Constraining Axion-Like Particles with HAWC Observations of TeV Blazars

Axion-like particles (ALPs) are a broad class of pseudo-scalar bosons that generically arise from broken symmetries in extensions of the standard model. In many scenarios, ALPs can mix with photons in regions with high magnetic fields. Photons from distant sources can mix with ALPs, which then travel unattenuated through the Universe, before they mix back to photons in the Milky Way galactic magnetic field. Thus, photons can traverse regions where their signals would normally be blocked or attenuated. In this paper, we study TeV $\gamma$-ray observations from distant blazars, utilizing the significant $\gamma$-ray attenuation expected from such signals to look for excess photon fluxes that may be due to ALP-photon mixing. We find no such excesses among a stacked population of seven blazars and constrain the ALP-photon coupling constant to fall below $\sim$3$\times$10$^{-11}$ GeV$^{-1}$ for ALP masses below 300 neV. These results are competitive with, or better than, leading terrestrial and astrophysical constraints in this mass range.