Analisi e stima della composizione in massa dei raggi cosmici ad altissima energia e le sue implicazioni astrofisiche usando i dati misurati dall'Osservatorio Pierre Auger

The most reliable technique to measure the mass composition of Ultra-High-Energy Cosmic Rays (UHECRs) is the simultaneous measurement of the depth, Xmax , at which the number of particles in an air shower reaches its maximum and the energy, E, of the shower. The most efficient way to detect these quantities is the fluorescence technique currently employed by the Pierre Auger Observatory and the Telescope Array. The data measured by the Pierre Auger Observatory of the first two moments (mean and standard deviation) of the X max distribution suggest that the composition of cosmic rays becomes lighter as the energy increases towards the ankle and then becomes heavier again when approaching ultra-high energies. To obtain a more quantitative interpretation of the expected mass composition of UHECRs, model predictions of four or five mass groups (p, He, N, Si Fe) to the Xmax distributions were fitted in each energy bin independently. The derived mass-fractions-to-energy curve revealed an interesting pattern of alternating dominance of certain mass groups. Above 10^18 eV the flux of cosmic rays is dominated by light primaries. As the energy increases, the protons are gradually replaced by helium, helium by nitrogen, and there might be an iron contribution emerging above 10^19.4 eV (bin characterized by low statistics). This trend has been obtained joining the combination of fitted fractions at each energy bin independently; on the contrary, a bin-to-bin relation should naturally exist. To explore this, a parametric model for each elemental spectrum at the top of the atmosphere has been assumed and the measurements of the energy spectrum and the composition, excluding all propagation effects have been fitted. This method improved the interpretation capability of the resulting mass-fractions-to-energy curve and allowed to test the current expected astrophysical scenarios.