Speaker
Description
The measurement of hadronic interaction cross-sections at the highest cosmic-ray energies provides a unique opportunity to probe soft QCD processes beyond the reach of existing accelerator experiments. A key observable for such studies is the atmospheric depth at which air showers reach their maximum development ($X_\mathrm{max}$), which is sensitive to both the mass of the primary cosmic rays and the particle interaction cross-section.
In this contribution, we present a measurement of the inelastic proton-proton cross-section at ultrahigh energies using data collected by the Fluorescence Detector of the Pierre Auger Observatory, based on a method that simultaneously fits the proton-proton cross-section, primary mass composition, and the $X_\mathrm{max}$ scale. This approach addresses the challenges of earlier analyses that inferred the cross-section from the tail of the $X_\mathrm{max}$ distribution by directly considering the interdependence between the three estimated quantities.
The measured cross-section is consistent with extrapolations from accelerator data, previous estimates from cosmic-ray data, and the results obtained from a tail-fit applied to the same dataset. In comparison with previous analyses, the current approach provides reduced overall uncertainties and extends the cross-section analysis to higher energies. The inferred mass composition is somewhat heavier than in earlier analyses, and the fitted $X_\mathrm{max}$ scale exhibits a small shift relative to model predictions. Overall, the results remain in agreement with previous estimates within the quoted systematic uncertainties, while offering improved precision and a weaker dependence on assumptions related to the mass composition and hadronic interaction models.