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We investigate the relation between the annihilation of dark matter (DM)particles into lepton pairs and into 2-body final states including one or twophotons. We parametrize the DM interactions with leptons in terms of contactinteractions, and calculate the loop-level annihilation into monochromaticgamma rays, specifically computing the ratio of the DM annihilation crosssections into two gamma rays versus lepton pairs. While the loop-levelprocesses are generically suppressed in comparison with the tree-levelannihilation into leptons, we find that some choices for the mediator spin andcoupling structure lead to large branching fractions into gamma-ray lines. Thisresult has implications for a dark matter contribution to the AMS-02 positronexcess. We also explore the possibility of mediators which are charged under adark symmetry and find that, for these loop-level processes, an effective fieldtheory description is accurate for DM masses up to about half the mediatormass.

Monochromatic Gamma Rays from Dark Matter Annihilation to Leptons