Interactions of Cosmic Rays in the primary energy range (0.1-1) PeV studied by the ARGO-YBJ Experiment

The ARGO-YBJ detector layout, features and location offer a unique possibility for a detailed study of several characteristics of the hadronic component of the cosmic ray flux in an energy range hardly accessed by direct measurements. The proton-air cross section has already been measured by ARGO-YBJ in an energy region up to similar to 100 TeV where the primary cosmic ray composition is sufficiently well known for that measure to be unbiased. The analog readout of the RPC signals now provides new tools to measure the lateral distribution of particle density ( LDF) as close as a few meters from the core and to investigate with unprecedented resolution the shower time structure. This allows to extend the hadronic interaction and mass composition studies up to similar to PeV energies in the laboratory rest frame. Moreover, it is shown that the LDF of detected showers can be properly described, even very close to the shower axis, by a NKG-like function, whose shape parameter is clearly related to the shower age and can be used for studying the mass composition of cosmic rays.