The cosmic ray spectrum extends to particles with energy
E ∼ 1020 eV, that corresponds (assuming that the primary particle is a proton) to a nucleon–nucleon c.m. energy
, 50 times higher than the current LHC
energy. These very high energy particles can be studied via the observation of the showers they generate in the atmosphere. The interpretation of the data requires therefore the modeling of hadronic interactions in an energy range beyond what can be studied in accelerator experiments. The theoretical problem of estimating the relevant properties of hadronic interactions in this energy range is therefore of central importance for the interpretation of the cosmic ray data. Viceversa, it is in principle possible to obtain information about hadronic interactions from the cosmic ray observations, but this program has to confront the fact that the (freely available) cosmic ray beam has an unknown energy
spectrum and an unknown mass composition.