Inference of the Mass Composition of Cosmic Rays with Energies from 10^{18.5} to 10^{20}  eV Using the Pierre Auger...

Area of Science:

• Astroparticle Physics
• Cosmic Ray Physics

Background:

• Understanding the origin and nature of ultra-high-energy cosmic rays (UHECRs) is a major challenge in astroparticle physics.
• The mass composition of UHECRs provides crucial clues to their sources and acceleration mechanisms.

Purpose of the Study:

• To measure the atmospheric depth of shower maximum (Xmax) for UHECRs on an event-by-event basis.
• To extend Xmax measurements to unprecedented energies (up to 100 EeV) and analyze their distributions.
• To gain new insights into the mass composition of cosmic rays at extreme energies.

Main Methods:

• Utilized the surface detector of the Pierre Auger Observatory.
• Applied deep learning techniques to infer Xmax on an event-by-event level.
• Analyzed Xmax distributions and their evolution with energy.

Main Results:

• Extended Xmax measurements to 100 EeV, significantly increasing statistics compared to fluorescence detector data.
• Identified three distinct breaks in the rate of change of average Xmax with energy at approximately 6.5, 11, and 31 EeV.
• Observed that the energy evolution of Xmax indicates a progressively heavier and purer cosmic ray composition.

Conclusions:

• The observed trend of increasing Xmax suggests a transition to heavier cosmic ray nuclei at energies above 50 EeV.
• The findings are incompatible with a significant fraction of light nuclei in the 50-100 EeV range.
• The breaks in Xmax evolution correlate with prominent features in the cosmic ray energy spectrum.