Search for photons above 10$^{19}$ eV with the surface detector of the Pierre Auger Observatory

TL;DR

This study uses the Pierre Auger Observatory's surface detector data from 2004 to 2020 to set upper limits on ultra-high energy photon fluxes above 10^19 eV, informing cosmic ray and dark matter models.

Contribution

First comprehensive search for photons above 10^19 eV using surface detector data, establishing new upper limits and implications for cosmic ray composition and dark matter.

Findings

Set upper limits on photon fluxes at 2.11×10^{-3}, 3.12×10^{-4}, 1.72×10^{-4} km^{-2} sr^{-1} yr^{-1} for energies above 10^{19}, 2×10^{19}, 4×10^{19} eV.

Sensitivity approaches cosmogenic flux expectations for pure-proton composition at 2×10^{19} eV.

Limits are above realistic mixed-composition model predictions, impacting dark matter searches.

Abstract

We use the surface detector of the Pierre Auger Observatory to search for air showers initiated by photons with an energy above $10^{19}$ eV. Photons in the zenith angle range from 30$^\circ$ to 60$^\circ$ can be identified in the overwhelming background of showers initiated by charged cosmic rays through the broader time structure of the signals induced in the water-Cherenkov detectors of the array and the steeper lateral distribution of shower particles reaching ground. Applying the search method to data collected between January 2004 and June 2020, upper limits at 95\% CL are set to an $E^{-2}$ diffuse flux of ultra-high energy photons above $10^{19}$ eV, $2{\times}10^{19}$ eV and $4{\times}10^{19}$ eV amounting to $2.11{\times}10^{-3}$, $3.12{\times}10^{-4}$ and $1.72{\times}10^{-4}$ km$^{-2}$ sr$^{-1}$ yr$^{-1}$, respectively. While the sensitivity of the present search around $2 \times 10^{19}$ eV approaches expectations of cosmogenic photon fluxes in the case of a pure-proton composition, it is one order of magnitude above those from more realistic mixed-composition models. The inferred limits have also implications for the search of super-heavy dark matter that are discussed and illustrated.