Extensive air shower asymmetry and cosmic ray mass composition with the upgraded Pierre Auger Observatory

Abstract

The Pierre Auger Observatory, the largest detector in the world for observing cosmic rays, is currently undergoing a significant upgrade. The addition of scintillator detectors to the observatory's ground array aims to improve our ability to determine cosmic ray mass composition at the highest energies. One phenomenon which may provide hints to mass composition is asymmetry, the systematic difference in signal between detectors of equal perpendicular distance from the axis of an inclined extensive air shower at different azimuthal angles. In this work, the asymmetry in the water Cherenkov and surface scintillator detectors of the Pierre Auger Observatory is parameterised in simulations for proton and iron primaries. The largest difference between the two parameterisations is found to be for the scintillator detector at zenith angles > 50° and energies ~ 2x10(19)eV. The causes of asymmetry in particle density are also investigated. For the electromagnetic component of extensive air showers, atmospheric attenuation is shown to give a non-negligible contribution to the overall asymmetry. A test of the understanding of these causes is demonstrated via a basic model for the asymmetry in muon particle density. Finally, the asymmetry parameterisations are utilised to improve upon the technique of reconstructing Xmax from the slope parameters of lateral distribution functions in simulations.