Preliminary results of the AMIGA engineering array at the Pierre Auger Observatory

The Auger Muons and Infill for the Ground Array (AMIGA) aims to both extend the detection range of the Pierre Auger Observatory down to energies $\sim 10^{16.5}~\mathrm{eV}$ and to measure the muon content of extensive air showers. To accomplish these goals, its detection system is composed of an array of coupled water-Cherenkov and scintillation detectors deployed in a graded triangular grid of 433 and 750\,m spacings. At each position, the scintillation detector is buried $2.3~\mathrm{m}$ deep so as to shield it from the air shower electromagnetic component and thus only measure the muon component. These muon detectors have $30~\mathrm{m^2}$ area split into modules, each of them highly segmented in 64 plastic-scintillator strips with an embedded wavelength-shifter optical fiber to transport light to an optical sensor located at the center of the module. During the engineering array phase (finished in November 2017) two module areas ($5~\mathrm{m^2}$ and $10~\mathrm{m^2}$) and two optical sensors (photo-multiplier tubes and silicon-photomultipliers) were tested. In this work, we present the final performance of the muon detectors equipped with silicon-photomultipliers which were thereafter selected as the baseline design for the AMIGA production phase. Analyses and results are based both on laboratory and field measurements.Comment: Proceeding of the UHECR 2018 conference, submitted to the forthcoming EPJ Web of Conference

Identification of rolling resistance as a shape parameter in sheared granular media

Using contact dynamics simulations, we compare the effect of rolling resistance at the contacts in granular systems composed of disks with the effect of angularity in granular systems composed of regular polygonal particles. In simple shear conditions, we consider four aspects of the mechanical behavior of these systems in the steady state: shear strength, solid fraction, force and fabric anisotropies, and probability distribution of contact forces. Our main finding is that, based on the energy dissipation associated with relative rotation between two particles in contact, the effect of rolling resistance can explicitly be identified with that of the number of sides in a regular polygonal particle. This finding supports the use of rolling resistance as a shape parameter accounting for particle angularity and shows unambiguously that one of the main influencing factors behind the mechanical behavior of granular systems composed of noncircular particles is the partial hindrance of rotations as a result of angular particle shape.Comment: Soumis a Physical Review E; Statistical, Nonlinear, and Soft Matter Physics http://link.aps.org/doi/10.1103/PhysRevE.84.01130

Prospects for the detection of GRBs with HAWC

The observation of Gamma Ray Bursts (GRBs) with very-high-energy (VHE) gamma rays can provide understanding of the particle acceleration mechanisms in GRBs, and can also be used to probe the extra-galactic background light and place constraints on Lorentz invariance violation. We present prospects for GRB detection by the ground-based HAWC (High Altitude Water Cherenkov) gamma-ray observatory. We model the VHE spectrum of GRBs by extrapolating observations by Fermi LAT and other observatories to higher energies. Under the assumption that only e-pair production associated with extra-galactic background light is responsible for high-energy cutoffs in the spectrum, we find that HAWC will have a detection rate as high as 1.65 GRBs/year. Most of the sensitivity of HAWC to GRBs is derived from short-hard GRBs during the prompt phase. We explore the possibility of universal high-energy cutoffs in GRB spectra and find that the GRB detection rate by HAWC should be at least half of this figure as long as the typical intrinsic cutoff is above 200-300 GeV in the rest frame