Upper limit on the cosmic-ray photon fraction at EeV energies from the Pierre Auger Observatory

TL;DR

This paper reports the first experimental upper limits on the fraction of ultra-high energy cosmic-ray photons below 10 EeV, using data from the Pierre Auger Observatory, which constrains models of cosmic-ray origins.

Contribution

It provides the first experimental limits on ultra-high energy photon fractions below 10 EeV, improving understanding of cosmic-ray composition and reducing uncertainties in shower data interpretation.

Findings

Photon fraction limits: 3.8%, 2.4%, 3.5%, 11.7% at 95% c.l.

First limits on photons below 10 EeV.

Constraints on top-down cosmic-ray models.

Abstract

From direct observations of the longitudinal development of ultra-high energy air showers performed with the Pierre Auger Observatory, upper limits of 3.8%, 2.4%, 3.5% and 11.7% (at 95% c.l.) are obtained on the fraction of cosmic-ray photons above 2, 3, 5 and 10 EeV (1 EeV = 10^18 eV) respectively. These are the first experimental limits on ultra-high energy photons at energies below 10 EeV. The results complement previous constraints on top-down models from array data and they reduce systematic uncertainties in the interpretation of shower data in terms of primary flux, nuclear composition and proton-air cross-section.