Speaker
Description
We present results on the mass composition of ultra-high-energy cosmic rays based on 17 years of data from the Pierre Auger Observatory. Measurements from the Fluorescence Detector of the average depth of shower maximum (Xmax) and the shower-to-shower fluctuations of Xmax provide evidence for a transition from a lighter to a heavier and less mixed composition above 10^18.4 eV. This transition is further characterized through fractional abundances of four primary mass groups (p, He, CNO, Fe), obtained from fits to Xmax distributions using post-LHC hadronic interaction models. These results show a progressive depletion of the proton component up to around 10^18.7 eV. By splitting the data into northern and southern sky regions at a declination of -15.7 degrees, we find no evidence for a declination dependence of the mass composition within the exposure of the Observatory. Additional constraints are obtained from the correlation between Xmax and the Surface Detector signal, allowing us to exclude pure compositions and mixtures of neighboring mass groups below 10^18.7 eV. Finally, machine learning applied to Surface Detector data enables Xmax inference with substantially increased statistics, corroborating fluorescence measurements and revealing indications of additional features in the energy evolution of Xmax above 10^18.5 eV.
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