Very High Energy Gamma Ray Astronomy and Cosmic Ray Physics with the ARGO-YBJ experiment

Gamma ray astronomyat energies 1011 1013 eV, which are strictly related to the identification and study of the cosmic ray acceleration sites, is the main scientific goal of the ARGO-YBJ experiment. The detector, which is now being assembled in Tibet (China) at 4300 m a.s.l., is a full coverage Extensive Air Showerarray consisting of a carpet of Resistive Plate Chambers covering a surface of about 7000 m2. The high altitude ( atmospheric depth 600 g/cm2 ) and the full coverage ensure a very low primary photon energy threshold at few hundreds GeV (close to the limits of the satellite technology), while the detector time resolution σt 1ns gives a good pointing accuracy, thus allowing a high sensitivity to γ-ray sources. Moreover the large field of view and the high duty-cycle ensure the continuous monitoring of the sky in the declination band 20 δ 80 . The detector layout, performance and location, offer a unique possibility to make also a deep study of several characteristics of the hadronic component of the cosmic ray flux up to energies of hundreds of TeV. In particular, the structure of the showercore, the lateral distribution, the energy spectra and the angular (e.g. anisotropies) and time (e.g. solar flares) flux modulations can be measured with high sensitivity. Moreover, the use of a full coverage detector with a high space granularity gives detailed images of the showerfront, that can be used to test different hypotheses on the cosmic ray interactions, the showerdevelopment in the atmosphere and particle physics at very high energies. In this work the general layout of the detector and its performance will be described, together with some of the first results coming from the data analysis of a relevant fraction of the apparatus that is already operating.