Photon-likeness of hadron showers and impact of Lorentz boosting

TL;DR

This study investigates how Lorentz boosting affects the likelihood of proton-induced air showers mimicking photon events at ultra-high energies, using simulations and data comparison.

Contribution

It provides a detailed analysis of the impact of Lorentz boosting on photon-like signatures in ultra-high-energy cosmic ray showers, incorporating simulation results and observational data.

Findings

Photon-like events are more probable at EeV energies due to neutral pion decay.

The probability decreases at higher energies because of increased hadronic interactions of boosted pions.

Current observations cannot be fully explained by neutral pion fluctuations alone.

Abstract

We examine the probability of proton-induced air showers at $E>10\,$EeV being misidentified as photon-induced due to neutral pions receiving a major part of the primary energy in the first interaction, thereby enhancing the electromagnetic shower component by their $\pi^0 \to \gamma \gamma$ decay. Using CORSIKA simulations, we demonstrate the relevance of this effect at EeV energies. However, the probability for such photon-like events drops down strongly at the highest energies due to the increasing probability of Lorentz boosted $\pi^0$'s suffering hadronic interactions before decay. Different hadronic interaction models suggest that photon-like hadronic events may be observed at current UHECR observatories. A quantitative comparison of the observed number of background events found in recent photon searches published by the Pierre Auger Collaboration allows us conclude that the hypothesis of upwards fluctuations of $\pi^0$-production alone is insufficient to explain the data.