Seeking dark matter with $\gamma$-ray attenuation

TL;DR

This paper investigates the attenuation of high-energy gamma rays by extragalactic background light, finding a possible excess that could be explained by dark matter axions, and sets new bounds on their properties.

Contribution

It provides the most current analysis of gamma-ray attenuation, exploring potential dark matter signatures and establishing the strongest bounds on photon-axion coupling in the 8-25 eV range.

Findings

Gamma-ray optical depth exceeds galaxy-based expectations at ~2σ.

An excess in EBL is suggested at 14-30%, with 2.7σ significance.

No definitive dark matter detection, but new constraints on axion-photon coupling.

Abstract

The flux of high-energy astrophysical $\gamma$ rays is attenuated by the production of electron-positron pairs from scattering off of extragalactic background light (EBL). We use the most up-to-date information on galaxy populations to compute their contributions to the pair-production optical depth. We find that the optical depth inferred from $\gamma$-ray measurements exceeds that expected from galaxies at the $\sim2\sigma$ level. If the excess is modeled as a frequency-independent re-scaling of the standard contribution to the EBL from galaxies, then an excess (an overall $14-30\%$ increase of the EBL) over the null hypothesis of no excess at the $2.7\sigma$ level. If the frequency dependence of the excess is instead modeled as a two-photon decay of a dark-matter axion, then the excess is favored over the null hypothesis at the $2.1\sigma$ confidence level. While we find no evidence for a dark-matter signal, the analysis sets the strongest current bounds on the photon-axion coupling over the $8-25$ eV mass range. This work highlights the sensitivity of $\gamma$-ray optical depth measurements to ALPs, which is expected to improve with new observatories and better EBL determinations from future observations.