Speaker
Description
In this highlight contribution, we review recent results from the Pierre Auger Observatory, indicating emerging connections between the energy spectrum and mass composition of ultra-high-energy cosmic rays and future implications for the interpretation of anisotropies in arrival directions. In this context, measurements of the energy spectrum over the full declination range covered by the surface array, with an exposure of approximately $105\,000\,\mathrm{km^2\,sr\,yr}$, reveal no significant declination dependence beyond that expected from the known large-scale anisotropy and strengthen the evidence for established spectral features, including the instep. Updated measurements of the depth of the shower maximum, $X_{\max}$, based on approximately $58\,000$ high-quality events observed simultaneously by the fluorescence and surface detectors, confirm a clear break in the evolution of the mean $X_{\max}$ near $2.5\,\mathrm{EeV}$ and a decrease of $X_{\max}$ fluctuations with energy, providing model-independent evidence for a transition toward a heavier and less mixed primary mass composition. A major advance comes from a novel machine-learning reconstruction of $X_{\max}$ from surface-detector data, extending composition-sensitive measurements up to $100\,\mathrm{EeV}$ by exploiting the much larger statistics of the surface array. The observed energy evolution of the mean $X_{\max}$ indicates additional breaks near the ankle, instep, and suppression features of the spectrum. The interpretation of these results relies on progress toward establishing a consistent mass scale from $X_{\max}$ and muon content measurements. Recent advances in this direction will be presented, using data-driven methods and approaches with reduced dependence on hadronic-interaction generators. The results indicate a systematically deeper $X_{\max}$ scale in the data and therefore a heavier mass composition than inferred from nominal model predictions. Progress toward a consistent mass scale, together with access to mass-sensitive observables from the surface detector, is expected to enable studies of composition-dependent signatures in arrival direction analyses, and their potential reach will be illustrated. Finally, the first results from AugerPrime will be presented, together with the prospects enabled by its enhanced mass sensitivity.