Investigating the hadron nature of high-energy photons with PeVatrons

Abstract

In high energy Gamma-Ray Astronomy with shower arrays the most discriminating signature of the photon-induced showers against the background of hadron-induced cosmic-ray is the content of muons in the observed events. In the electromagnetic $\gamma$-showers the muon production is mainly due to the photo-production of pions followed by the decay $\pi\to\mu\nu$. In high energy photo-production process the photon exhibits an internal structure which is very similar to that of hadrons. Indeed, photon-hadron interactions can be understood if the physical photon is viewed as a superposition of a bare photon and an accompanying small hadronic component which feels conventional hadronic interactions. Information on photo-production $\gamma$p and $\gamma\gamma$ cross-sections are limited to $\sqrt{s}\leq$ 200 GeV from data collected at HERA. Starting from $E_{lab}\approx$100 TeV the difference between different extrapolations of the cross sections increases to more than 50\% at $E_{lab}\approx$10$^{19}$ eV, with important impact on a number of shower observables and on the selection of the photon-initiated air showers. Recently, the LHAASO experiment opened the PeV-sky to observations detecting 40 PeVatrons in a background-free regime starting from about $E_{lab}\approx$ 100 TeV. This result provides a beam of pure high energy primary photons allowing to measure for the first time the photo-production cross section even at energies not explored yet. The future air shower array SWGO in the Southern Hemisphere, where the existence of Super-Pevatrons emitting photons well above the PeV is expected, could extend the study of the hadron nature of the photons in the PeV region. In this contribution the opportunity for a measurement of the photo-production cross section with air shower arrays is presented and discussed.