MASS COMPOSITION STUDIES OF ULTRA HIGH ENERGY COSMIC RAYS THROUGH THE MEASUREMENT OF THE MUON PRODUCTION DEPTHS AT THE PIERRE AUGER OBSERVATORY

The Pierre Auger Observatory in Argentina studies Ultra High Energy Cosmic Rays (UHECRs) physics. The flux of CRs at these energies (above 1 EeV) is very low and UHECRs properties must be inferred from the measurements of the Extensive Air Showers (EAS) that the cosmic ray primary produces in the atmosphere. Two complementary techniques are used at Auger to detect EAS: a surface detector (SD) array of water Cherenkov tanks, which sample particle at ground level, and fluorescence detectors (FD), which collect the UV light emitted by the de-excitation of nitrogen nuclei in the atmosphere, and can operate only in clear, moonless nights. Auger is the largest cosmic rays detector ever built and it provides high- quality data together with unprecedented statistics. The main goal of this thesis is the measurement of UHECRs mass composition using data from Auger SD. Measuring the cosmic ray composition at the highest energies is of fundamental importance from Mass Composition Studies of Ultra High the astrophysical point of view, since it could discriminate between different scenarios of origin and propagation of cosmic rays. Moreover, mass composition studies are of utmost importance for particle physics. As a matter of fact, knowing the composition helps in exploring the hadronic interactions at ultra-high energies, inaccessible to present accelerator experiments.