Hadronic interactions at ultra high energies -- tests with the Pierre Auger Observatory

The Pierre Auger Observatory is a hybrid detector for cosmic rays with E > 1EeV. From the gathered data we estimated the proton-proton cross-section at sqrt(s) = 55 TeV and tested other features of the hadronic interaction models, which use extrapolations from the LHC energy. The electromagnetic component, carrying most of the energy of the shower, is precisely measured using fluorescence telescopes, while the hadronic back- bone of the shower is indirectly tested by measuring the muons arriving to the surface detector. The analyses show that models fail to describe these two components consistently, predicting too few muons at the ground.