Study of the shower front structure at few meters from the core with ARGO-YBJ

The peculiar features of the ARGO-YBJ detector, i.e. the full-coverage and charge readout segmentation, together with the linearity range of the analog charge readout system, allow the analysis of the detailed structure of the EAS particle density distribution (LDF) around its axis. Particle densities up to 10000/m2 can be measured without detector saturation. This allows detailed studies in a previously unexplored range, thus giving new inputs to hadronic interaction models in the very forward region up to TeV center of mass energy. Several LDF types are compared with measurements and detector simulation, showing that the best performance in terms of data fitting, number of free parameters and relevant physics information is given by a suitably modified Nishimura-Kamata-Greisen function. Furthermore, the lateral slope parameter is shown to be related to the shower age by a universal behavior which is independent from the primary mass. This allows mass composition and hadronic interaction studies in an energy region marked by the transition from direct to indirect measurements, where new interesting features are now being suggested by last generation experiments.