33RD INTERNATIONAL COSMIC RAY CONFERENCE, RIO DE JANEIRO 2013 THE ASTROPARTICLE PHYSICS CONFERENCE Mass composition and hadronic interaction studies with ARGO-YBJ

(high resolution space-time pixels) and location (about 600g/cm2 of atmospheric depth) offer a unique opportunity for a detailed study of several characteristics of the hadronic component of the cosmic ray flux in the 1012-1015 eV energy range. The analog readout of the RPC signals indeed provides a powerful tool to study, with unprecedented resolution and without saturation, the extensive air shower space-time structure very close to its axis. The distribution of charged particles at ground and the time structure of the shower front allow estimating the shower age at the detection level independently from the primary mass. Furthermore the truncated size, measured within few meters from the core, gives a reliable energy measurement without biases introduced by finite detector effects. Fluctuations are also reduced thanks to the proximity of the shower maximum to the high altitude detection level. These features allows mass composition studies with an EAS detector in an energy region where a comparison with space or balloon born experiments are now possible for the first time, thus giving a further cross checks on the systematics of the adopted analysis procedures. Moreover, measurements of the proton-air cross section, of the particle distribution close to the shower axis, etc., give new inputs, in the very forward region, to the hadronic interaction models currently used for the study of the cosmic ray flux and its origin up to the highest energies