Measurement of the Depth of Maximum of Air-Shower Profiles with energies between 10^18.5 and 10^20 eV using the Surface Detector of the Pierre Auger Observatory and Deep Learning

We report an investigation of the mass composition of cosmic rays with energies from 3 to 100 EeV (1 EeV=10^18 eV) using the distributions of the depth of shower maximum X_max. The analysis relies on ∼50,000 events recorded by the Surface Detector of the Pierre Auger Observatory and a deep-learning-based reconstruction algorithm. Above energies of 5 EeV, the data set offers a 10-fold increase in statistics with respect to fluorescence measurements at the Observatory. After cross-calibration using the Fluorescence Detector, this enables the first measurement of the evolution of the mean and the standard deviation of the X_max distributions up to 100 EeV. Our findings are threefold: (1.) The evolution of the mean logarithmic mass towards a heavier composition with increasing energy can be confirmed and is extended to 100 EeV. (2.) The evolution of the fluctuations of X_max towards a heavier and purer composition with increasing energy can be confirmed with high statistics. We report a rather heavy composition and small fluctuations in X_max at the highest energies. (3.) We find indications for a characteristic structure beyond a constant change in the mean logarithmic mass, featuring three breaks that are observed in proximity to the ankle, instep, and suppression features in the energy spectrum.