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

We use the surface detector of the Pierre Auger Observatory to search for air
showers initiated by photons with an energy above 1019 eV. Photons in the zenith angle range
from 30◦
to 60◦
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 waterCherenkov 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 1019 eV, 2×1019 eV and 4×1019 eV amounting to 2.11×10−3
, 3.12×10−4 and
1.72×10−4 km−2
sr−1 yr−1
, respectively. While the sensitivity of the present search around
2×1019 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.