Cosmic Ray Interaction Models: an Overview

Abstract

I review the state-of-the-art concerning the treatment of high energy cosmic ray interactions in the atmosphere, discussing in some detail the underlying physical concepts and the possibilities to constrain the latter by current and future measurements at the Large Hadron Collider. The relation of basic characteristics of hadronic interactions to the properties of nuclear-electromagnetic cascades induced by primary cosmic rays in the atmosphere is addressed. Experimental studies of high energy cosmic rays (CR) are traditionally performed using the so-called extensive air shower (EAS) techniques: the properties of the primary CR particles are reconstructed from measured characteristics of nuclear-electro-magnetic cascades induced by their interactions in the atmosphere. This naturally implies the importance of detailed Monte Carlo simulations of EAS development, particularly, of its backbone - the cascade of nuclear interactions of both the primary particles and of the secondary hadrons produced. Thus, the very success of experimental studies depends crucially on the validity of hadronic interaction models used in the analysis. Typically, one chooses between two main experimental procedures (1). In the first case, one deals with the information obtained with scintillation detectors positioned at ground. The energy of the primary particle is reconstructed from the measured lateral density of charged particles (mostly, elec- trons and positrons) while the particle type is inferred from the relative fraction of muons, compared to all charged particles at ground. Alternatively, one may study the longitudinal EAS development by measuring fluorescence light produced by excited air molecules at different heights in the atmo- sphere, for which purpose dedicated fluorescence telescopes are employed. In the latter case, the primary energy is related to the total amount of fluorescence light emitted. In turn, the particle type may be determined from the measured position of the so-called shower maximum Xmax - the depth in the atmosphere (in g/cm 2 ) where the number of ionizing particles reaches its maximal value (which is thus the brightest spot on the shower image). Not surprisingly, the observables used to determine the primary particle type - the lateral muon density ρμ and the EAS maximum position Xmax - appear to be very sensitive to details of high energy hadronic interactions. More precisely, Xmax depends strongly on the properties of the primary particle interaction with air nuclei: the inelastic cross section and the forward spectra of secondary hadrons produced. In turn, the EAS muon content is formed in a multi-step cascade process, driven mostly