ARCHITECTURE AND PERFORMANCE OF THE DELPHI TRIGGER SYSTEM

This paper describes the trigger system of the DELPHI detector at LEP. It reports on the most relevant aspects of the hardware and shows the software strategies that have been developed to optimize its use. In 1993 the structure of the trigger in four decision levels has become f ully operational and data collected during this period have been used to study the trigger performance. Various final state channels such as mu(+)mu(-), e(+)e(-) and hadronic events were selected and their trig ger efficiencies were calculated as a function of the polar angle thet a. The results obtained indicate that, for any of the event topologies considered, the DELPHI trigger efficiency is independent of the a ang le, and, furthermore, the attained efficiency values are determined to be very close to 100% within an extremely good precision. This is a c onsequence of the high redundancy presently provided by all the DELPHI subdetectors. In addition to this analysis, events containing isolate d particles either in the barrel or in the forward regions have been s elected to evaluate the trigger response to single particles. Hence, t rigger efficiencies for single particles have also been computed for c harged tracks as a function of the momentum and for photons as a funct ion of the deposited electromagnetic energy.