Photon-likeness of Hadron Showers and Impact of Lorentz Boosting

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 π 0 → γγ 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 π 0 ’s suffering hadronic interactions before decay. Different hadronic interaction models suggest that photon-like hadronic events may be observed at current ultra-high-energy cosmic-ray observatories. A quantitative comparison of the observed number of background events found in recent photon searches published by the Pierre Auger Collaboration suggests that the hypothesis of upward fluctuations in π 0 production alone is insufficient to explain the data. Detailed detector simulations will be needed to quantify the significance of these differences.