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 $\ddot{I}{€}^0\to {\hat{I}}^3{\hat{I}}^3$ 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 $\ddot{I}{€}^0$'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 $\ddot{I}{€}^0$-production alone is insufficient to explain the data.

8 pages, 9 figures, accepted for publication in ApJ