The ZEUS calorimeter first level trigger

Abstract An overview of the ZEUS calorimeterfirst level trigger is presented. The CFLT uses a pipelined architecture to accept and analyze calorimeterdata for every 96 ns beam crossinginterval. PMT signals are combined by analog electronics into electromagnetic and hadronic sums for 896 trigger towers. The analog sums are then digitized and analyzed. The CFLT determines the total, transverse, and missing transverse energy, identifies isolated electrons and muons, and sums energies in programmable subregions. Calculations are performed in 96 ns steps, and new data are accepted for every beam crossing. Trigger data are forwarded to the global first level trigger (GFLT) after 2 μs, allowing a GFLT accept to be issued 5 μs after the beam crossingwhich produced the event. Important features of the CFLT include a 12-biteffective dynamic range, extensive use of memory lookup tablesfor trigger calculations, fast pattern searchesfor isolated leptons, and low electronics noise. During the 1993 HERArun, the CFLT reduced a 50 kHz background rate to around 100 Hz.