Monochromatic Gamma Rays from Dark Matter Annihilation to Leptons
arXiv:1504.05187 · doi:10.1007/JHEP08(2015)074
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.
21 pages plus appendices, 7 figures. v2: added experimental constraints from CMB and Fermi, expanded and reorganized discussion throughout. Accepted by JHEP
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