Observation of the cosmic ray moon shadowing effect with the ARGO-YBJ experiment

Abstract

Cosmic rays are hampered by the Moon and a deficit in its direction is expected (the so-called Moon shadow). The Moon shadow is an important tool to determine the performance of an air shower array. Indeed, the westward displacement of the shadow center, due to the bending effect of the geomagnetic field on the propagation of cosmic rays, allows the setting of the absolute rigidity scale of the primary particles inducing the showers recorded by the detector. In addition, the shape of the shadow permits to determine the detector point spread function, while the position of the deficit at high energies allows the evaluation of its absolute pointing accuracy. In this paper we present the observation of the cosmic ray Moon shadowing effect carried out by the ARGO-YBJ experiment in the multi-TeV energy region with high statistical significance (55 standard deviations). By means of an accurate Monte Carlo simulation of the cosmic rays propagation in the Earth-Moon system, we have studied separately the effect of the geomagnetic field and of the detector point spread function on the observed shadow. The angular resolution as a function of the particle multiplicity and the pointing accuracy have been obtained. The primary energy of detected showers has been estimated by measuring the westward displacement as a function of the particle multiplicity, thus calibrating the relation between shower size and cosmic ray energy. The stability of the detector on a monthly basis has been checked by monitoring the position and the deficit of the Moon shadow. Finally, we have studied with high statistical accuracy the shadowing effect in the day/''night'' time looking for possible effect induced by the solar wind

Light-component spectrum of the primary cosmic rays in the multi-TeV region measured by the ARGO-YBJ experiment

“…The detector performance is discussed in [12,16]. The angular resolution has been estimated using different methods and is better than 0.6 [17] for events with more than 200 fired pads. The installation of the central carpet was completed in June 2006.…”

Cosmic ray proton plus helium energy spectrum measured by the ARGO-YBJ experiment in the energy range 3–300 TeV

“…A dedicated procedure was implemented to calibrate the detector in order to achieve high pointing accuracy [7]. The angular and core reconstruction resolution are, respectively, 0.4°and 5 m for events with at least 500 fired pads [8,9]. The installation of the central carpet was completed in June 2006.…”

“…This means that the absolute energy scale difference of the two experiments is within 4% and 6%. The uncertainty on the absolute energy scale has been evaluated by exploiting the Moon shadow tool at a level of 10% for energies below 30 TeV [8]. The ARGO-YBJ experiment is a ground-based detector able to investigate the cosmic ray energy spectrum in an energy region so far accessible to direct detection experiment.…”

Results from the ARGO-YBJ experiment

“…by observing the CR deficit towards the Moon ( figure 3, left). It allows to evaluate the angular resolution, the pointing accuracy and to energy-calibrate the detector, as well as to check its operation stability [3]. Analog read-out.…”

“…Once the coincidence of the secondary particles has been recorded, the main parameters of the detected shower are reconstructed from the digital information, by following the procedure described in [3]: among them, the shower size (proportional to the primary CR energy) and the arrival direction. Since December 2009, the analog information about the event is recorded too.…”