Inference of the Mass Composition of Cosmic Rays with Energies from 10^18.5 to 10^20 eV Using the Pierre Auger Observatory and Deep Learning

Abstract

We present measurements of the atmospheric depth of the shower maximum 𝑋�max, inferred for the first time on an event-by-event level using the surface detector of the Pierre Auger Observatory. Using deep learning, we were able to extend measurements of the 𝑋�max distributions up to energies of 100 EeV (1020 eV), not yet revealed by current measurements, providing new insights into the mass composition of cosmic rays at extreme energies. Gaining a 10-fold increase in statistics compared to the fluorescence detector data, we find evidence that the rate of change of the average 𝑋�max with the logarithm of energy features three breaks at 6.5±0.6⁢(stat)±1⁢(syst) EeV, 11 ±2⁢(stat) ±1⁢(syst) EeV, and 31 ±5⁢(stat) ±3⁢(syst) EeV, in the vicinity to the three prominent features (ankle, instep, suppression) of the cosmic-ray flux. The energy evolution of the mean and standard deviation of the measured 𝑋�max distributions indicates that the mass composition becomes increasingly heavier and purer, thus being incompatible with a large fraction of light nuclei between 50 and 100 EeV.