Bayesian analysis of the mass composition of Ultra-High Energy Cosmic Rays using Xmax data recorded at the Pierre Auger Observatory

Résumé

Understanding the composition of Ultra High Energy Cosmic Rays (UHECRs) at Earth is crucial to unveil their origin and propagation mechanisms, and it is thus one of the main current challenges in the field of Astroparticle Physics. This thesis is aimed to study the mass composition of UHECRs using data measured at the Pierre Auger Observatory in a Bayesian approach. An innovative method for composition analysis based on Bayesian statistics is developed. This is then applied to Auger data to infer the composition of UHECRs under different primary cosmic ray mass scenarios and assuming different hadronic interaction models. As a result a transition from intermediate to lighter masses from 0.63 EeV to 2 EeV is found. Above this energy the composition becomes heavier as energy increases. Using these results and the all particle spectrum measured at the Auger Observatory a first estimate of the proton flux is inferred and its astrophysical interpretations are discussed.