Toward a Minimum Branching Fraction for Dark Matter Annihilation into Electromagnetic Final States

TL;DR

This paper investigates the electromagnetic signals resulting from dark matter annihilation into neutrinos, showing that loop-induced charged lepton production leads to observable synchrotron radiation, which can impose stronger constraints than neutrino observations.

Contribution

It calculates the loop-induced branching ratio into charged leptons in effective field theory, revealing significant electromagnetic signatures for dark matter masses above the electroweak scale.

Findings

Loop-induced branching ratios >1% for dark matter mass > M_W

Synchrotron bounds surpass neutrino bounds for >3% branching fraction

Framework for 'neutrinos only' dark matter models below M_W

Abstract

Observational limits on the high-energy neutrino background have been used to place general constraints on dark matter that annihilates only into standard model particles. Dark matter particles that annihilate into neutrinos will also inevitably branch into electromagnetic final states through higher-order tree and loop diagrams that give rise to charged leptons, and these charged particles can transfer their energy into photons via synchrotron radiation or inverse Compton scattering. In the context of effective field theory, we calculate the loop-induced branching ratio to charged leptons and show that it is generally quite large, typically >1%, when the scale of the dark matter mass exceeds the electroweak scale, M_W. For a branching fraction >3%, the synchrotron radiation bounds on dark matter annihilation are currently stronger than the corresponding neutrino bounds in the interesting mass range from 100 GeV to 1 TeV. For dark matter masses below M_W, our work provides a plausible framework for the construction of a model for "neutrinos only" dark matter annihilations.