Narrow Results

The understanding of the basic properties of the ultrahigh-energy extensive air showers is dependent on the description of hadronic interactions in an energy range beyond that probed by the LHC. One of the uncertainties present in the modeling of air showers is the treatment of diffractive interactions, which are dominated by nonperturbative physics and usually described by phenomenological models. These interactions are expected to affect the development of the air showers, since they provide a way of transporting substantial amounts of energy deep in the atmosphere, modifying the global characteristics of the shower profile. In this paper, we investigate the impact of diffractive interactions in the observables that can be measured in hadronic collisions at high energies and ultrahigh-energy cosmic ray interactions. We consider three distinct phenomenological models for the treatment of diffractive physics and estimate the influence of these interactions on the elasticity, number of secondaries, longitudinal air shower profiles and muon densities for proton-air and iron-air collisions at different primary energies. Our results demonstrate that even for the most recent models, diffractive events have a non-negligible effect on the observables and that the distinct approaches for these interactions, present in the phenomenological models, still are an important source of theoretical uncertainty for the description of the extensive air showers.

The ARGO-YBJ experiment has been in stable data taking for more than 5 years at the YangBaJing Cosmic Ray Observatory (Tibet, P. R. China, 4300 m a.s.l., 606 g/cm²). With a duty-cycle greater than 86%, the detector collected about 5 × 10¹¹ events in a wide energy range, from few hundred GeV up to about 10 PeV. High altitude location and detector features make ARGO-YBJ capable of investigating a wide range of important issues in Cosmic Ray and Astroparticle Physics by imaging the front of atmospheric showers with unprecedented resolution and detail. In this paper, the main physics results in gamma-ray astronomy and in cosmic ray physics are summarized.

Very high energy gamma ray astronomy is one of the scientific goals of the ARGO-YBJ experiment. The detector, 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 7000 m². 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 (σ_t ≃ 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 some of 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.