Search for a diffuse flux of photons with energies above tens of PeV at the Pierre Auger Observatory

TL;DR

This study searches for diffuse high-energy photons above 50 PeV at the Pierre Auger Observatory, setting upper limits on their flux and enhancing the cosmic-ray energy range coverage for future dark matter and cosmic-ray research.

Contribution

First combined air-shower and muon measurements at Pierre Auger Observatory to set limits on diffuse photon flux above 50 PeV.

Findings

Upper limits on photon flux range from 13.3 to 13.8 km$^{-2}$ sr$^{-1}$ yr$^{-1}$.

Extended photon search to lower energies, covering over three decades of cosmic-ray energy.

Future data expected to improve flux limits by a factor of 20.

Abstract

Diffuse photons of energy above 0.1 PeV, produced through the interactions between cosmic rays and either interstellar matter or background radiation fields, are powerful tracers of the distribution of cosmic rays in the Galaxy. Furthermore, the measurement of a diffuse photon flux would be an important probe to test models of super-heavy dark matter decaying into gamma-rays. In this work, we search for a diffuse photon flux in the energy range between 50 PeV and 200 PeV using data from the Pierre Auger Observatory. For the first time, we combine the air-shower measurements from a 2 km$^2$ surface array consisting of 19 water-Cherenkov surface detectors, spaced at 433 m, with the muon measurements from an array of buried scintillators placed in the same area. Using 15 months of data, collected while the array was still under construction, we derive upper limits to the integral photon flux ranging from 13.3 to 13.8 km$^{-2}$ sr$^{-1}$ yr$^{-1}$ above tens of PeV. We extend the Pierre Auger Observatory photon search program towards lower energies, covering more than three decades of cosmic-ray energy. This work lays the foundation for future diffuse photon searches: with the data from the next 10 years of operation of the Observatory, this limit is expected to improve by a factor of $\sim$20.