Probing axion-like particles through the gamma-ray production from cosmic-ray scattering in the Milky Way dark matter halo

Abstract: Axion-like particles (ALP) are promising candidates to comprise all the dark matter in the universe. We investigate the ALP couplings to photons and electrons via astrophysical measurements through the search for very-high-energy gamma rays arising from high-energy cosmic-ray scattering off ALP populating the halo of the Milky Way. We show that gamma-ray signals from ALP couplings to photons and electrons via inverse Primakoff and Compton processes respectively, can be probed by very-high-energy ($\gtrsim$100 GeV) gamma-ray ground-based observatories, providing an alternative and complementary avenue to probe ALP couplings in the eV mass range. Sensitivities of current and near-future ground-based gamma-ray observatories improves upon one order of magnitude the current constraints from gamma-ray satellite experiments for the ALP-photon couplings in the region of masses below 10$^{-9}$ GeV. Their sensitivities reached on the ALP-electron couplings allow probing masses below 10$^{-8}$ GeV, which are lower than the masses probed in gamma-ray satellite experiments.