ARGO-YBJ: Status and First Results

Very high energy gamma ray astronomy is one of the scientific goals of the ARGO-YBJ experiment. The detector, which is located in Tibet (China) at 4300 m a.s.l., is a full coverage Extensive Air Shower array consisting of a carpet of Resistive Plate Chambers (RPCs) of about 7000m2. The high altitude and the full coverage ensure a very low energy threshold (few hundreds of GeV for primary photons), while the detector time resolution (σt ≃ 1 ns) gives a good pointing accuracy, thus allowing a high sensitivity to γ-ray sources, with a field of view of more than 2 sr and a duty cycle close to 100%. 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 several hundreds of TeV. In particular, the topological structure of the shower, the lateral distribution, the energy spectrum and the space and time 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 shower front, that can be used to test different hypotheses on the cosmic ray interaction model, the shower development 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 the first results coming from the analysis of a data sample collected with a relevant fraction of the apparatus that is already in continuous data taking.