Search for the imprint of axion-like particles in the highest-energy photons of hard γ-ray blazars

Abstract

Axion-like particles (ALPs), predicted in theories beyond the Standard Model, can have observational effects on the transparency of the Universe to γ rays in the presence of magnetic fields. In this work, we search for effects compatible with the existence of ALPs with 80 months of data from the Fermi Large Area Telescope, by comparing the distributions of observed highest energy photons from sources beyond redshifts of z ⩾ 0.1 with theoretical predictions in the presence of ALPs. We find no evidence for an increased γ-ray transparency due to ALPs and therefore we set limits on the ALPs parameters assuming a value of the intergalactic magnetic field strength of 1 nG. Photon-ALP couplings above 10−11 GeV−1 are excluded for ALP masses ma ≲ 3.0 neV . As the allowed magnetic field parameter space is large, we also test lower magnetic field strengths and no constraints can be set for B⩽0.1 nG below the CAST limit. These constraints exclude a region of the parameter space not covered by other γ-ray telescopes and are compatible with limits imposed by other experiments.