Constraints on the flux of primary cosmic-ray photons at energies E > 10^18 eV from Yakutsk muon data

TL;DR

This study uses Yakutsk muon data to set stringent upper limits on the flux and fraction of primary cosmic-ray photons at energies above 10^18 eV, improving sensitivity to photon fractions below 1%.

Contribution

It provides new upper limits on ultra-high-energy cosmic-ray photon fluxes using combined surface and underground detector data, achieving unprecedented sensitivity.

Findings

Upper limits on photon flux at E > 10^18 eV and higher energies.

Photon fraction sensitivity below 1%.

Enhanced constraints on cosmic-ray composition at ultra-high energies.

Abstract

Comparing the signals measured by the surface and underground scintillator detectors of the Yakutsk Extensive Air Shower Array, we place upper limits on the integral flux and the fraction of primary cosmic-ray photons with energies E > 10^18 eV, E > 2*10^18 eV and E > 4*10^18 eV. The large collected statistics of the showers measured by large-area muon detectors provides a sensitivity to photon fractions < 10^-2, thus achieving precision previously unreachable at ultra-high energies.