First constraints on axion-like particles from Galactic sub-PeV gamma
  rays
  Experimental refinements and technical innovations in the field of extensive
air shower telescopes have enabled measurements of Galactic cosmic-ray
interactions in the sub-PeV (100 TeV to 1 PeV) range, providing new avenues for
the search for new physics and dark matter. For the first time, we exploit
sub-PeV (from 10 TeV to 1 PeV) observations of Galactic diffuse gamma rays by
Tibet AS$\gamma$ and HAWC to search for an axion-like-particle (ALP) induced
gamma-ray signal directly linked to the origin of the IceCube extragalactic
high-energy neutrino flux. Indeed, the production of high-energy neutrinos in
extragalatic sources implies the concomitant production of gamma rays at
comparable energies. Within the magnetic field of the neutrino emitting
sources, gamma rays may efficiently convert into ALPs and escape their host
galaxy unattenuated, propagate through intergalactic space, and reconvert into
gamma rays in the magnetic field of the Milky Way. Such a scenario creates an
all-sky diffuse high-energy gamma-ray signal in the sub-PeV range. Accounting
for the guaranteed Galactic astrophysical gamma-ray contributions from
cosmic-ray interactions with gas and radiation and from sub-threshold sources,
we set competitive upper limits on the photon-ALP coupling constant
$g_{a\gamma\gamma}$. We find $g_{a\gamma\gamma} < 2.1\times10^{-11}$ GeV$^{-1}$
for ALP masses $m_a \leq 2\times10^{-7}$ eV at a 95$\%$ confidence level,
progressively closing the mass gap towards ADMX limits.
