Exciting dark matter in the galactic center

TL;DR

This paper reevaluates excited dark matter models as an explanation for the 511 keV gamma-ray excess in the galactic center, focusing on cross section calculations and model constraints.

Contribution

It provides a detailed computation of dark matter annihilation cross sections mediated by a light gauge boson and explores conditions for matching observed positron rates.

Findings

Standard models are insufficient to explain the gamma-ray excess without extreme assumptions.

A metastable dark matter population can produce the observed positron rate under certain mass gap conditions.

Predicted positron production rates are generally too low unless specific model parameters are met.

Abstract

We reconsider the proposal of excited dark matter (DM) as an explanation for excess 511 keV gamma rays from positrons in the galactic center. We quantitatively compute the cross section for DM annihilation to nearby excited states, mediated by exchange of a new light gauge boson with off-diagonal couplings to the DM states. In models where both excited states must be heavy enough to decay into e^+ e^- and the ground state, the predicted rate of positron production is never large enough to agree with observations, unless one makes extreme assumptions about the local circular velocity in the Milky Way, or alternatively if there exists a metastable population of DM states which can be excited through a mass gap of less than 650 keV, before decaying into electrons and positrons.