Measurement and Interpretation of UHECR Mass Composition at the Pierre Auger Observatory

TL;DR

This paper reviews recent measurements of ultra-high-energy cosmic ray (UHECR) mass composition at the Pierre Auger Observatory, highlighting complex evolution, robust findings, and implications for cosmic ray origins, using multiple detection methods.

Contribution

It presents new results on UHECR mass composition from various detectors, challenging proton dominance and refining the understanding of cosmic ray flux features.

Findings

UHECRs are not proton-dominated at high energies.

Mass composition varies with declination.

Results support a complex evolution of UHECRs.

Abstract

The Pierre Auger Observatory has driven the field of ultra-high-energy cosmic ray (UHECR) physics, producing several groundbreaking observations over the last 20 years. One of the most striking findings has been the complex evolution of UHECR mass composition, as revealed by detailed analyses of observables such as the depth of shower maximum ($X_{\rm max}$) and the muon content of showers. As more data are collected and sophisticated analyses are undertaken, not only are new fine details emerging, but the general picture of UHECR mass composition is becoming increasingly robust. This contribution presents recent results on the mass composition of UHECRs derived from surface, fluorescence, and radio detectors. Together with other key findings from the Observatory, these results converge to present a coherent picture of UHECR mass composition, effectively ruling out proton dominance and challenging the interpretation of the observed flux features as purely proton-induced propagation effects. To finish the contribution, we compare the $X_{\rm max}$ data from the southern and northern equatorial bands of the exposure of the Pierre Auger Observatory fluorescence detector to evaluate the possibility of changes in composition as a function of declination.