Adjustments to Model Predictions of Depth of Shower Maximum and Signals at Ground Level using Hybrid Events of the Pierre Auger Observatory

PIERRE AUGER COLLABORATION; A. MARIAZZI

Berlin

Conferencia; 37th International Cosmic Ray Conference (ICRC2021); 2021

We present a new method to explore simple ad-hoc adjustments to the predictions of hadronic interaction models to improve their consistency with observed two-dimensional distributions of the depth of shower maximum, 𝑋max, and signal at ground level, as a function of zenith angle. The method relies on the assumption that the mass composition is the same at all zenith angles, while the atmospheric shower development and attenuation depend on composition in a correlated way. In the present work, for each of the three leading LHC-tuned hadronic interaction models, we allow a global shift Δ𝑋max of the predicted shower maximum, which is the same for every mass and energy, and a rescaling 𝑅Had of the hadronic component at ground level which depends on the zenith angle.We apply the analysis to 2297 events reconstructed by both fluorescence and surface detectors at the Pierre Auger Observatory with energies 1018.5 − 1019.0 eV. Given the modeling assumptions made in this analysis, the best fit reaches its optimum value when shifting the 𝑋max predictions of hadronic interaction models to deeper values and increasing the hadronic signal at both extreme zenith angles. The resulting change in the composition towards heavier primaries alleviates the previously identified model deficit in the hadronic signal (commonly called the muon deficit), but does not remove it. Because of the size of the required corrections Δ𝑋max and 𝑅Had and the large number of events in the sample, the statistical significance of the corrections is large, greater than 5𝜎stat even for the combination of experimental systematic shifts within 1𝜎sys that are the most favorable for the models.