Speaker
Description
For the past 20 years, the Pierre Auger Observatory has collected the largest dataset of ultra-high-energy cosmic rays (UHECRs) ever achieved using a hybrid detector. The study of this dataset has led to numerous unexpected discoveries that enhance our understanding of the origins of UHECRs. One of the key points in this study is their mass composition. In this presentation, we will present the most recent results regarding the mass composition of UHECRs at the Pierre Auger Observatory. In particular, we will focus on the measurement of the depth of the maximum of air-shower profiles, denoted as Xmax. This determination has been achieved through both direct measurements from the Fluorescence Detector data and the application of machine learning for estimating Xmax on an event-by-event basis using the Surface Detector data. The latter has allowed us to extend the measurement to energies up to 100 EeV and indicates a correlation between changes in composition and three features of the energy spectrum (ankle, instep, steepening). Moreover, the results provide evidence of a heavy and nearly pure primary beam for energies greater than 50 EeV that is independent of the hadronic interaction model. The implications of these findings for astrophysics and for modelling hadronic interactions will be discussed.
