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

THE PIERRE AUGER COLLABORATION, INCLUYE S. MOLLERACH

PHYSICAL REVIEW D - PARTICLE AND FILDS

American Physical Society

Año: 2016 vol. 93

0556-2821

The azimuthal asymmetry in the risetime of signals in Auger surfacedetector stations is a source of information on shower development. Theazimuthal asymmetry is due to a combination of the longitudinalevolution of the shower and geometrical effects related to the angles ofincidence of the particles into the detectors. The magnitude of theeffect depends upon the zenith angle and state of development of theshower and thus provides a novel observable, $(secheta)_mathrm{max}$, sensitive to the mass composition of cosmic raysabove $3 imes 10^{18}$ eV. By comparing measurements with predictionsfrom shower simulations, we find for both of our adopted models ofhadronic physics (QGSJETII-04 and EPOS-LHC) an indication that the meancosmic-ray mass increases slowly with energy, as has been inferred fromother studies. However, the mass estimates are dependent on the showermodel and on the range of distance from the shower core selected. Thusthe method has uncovered further deficiencies in our understanding ofshower modelling that must be resolved before the mass composition canbe inferred from $(sec heta)_mathrm{max}$.