A study of photons at ultra high energy using Auger surface detector

TL;DR

This paper introduces a new event-by-event simulation method to set upper limits on ultra-high-energy photon flux using Auger surface detector data, improving accuracy with fewer simulations.

Contribution

It presents an innovative approach that uses adapted photon shower simulations for each observed event, reducing computational effort while maintaining near-ideal upper limit accuracy.

Findings

Achieved an upper limit on photon flux above 150 EeV.

Demonstrated that 10 simulations per event suffice for near-ideal limits.

Method can be extended to lower energies for broader constraints.

Abstract

We present a new method to derive an upper limit on the ultra-high-energy (UHE) photon flux above a given energy, using the Auger SD events. As previous studies, it takes advantage of the fundamental differences between showers induced by hadrons and showers induced by photons (signal rise time, slope of the Lateral Distribution Function, radius of curvature), but it uses photon shower simulations adapted to the actually observed SD events, on an event-by-event basis. In particular, we find that a limited number of simulations per event allows to reach an upper limit that is very close to the ideal case that would be obtained with an infinite number of simulations. For instance, 10 simulated showers per event are sufficient to reach an upper limit only 10 percent higher than the ideal one. As an illustration, we apply this method to the SD events reconstructed above 55 EeV, which provides an optimized upper limit on the photon flux above 150 EeV, and indicate how this can be easily extended to the lower-energy events, to constrain the photon flux down to much lower energies.