On the need for unbiasing azimuthal asymmetries in signals measured by surface detector arrays

Abstract

A surface detector array samples the lateral distribution of an extensive air-shower (EAS) at the ground, i.e. the density of particles as a function of the distance from the axis of the shower. The azimuthal symmetry of this measured lateral distribution is broken for EAS with a nonzero zenith angle. The resulting asymmetry, caused by atmospheric attenuation and geometrical effects, increases with the inclination of the shower and introduces a bias in the reconstruction of the shower parameters. Using simulated sets of air-showers, we present a model to correct the azimuthal asymmetry in signals measured by water-Cherenkov detectors and exemplified using the geometry and detector response of the Pierre Auger Observatory. Testing showers initiated by proton and iron primaries using E -LHC and QGSJ II-04 as hadronic models, we developed a fine-tuned model of the amplitude of the asymmetry as a function of the zenith angle, shower size and distance of a detector from the shower axis. The improvements resulting from the application of the correction are quantified in terms of the biases and resolutions in the impact-point and arrival direction.

A Catalog of the Highest-energy Cosmic Rays Recorded during Phase I of Operation of the Pierre Auger Observatory

“…The consequences of asymmetries of the signal sizes have been studied in some detail using simulations. Luce et al (2022) examined the impact on the electromagnetic component. At 1000 m from the shower axis, the amplitude of the asymmetry of the signal size is ∼50% in a shower produced by a primary of 10 EeV at a zenith angle of 45°.…”