Multiple Gamma Lines from Semi-Annihilation

TL;DR

This paper proposes that dark matter semi-annihilation among multiple states can produce multiple gamma lines, offering an alternative explanation for observed signals and enabling dark sector spectroscopy.

Contribution

It introduces the concept of semi-annihilation as a mechanism for gamma line production, expanding the potential dark matter mass range and predicting multiple gamma lines.

Findings

Semi-annihilation enhances gamma line signals compared to ordinary annihilation.

Multiple gamma lines can arise, with up to N^3 lines for N dark sector states.

A specific case predicts a 130 GeV line with a weaker 173 GeV line.

Abstract

Hints in the Fermi data for a 130 GeV gamma line from the galactic center have ignited interest in potential gamma line signatures of dark matter. Explanations of this line based on dark matter annihilation face a parametric tension since they often rely on large enhancements of loop-suppressed cross sections. In this paper, we pursue an alternative possibility that dark matter gamma lines could arise from "semi-annihilation" among multiple dark sector states. The semi-annihilation reaction with a single final state photon is typically enhanced relative to ordinary annihilation into photon pairs. Semi-annihilation allows for a wide range of dark matter masses compared to the fixed mass value required by annihilation, opening the possibility to explain potential dark matter signatures at higher energies. The most striking prediction of semi-annihilation is the presence of multiple gamma lines, with as many as order N^3 lines possible for N dark sector states, allowing for dark sector spectroscopy. A smoking gun signature arises in the simplest case of degenerate dark matter, where a strong semi-annihilation line at 130 GeV would be accompanied by a weaker annihilation line at 173 GeV. As a proof of principle, we construct two explicit models of dark matter semi-annihilation, one based on non-Abelian vector dark matter and the other based on retrofitting Rayleigh dark matter.