1. Hadronic interaction studies with the ARGO-YBJ experiment (5,800 m 2 ) 10 Pads (56 x 62 cm 2 ) for each RPC 8 Strips (6.5 x 62 cm 2 ) for each Pad (  43 m 2 ) 1 CLUSTER = 12 RPC Fig.1 Detector layout. The detector Layout:modular, full coverage extensive air shower array (Fig.1) made by a single layer of Resistive Plate Chambers (RPCs). Location: high altitude CR Laboratory of Yangbaijing (Tibet, China), 4300 m a.s.l. (~606 g/cm 2 of vertical atmospheric depth). The main physics items   ray astronomy with few hundreds GeV energy threshold   ray bursts in the full GeV – TeV energy range  cosmic ray physics from 1 TeV up to few PeV primary energy (range made possible by the analog RPC charge readout system) Main detector features: (a) setup granularity and time resolution (1-2 ns) provide a detailed 3D reconstruction of the detected shower front with unprecedented space-time resolution (b) the RPC analog charge readout from large pads allows to measure particle densities up to ~10 4 /m 2, thus overriding the saturated digital information, and inspect the shower lateral distribution function (LDF) near the core position (Fig.2) Possibility to investigate several characteristics of the hadronic interactions and to extend the p-air cross section measurement up to ~ PeV proton energy. ARGO-YBJ already measured the p-air cross section in the energy range (1-100) TeV, thus providing an estimate of the total p-p cross section at  s ~ (70-500) GeV [1]. Fig.2 Particle distribution in a real event detected by ARGO-YBJ. MC simulation: observables and physics parameters Samples of proton and iron initiated showers produced with CORSIKA code (  zenith =0-30°,  E=1-3000TeV), using two hadronic interaction models (QGSjet and Sibyll). Detector response simulated by a GEANT package based program. Same format and reconstruction code used for real data. Selection: events triggering the analog RPC readout (≥ 73 pad/cl), with core in a fiducial area 64x64m 2 and  zenith <15°. Data analysis: preliminary results  A sample of 3.5·10 6 events triggering the analog RPC readout system (trigger rate ~8 Hz) has been analyzed.  The same selections applied in the MC reduce the data to 5·10 5 events.  A calibration procedure (accounting for both detector and electronics response) has been used to convert ADC counts to particles [2]. Estimator of the primary E Fig.3 N p8, the number of particles detected within a distance of 8m from the shower core results well correlated with primary energy E. Fig.4 Systematics introduced on LDF by the used hadronic interaction model. Difference is within few %. Fig.5 Correlation between X dm (the distance of shower maximum from the detection level) and LDF slope 1m from the core. Fig.6 Difference of particle densities  0 and  1 (at 0m and 1m from the core) as a function of N p8 for p and iron initiated showers. Fig.7 Curvature  of the shower front vs R q70, the radius of the circle including ≥ 70% of detected particles, for p and iron initiated showers. Fig.8 Experimental particle spectrum: digital and analog systems agree fairly well so as the estimated particle multiplicity spectra can be fitted with a single power law. Fig.9 Experimental LDF for two N p8 intervals and events with core located in a 48x48m 2 fiducial area. The results are in good agreement with MC. LDF and hadronic modelsShower age selection Primary mass discrimination [1] G. Aielli et al. (ARGO-YBJ coll.), Phys Rev. D, 2009, 80 : 092004 [2] M. Iacovacci et al. (ARGO-YBJ coll.), this Conference. Particle spectrum LDF I. De Mitri 1,2, G. Marsella 3,2, L. Perrone 3,2, A. Surdo 2, G. Zizzi 4 for the ARGO-YBJ Collaboration 1 Dipartimento di Fisica, Università del Salento, Lecce, Italy 2 Istituto Nazionale di Fisica Nucleare (INFN), Sezione di Lecce, Italy 3 Dipartimento di Ingegneria dell’Innovazione, Università del Salento, Lecce, Italy 4 Istituto Nazionale di Fisica Nucleare, CNAF, Bologna, Italy 754