The ARGO-YBJ detector layout (a full coverage Resistive Plate Chamber, RPC, carpet), features
(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
