Diffuse axion-like particle searches

We propose a new method to search for axion-like particles (ALPs) based on the gamma-rays produced concomitant with high-energy astrophysical neutrinos. The existence of high-energy neutrinos implies production of gamma-rays in the same sources. Photons can convert into ALPs in the sources’ magnetic fields, and will travel as ALPs through extragalactic space. Back-conversion in the Milky Way’s magnetic field leads to a diffuse anisotropic high-energy photon flux that existing and upcoming gamma-ray detectors, like HAWC, CTA, and LHAASO can detect. This method probes unexplored ALP parameter space, with LHAASO being realistically sensitive to couplings above ${10}^{-11}\mathrm{G}\mathrm{e}{\mathrm{V}}^{-1}$ and masses up to $3\times {10}^{-6}\mathrm{e}\mathrm{V}$ in ten years. Our technique also explores viable ALP dark matter parameter space.