Ultra-high energy cosmic rays (UHECR) can be used to study hadronic interactions beyond LHC energies. In this contribution we summarize relevant data from the Pierre Auger Observatory.
While the proton-air cross section has been measured at sqrt(s) = 57 TeV and found to be in good agreement with extrapolations from LHC energies, other observables are significantly different to what is predicted using current models. In particular, the predictions from models of showers based on LHC data are in strong contradiction with the observed number of muons. More muons are detected than predicted with the magnitude of the effect being model-dependent. Another observable from the Auger Observatory, the distribution of the depths of muon production, is also poorly described. Indeed no current model is capable of describing the full range of data from the Observatory, thus highlighting deficiencies in extrapolations beyond LHC energies.
The discrepancy between hadronic models can be examined further by measuring separately the muon and electromagnetic components of the signal recorded by detectors on the ground. The Auger Observatory is being upgraded by the addition of plastic scintillators above the water-Cherenkov detectors to achieve this. The new observations will enable rigorous testing of hadronic models up to sqrt(s) ~100 TeV and are crucial to the quest of determining the composition of UHECR.