Ponente
Descripción
One of the major challenges in astrophysics is the estimation of the mass composition of ultra-high-energy cosmic rays (UHECRs), ions with energies exceeding $1\,\text{EeV}$ of extraterrestrial origin. Due to the low flux of UHECRs, the direct detection of the particles is infeasible, which poses several challenges for estimating their mass. When a UHECR enters Earth's atmosphere, it will eventually interact with an air molecule, inducing a cascade of secondary particles, referred to as an extensive air shower (EAS). The mass information of the UHECR is encoded stochastically in the development of the shower, in particular, the shower depth of the shower maximum, $X_\mathrm{max}$. The Pierre Auger Observatory is the largest detector for measuring UHECRs in the southern hemisphere with an area of $3000\,\mathrm{km}^2$. The Observatory employs multiple sub-detectors to measure different components of the EAS, arranged into two main parts, the Surface Detector (SD) and the Fluorescence Detector (FD). The SD consists of $1\,600$ water-Cherenkov detectors with $1.5\,\mathrm{km}$ spacing. The FD, composed of 27 fluorescence telescopes, overlooks the area above the SD. The highly sensitive FD measures $X_\mathrm{max}$ directly, while having a duty cycle about 10\% as it operates only during moonless nights. The SD, having a significantly larger duty cycle, allows for reconstructing $X_\mathrm{max}$ from the EAS footprint using machine learning techniques. Using data recorded over 20 years, we present an analysis of the $X_\text{max}$ measurements by the SD and FD. In particular, we show that the UHECR mass composition evolution excludes a large proton abundance.
