-level gamma-ray production can be significant for certain mediator properties.

Effective field theory accurately describes processes up to half the mediator mass.

Implications for dark matter contributions to AMS-02 positron excess.

Abstract

We investigate the relation between the annihilation of dark matter (DM) particles into lepton pairs and into 2-body final states including one or two photons. We parametrize the DM interactions with leptons in terms of contact interactions, and calculate the loop-level annihilation into monochromatic gamma rays, specifically computing the ratio of the DM annihilation cross sections into two gamma rays versus lepton pairs. While the loop-level processes are generically suppressed in comparison with the tree-level annihilation into leptons, we find that some choices for the mediator spin and coupling structure lead to large branching fractions into gamma-ray lines. This result has implications for a dark matter contribution to the AMS-02 positron excess. We also explore the possibility of mediators which are charged under a dark symmetry and find that, for these loop-level processes, an effective field theory description is accurate for DM masses up to about half the mediator mass.