Hadronic interactions and cosmic rays at ultra high energies

Abstract

Observations of ultra high energy cosmic rays allow in principle to obtain information about the properties of hadronic interactions in an energy range not accessible with particle accelerators. This task is however complicated by the fact that the composition of the primary CR is not known and must be estimated from the same data set. Solving the ambiguity between composition and hadronic interaction modeling is a central problem for UHECR observations. Recently the Pierre Auger collaboration has presented an estimate of the p-air interaction length at a laboratory energy E0 � 10 18.24 eV. The interaction length is inferred from the shape of the tail of the Xmax distribution of the observed showers, following a method pioneered by the Fly's Eye group. In this work we want to analyse critically this method, and discuss its potential for further studies. composition of these particles, and at the same time to obtain information about the properties of hadronic interaction in an energy range not accessible to accelerators. These two goals are unfortunately in conflict with each other. Uncertainties in the modeling of the properties of hadronic interactions at high energy limit the precision that can be achieved in the measurement of the composition of the primary radiation; viceversa, uncertainties in the composition of the primary particles do not allow an easy determination of the properties of the showers generated by protons (or other nuclear species). The best strategy to solve this ambiguity is the object of an active field of investigations. At the present moment there is still a significant uncertainty both on the composition of cosmic rays in the UHE range, and on the quality of the description of their showers by the existing montecarlo codes. The analysis of the current situation is difficult also because the observations made by the Pierre Auger (1, 2), HiRes (3) and Telescope Array (4) collaborations are not in agreement. The HiRes and Telescope Array results appear to be consistent with a composition that is proton dominated in the entire energy range, while the data of the Pierre Auger Observatory seem to imply a composition that becomes gradually enriched in heavy nuclei with increasing energy. This disagreement is a serious obstacle in the progress of the field, since a consistent interpretation of all data is currently impossible. It is clear that a satisfactory resolution of these discrepancies is urgent and important. Recently the Pierre Auger collaboration (5) has presented a measurement of the interaction length for protons with energy E0 � 10 18.24 eV (that corresponds to a c.m. energy √ s � 57TeV for nucleon- nucleon interactions, that corresponds to a p-air cross section 505 ± 23(stat) + 28 − 36(sys)mb (this must be understood as the production cross section, that refers to interactions where at least one additional pion is produced, excluding the elastic and target fragmentation processes). Converting the