Trigger and Aperture of the Surface Detector Array of the Pierre Auger Observatory

TL;DR

This paper details the trigger system of the Pierre Auger Observatory's surface detector array, ensuring full efficiency for ultra-high-energy cosmic ray detection above 3×10^18 eV across various angles and positions.

Contribution

It introduces a trigger hierarchy that guarantees complete detection efficiency for extensive air showers above a specific energy threshold, regardless of impact point or primary particle mass.

Findings

Trigger system achieves full efficiency above 3×10^18 eV

Array's exposure is determined solely by geometrical acceptance

Detection is effective for all zenith angles between 0° and 60°

Abstract

The surface detector array of the Pierre Auger Observatory consists of 1600 water-Cherenkov detectors, for the study of extensive air showers (EAS) generated by ultra-high-energy cosmic rays. We describe the trigger hierarchy, from the identification of candidate showers at the level of a single detector, amongst a large background (mainly random single cosmic ray muons), up to the selection of real events and the rejection of random coincidences. Such trigger makes the surface detector array fully efficient for the detection of EAS with energy above $3\times 10^{18}$ eV, for all zenith angles between 0$^\circ$ and 60$^\circ$, independently of the position of the impact point and of the mass of the primary particle. In these range of energies and angles, the exposure of the surface array can be determined purely on the basis of the geometrical acceptance.