Seeking dark matter with <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>γ</mml:mi></mml:math>-ray attenuation
José Luis Bernal, Andrea Caputo, Gabriela Sato-Polito, Jordan Mirocha, Marc Kamionkowski
Abstract
The flux of high-energy astrophysical $\ensuremath{\gamma}$ 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 $\ensuremath{\gamma}$-ray measurements exceeds that expected from galaxies at the $\ensuremath{\sim}2\ensuremath{\sigma}$ 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) is favored over the null hypothesis of no excess at the $2.7\ensuremath{\sigma}$ 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 $2.1\ensuremath{\sigma}$ 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 $\ensuremath{\gamma}$-ray optical depth measurements to ALPs, which is expected to improve with new observatories and better EBL determinations from future observations.