Gamma-ray Constraints on Dark Matter Annihilation into Charged Particles

TL;DR

This paper derives model-independent gamma-ray based upper limits on dark matter annihilation into charged leptons, especially electrons, using Milky Way gamma-ray data across a wide energy range, highlighting the impact of astrophysical assumptions.

Contribution

It provides robust, conservative upper bounds on dark matter annihilation cross sections into charged leptons based on gamma-ray observations, including the effects of radiative corrections.

Findings

Upper bounds on annihilation cross sections into electrons and other leptons are established.

Gamma-ray signals near the endpoint are only two orders of magnitude lower than monoenergetic gamma-ray signals.

Bounds are compared with total annihilation cross section limits from neutrino data.

Abstract

Dark matter annihilation into charged particles is necessarily accompanied by gamma rays, produced via radiative corrections. Internal bremsstrahlung from the final state particles can produce hard gamma rays up to the dark matter mass, with an approximately model-independent spectrum. Focusing on annihilation into electrons, we compute robust upper bounds on the dark matter self annihilation cross section $<\sigma_A v >_{e^+e^-}$ using gamma ray data from the Milky Way spanning a wide range of energies, $\sim10^{-3} - 10^4$ GeV. We also compute corresponding bounds for the other charged leptons. We make conservative assumptions about the astrophysical inputs, and demonstrate how our derived bounds would be strengthened if stronger assumptions about these inputs are adopted. The fraction of hard gamma rays near the endpoint accompanying annihilation to $e^+e^-$ is only a factor of $\alt 10^2$ lower than for annihilation directly to monoenergetic gamma rays. The bound on $<\sigma_A v >_{e^+e^-}$ is thus weaker than that for $<\sigma_A v >_{\gamma\gamma}$ by this same factor. The upper bounds on the annihilation cross sections to charged leptons are compared with an upper bound on the {\it total} annihilation cross section defined by neutrinos.