Composition-sensitive parameters measured with the surface detector of the Pierre Auger Observatory

TL;DR

This paper explores surface detector observables at the Pierre Auger Observatory that can help determine the mass composition of ultra-high energy cosmic rays, especially when fluorescence data is limited.

Contribution

It identifies and demonstrates the effectiveness of surface detector observables for cosmic ray composition analysis, enabling studies with limited fluorescence data.

Findings

Risetime and curvature relate to shower development and primary mass.

Muon-to-electromagnetic ratio depends on primary energy and nucleon number.

Azimuthal asymmetry is influenced by geometric effects and attenuation.

Abstract

A key step towards the understanding of the origin of ultra-high energy cosmic rays is their mass composition. Primary photons and neutrinos produce markedly different showers from nuclei, while showers of different nuclear species are not easy to distinguish. To maximise the discrimination with the Pierre Auger Observatory ideally all mass-sensitive observables should be combined, but the 10% duty cycle of the fluorescence detector limits the use of direct measurements of shower maximum at the highest energies. Therefore, we investigate mass-sensitive observables accessible with the surface detectors alone. These are the signal risetime in the Cherenkov stations, the curvature of the shower front, the muon-to-electromagnetic ratio, and the azimuthal signal asymmetry. Risetime and curvature depend mainly on the depth of the shower development in the atmosphere, and thus on primary energy and mass. The muon content of a shower depends on the primary energy and the number of nucleons, while asymmetry about the shower core is due to geometric effects and attenuation, which are dependent on the primary mass. The mass sensitivity of these variables is demonstrated and their application for composition studies is discussed.