Search for photons above <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow><mml:mn>10</mml:mn></mml:mrow><mml:mrow><mml:mn>18</mml:mn></mml:mrow></mml:msup></mml:mrow><mml:mtext> </mml:mtext><mml:mtext> </mml:mtext><mml:mi>eV</mml:mi></mml:math> by simultaneously measuring the atmospheric depth and the muon content of air showers at the Pierre Auger Observatory

The Pierre Auger Observatory is the most sensitive instrument to detect photons with energies above <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mrow><a:mn>1</a:mn><a:msup><a:mrow><a:mn>0</a:mn></a:mrow><a:mrow><a:mn>17</a:mn></a:mrow></a:msup><a:mtext> </a:mtext><a:mtext> </a:mtext><a:mi>eV</a:mi></a:mrow></a:math>. It measures extensive air showers generated by ultrahigh energy cosmic rays using a hybrid technique that exploits the combination of a fluorescence detector with a ground array of particle detectors. The signatures of a photon-induced air shower are a larger atmospheric depth of the shower maximum (<c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:msub><c:mi>X</c:mi><c:mi>max</c:mi></c:msub></c:math>) and a steeper lateral distribution function, along with a lower number of muons with respect to the bulk of hadron-induced cascades. In this work, a new analysis technique in the energy interval between 1 and 30 EeV (<e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:mrow><e:mn>1</e:mn><e:mtext> </e:mtext><e:mtext> </e:mtext><e:mi>EeV</e:mi><e:mo>=</e:mo><e:mn>1</e:mn><e:msup><e:mrow><e:mn>0</e:mn></e:mrow><e:mrow><e:mn>18</e:mn></e:mrow></e:msup><e:mtext> </e:mtext><e:mtext> </e:mtext><e:mi>eV</e:mi></e:mrow></e:math>) has been developed by combining the fluorescence detector-based measurement of <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"><g:msub><g:mi>X</g:mi><g:mi>max</g:mi></g:msub></g:math> with the specific features of the surface detector signal through a parameter related to the air shower muon content, derived from the universality of the air shower development. No evidence of a statistically significant signal due to photon primaries was found using data collected in about 12 years of operation. Thus, upper bounds to the integral photon flux have been set using a detailed calculation of the detector exposure, in combination with a data-driven background estimation. The derived 95% confidence level upper limits are 0.0403, 0.01113, 0.0035, 0.0023, and <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"><i:mrow><i:mn>0.0021</i:mn><i:mtext> </i:mtext><i:msup><i:mrow><i:mi>km</i:mi></i:mrow><i:mrow><i:mo>−</i:mo><i:mn>2</i:mn></i:mrow></i:msup><i:mtext> </i:mtext><i:msup><i:mrow><i:mi>sr</i:mi></i:mrow><i:mrow><i:mo>−</i:mo><i:mn>1</i:mn></i:mrow></i:msup><i:mtext> </i:mtext><i:msup><i:mrow><i:mi>yr</i:mi></i:mrow><i:mrow><i:mo>−</i:mo><i:mn>1</i:mn></i:mrow></i:msup></i:mrow></i:math> above 1, 2, 3, 5, and 10 EeV, respectively, leading to the most stringent upper limits on the photon flux in the EeV range. Compared with past results, the upper limits were improved by about 40% for the lowest energy threshold and by a factor 3 above 3 EeV, where no candidates were found and the expected background is negligible. The presented limits can be used to probe the assumptions on chemical composition of ultrahigh energy cosmic rays and allow for the constraint of the mass and lifetime phase space of super-heavy dark matter particles. Published by the American Physical Society 2024