Primary particle type of the most energetic Flyʹs Eye air shower

The longitudinal profile of the most energetic cosmic-ray air shower measured so far, the event recorded by the Flyʹs Eye detector with a reconstructed primary energy of about 3.2 × 1020 eV, is compared to simulated shower profiles. The calculations are performed with the CORSIKA code and include primary photons and different hadron primaries. For primary photons, preshower formation in the geomagnetic field is additionally treated in detail. For primary hadrons, the hadronic interaction models QGSJET 01 and SIBYLL 2.1 have been employed. The predicted longitudinal profiles are compared to the observation. A method for testing the hypothesis of a specific primary particle type against the measured profile is described which naturally takes shower fluctuations into account. The Flyʹs Eye event is compatible with any assumption of a hadron primary between proton and iron nuclei in both interaction models, although differences between QGSJET 01 and SIBYLL 2.1 in the predicted profiles of lighter nuclei exist. The primary photon profiles differ from the data on a level of ≃1.5σ. Although not favoured by the observation, the primary photon hypothesis can not be rejected for this particular event.