Study of the shower front structure at few meters from the core with ARGO-YBJ

The peculiar features of the ARGO-YBJ detector, i.e. the full-coverage and charge readout segmentation, together with the linearity range of the analog charge readout system, allow the analysis of the detailed structure of the EAS particle density distribution (LDF) around its axis. Particle densities up to 10000/m2 can be measured without detector saturation. This allows detailed studies in a previously unexplored range, thus giving new inputs to hadronic interaction models in the very forward region up to TeV center of mass energy. Several LDF types are compared with measurements and detector simulation, showing that the best performance in terms of data fitting, number of free parameters and relevant physics information is given by a suitably modified Nishimura-Kamata-Greisen function. Furthermore, the lateral slope parameter is shown to be related to the shower age by a universal behavior which is independent from the primary mass. This allows mass composition and hadronic interaction studies in an energy region marked by the transition from direct to indirect measurements, where new interesting features are now being suggested by last generation experiments

Mass composition and hadronic interaction studies with ARGO-YBJ

The ARGO-YBJ detector layout (a full coverage Resistive Plate Chamber, RPC, carpet), features (high resolution space-time pixels) and location (about 600g/cm2 of atmospheric depth) offer a unique opportunity for a detailed study of several characteristics of the hadronic component of the cosmic ray flux in the 1012-1015 eV energy range. The analog readout of the RPC signals indeed provides a powerful tool to study, with unprecedented resolution and without saturation, the extensive air shower space-time structure very close to its axis. The distribution of charged particles at ground and the time structure of the shower front allow estimating the shower age at the detection level independently from the primary mass. Furthermore the truncated size, measured within few meters from the core, gives a reliable energy measurement without biases introduced by finite detector effects. Fluctuations are also reduced thanks to the proximity of the shower maximum to the high altitude detection level. These features allows mass composition studies with an EAS detector in an energy region where a comparison with space or balloon born experiments are now possible for the first time, thus giving a further cross checks on the systematics of the adopted analysis procedures. Moreover, measurements of the proton-air cross section, of the particle distribution close to the shower axis, etc., give new inputs, in the very forward region, to the hadronic interaction models currently used for the study of the cosmic ray flux and its origin up to the highest energies

Intrinsic linearity of bakelite Resistive Plate Chambers operated in streamer mode

Resistive Plate Chambers have largely been used in High Energy Physics and Cosmic Ray research. In view of using this detector for calorimetry applications it is important to know the maximum measurable particle density, or its intrinsic linearity limit, which is tightly related to the dimension of the discharge region. In this paper we report the results of measurements performed at the Beam Test Facility (INFN National Laboratory of Frascati, Italy) where the intrinsic linearity of bakelite RPCs operated in streamer mode has been tested at different impinging particle densities

Intrinsic linearity of bakelite Resistive Plate Chambers operated in streamer mode

Resistive Plate Chambers have largely been used in High Energy Physics and Cosmic Ray research. In view of using this detector for calorimetry applications it is important to know the maximum measurable particle density, or its intrinsic linearity limit, which is tightly related to the dimension of the discharge region. In this paper we report the results of measurements performed at the Beam Test Facility (INFN National Laboratory of Frascati, Italy) where the intrinsic linearity of bakelite RPCs operated in streamer mode has been tested at different impinging particle densities

Intrinsic linearity of bakelite Resistive Plate Chambers operated in streamer mode

Resistive Plate Chambers have largely been used in High Energy Physics and Cosmic Ray research. In view of using this detector for calorimetry applications it is important to know the maximum measurable particle density, or its intrinsic linearity limit, which is tightly related to the dimension of the discharge region. In this paper we report the results of measurements performed at the Beam Test Facility (INFN National Laboratory of Frascati, Italy) where the intrinsic linearity of bakelite RPCs operated in streamer mode has been tested at different impinging particle densities

Medium scale anisotropy in the TeV cosmic ray flux observed by ARGO-YBJ

Measuring the anisotropy of the arrival direction distribution of cosmic rays provides important information on the propagation mechanisms and the identification of their sources. In fact, the flux of cosmic rays is thought to be dependent on the arrival direction only due to the presence of nearby cosmic ray sources or particular magnetic-field structures. Recently, the observation of unexpected excesses at TeV energy down to an angular scale as narrow as 10° raised the possibility that the problem of the origin of Galactic cosmic rays may be addressed by studying the anisotropy. The ARGO-YBJ experiment is a full-coverage extensive air showers array, sensitive to cosmic rays with the energy threshold of a few hundred GeV. Searching for small-size deviations from the isotropy, the ARGO-YBJ Collaboration explored the declination region 20°–80°, making use of about 3.7x10**11 events collected from November 2007 to May 2012. In this paper, the detection of different significant (up to 13 standard deviations) medium-scale anisotropy regions in the arrival directions of cosmic rays is reported. The observation was performed with unprecedented detail. The relative excess intensity with respect to the isotropic flux extends up to 10**-3. The maximum excess occurs for proton energies of 10–20 TeV, suggesting the presence of unknown features of the magnetic fields the charged cosmic rays propagate through, or some contribution of nearby sources never considered so far. The observation of new weaker few-degree excesses throughout the sky region 195° < R.A. < 290° is reported for the first time

Detection of thermal neutrons with the PRISMA-YBJ array in extensive air showers selected by the ARGO-YBJ experiment

We report on a measurement of thermal neutrons, generated by the hadronic component of extensive air showers (EAS), by means of a small array of EN-detectors developed for the PRISMA project (PRImary Spectrum Measurement Array), novel devices based on a compound alloy of ZnS(Ag) and (LiF)-Li-6. This array has been operated within the ARGO-YBJ experiment at the high altitude Cosmic Ray Observatory in Yangbajing (Tibet, 4300 m a.s.l.). Due to the tight correlation, between the air shower hadrons and thermal neutrons, this technique can be envisaged as a simple way to estimate the number of high energy hadrons in EAS. Coincident events generated by primary cosmic rays of energies greater than 100 TeV have been selected and analyzed. The EN-detectors have been used to record simultaneously thermal neutrons and the air shower electromagnetic component. The density distributions of both components and the total number of thermal neutrons have been measured. The correlation of these data with the measurements carried out by ARGO-YBJ confirms the excellent performance of the EN-detector. (C) 2016 Elsevier B.V. All rights reserved