Cosmic Rays and the Hunt for Ultra-High-Energy Photons at the the Pierre Auger Observatory

**Introduction*** The podcast discusses the search for **diffuse photons** with energies above tens of PeV, using data from the **Pierre Auger Observatory**.* These photons are produced by interactions between cosmic rays and interstellar matter or background radiation.* The measurement of a diffuse photon flux can help us understand the distribution of cosmic rays in the Galaxy and probe models of super-heavy dark matter.**The Pierre Auger Observatory*** The observatory uses a surface detector (SD) and an underground muon detector (UMD).* The SD array consists of **water-Cherenkov detectors (WCDs)**, and the UMD uses **buried scintillators**.* The study focuses on data from a 2 km² area with 19 WCDs and 11 UMD stations.* The combination of SD and UMD measurements allows for a more accurate analysis of air showers.**The Search for Photons*** Primary photons are difficult to distinguish from the background of charged cosmic rays.* Photon-initiated air showers are mostly electromagnetic, while hadron-initiated showers have more muons.* The analysis uses a **muon content estimator (Mb)** to discriminate between photon and hadron events.* The study uses **15 months of data** collected during the construction of the array.* A photon-equivalent energy scale is developed for comparing events initiated by different primary species.**Results and Implications*** No photon candidate events were identified in the data.* Upper limits on the integral photon flux were set between 13.3 and 13.8 km−2 sr−1 yr−1 above tens of PeV.* These limits are the only ones based on measurements from the **Southern Hemisphere** in this energy domain.* The analysis extends the Pierre Auger Observatory photon search program to lower energies.* The results provide constraints on models of super-heavy dark matter.* Future data from the observatory is expected to improve the upper limit by a factor of ~20.**Article Reference:*** A. Abdul Halim et al., Search for a diffuse flux of photons with energies above tens of PeV at the Pierre Auger Observatory, 2024,Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: Pierre Auger Observatory