The trigger system of the ARGO-YBJ experiment

ARGO-YBJ is a ground-based gamma and cosmic ray telescope presently tinder construction at the Yangbajing High Altitude Cosmic Ray Laboratory (4300 a.s.l.) nearby Lhasa (Tibet, People's Republic of China). The apparatus has been designed to detect air showers with an energy threshold of it few hundreds GeV and it has a total active area of 6400 m(2). The full-coverage central carpet of 5772 m(2) is equipped with an array of Resistive Plate Counters. This array is surrounded by a partially instrumented guard ring to improve the reconstruction of showers partially outside the detector's core. The ARGO trigger logic implements simple yet robust algorithms, based on the timing distribution of the hits and their multiplicity on the central carpet. In this paper, we describe the hardware architecture and the main features of the trigger system.