Triggers for the Pierre Auger Observatory, the current status and plans for the future

The Pierre Auger Observatory is a multi-national organization for research on ultra-high energy cosmic rays. The Southern Auger Observatory (Auger-South) in the province of Mendoza, Argentina, has been completed in 2008. First results on the energy spectrum, mass composition and distribution of arrival directions on the southern sky are really impressive. The planned Northern Auger Observatory in Colorado, USA, (Auger-North) will open a new window into the universe and establish charged particle astronomy to determine the origin and nature of ultra-high energy cosmic rays. These cosmic particles carry information complementary to neutrinos and photons and to gravitational waves. They also provide an extremely energetic beam for the study of particle interactions at energies that thirty times higher than those reached in terrestrial accelerators. The Auger Observatory is a hybrid detector consisting of a Surface Detector (SD) and an atmospheric Fluorescence Detector (FD). The hybrid data set obtained when both detectors are working together will be especially important for evaluating the systematics of both detectors. It will also provide an energy spectrum with small energy uncertainties. The hybrid data set will also provide the best evaluation of the primary particle composition utilizing all of the known parameters sensitive to the primary particle type. The book describes the crucial components for the Pierre Auger Observatory: triggers - necessary in order to detect a very rare cosmic rays stream, estimated as a single particle per km2 per steradian per century. A reasonable statistics requires a huge detection area, in Auger South 3000 km2, in Auger North planned up to 22000 km2. The very rare stream of ultra-energetic particles is deeply hidden in a huge background of lower-energy particles. The main goal of triggers is recognition and selection of interesting, high-energy events and a rejection of the background. The author is the designer of the Second Level Trigger for the Fluorescence Detector and also the designer of four generations of the First Level Trigger for the Surface Detector. The author’s trigger designs have been successively implemented in the real Pierre Auger detectors and they have been working in both types of detectors since 2000. Extensive Air Showers initiated in the atmosphere by a single cosmic ray particle (proton or a nucleus) with a macroscopic energy of ~50 joules (equivalent to the kinetic energy of a tennis ball at 160 km/h) and spread on the large area of tens km2 are registered by the two independent detectors. They measure the fluorescence light emitted by nitrogen in the air (FD) and Cherenkov light in super–pure water, emitted by shower particles, in 1600 surface detectors spread on 3000 km2. To improve the signal to noise ratio each detector contains a hierarchical trigger. The book presents the concept and technical details of triggers developed by the author in 1998–2009. Although the “standard” (more than 1600 SD and 24 FD telescopes) Auger South has been completed, new techniques: underground muon counters (AMIGA), detection of the geo-synchrotron radiation (AREA), measurements of EAS in an early stage of their development (HEAT) are being implemented on the Infill Array (dedicated test area in Auger South). Since 2008, the author develops the underground trigger system for AMIGA (Auger Muons and Infill for the Ground Array) synchronized with the standard Auger surface detector, trigger system for AERA, new spectral triggers for the Auger North as well as new electronics with much higher sampling than in the standard Auger surface detectors.