Azimuthal asymmetry in the risetime of the surface detector signals of the Pierre Auger Observatory

THE PIERRE AUGER COLLABORATION: A.C. ROVERO

PHYSICAL REVIEW D

AMER PHYSICAL SOC

Lugar: New York; Año: 2016 vol. 93 p. 72006 - 72022

1550-7998

The azimuthal asymmetry in the risetime of signals in Auger surface detector stations is a source of information on shower development. The azimuthal asymmetry is due to a combination of the longitudinalevolution of the shower and geometrical effects related to the angles of incidence of the particles into the detectors. The magnitude of the effect depends upon the zenith angle and state of development of theshower and thus provides a novel observable, (sec θ)_max, sensitive to the mass composition of cosmic rays above 3 × 10^18 eV. By comparing measurements with predictions from shower simulations, we find forboth of our adopted models of hadronic physics (QGSJETII-04 and EPOS-LHC) an indication that the mean cosmic-ray mass increases slowly with energy, as has been inferred from other studies. However, themass estimates are dependent on the shower model and on the range of distance from the shower core selected. Thus the method has uncovered further deficiencies in our understanding of shower modeling thatmust be resolved before the mass composition can be inferred from (sec θ)_max .