Time asymmetries in the Surface Detector signals of the Pierre Auger Observatory

I. MINAYA, H. WAHLBERG, ET AL. POR LA COLABORACION PIERRE AUGER

Conferencia; 34th International Cosmic Ray Conference (ICRC 2015); 2013

The azimuthal asymmetry in the risetime of signals in Auger surface detector stations is a sourceof information on shower development. The azimuthal asymmetry is due to a combination of thelongitudinal evolution of the shower and geometrical effects related to the angles of incidence ofthe particles into the detectors. The magnitude of the effect depends upon the zenith angle andstate of development of the shower and thus provides a novel observable sensitive to the masscomposition of cosmic rays above 3 × 1018 eV. By comparing measurements with predictionsfrom shower simulations, we find for both of our adopted models of hadronic physics (QGSJetII-04 and Epos-LHC) an indication that the mean cosmic-ray mass increases with energy, as hasbeen inferred from other studies. However the absolute values derived for the mass are dependenton the shower model and on the range of distance from the shower core selected. Thus the methodhas uncovered further deficiencies in our understanding of shower modelling that must be resolvedbefore the mass composition can be inferred from (secθ)max.