THE ZEUS CALORIMETER FIRST LEVEL TRIGGER

The design of the ZEUS Calorimeter First Level Trigger (CFLT) is prese nted. The CFLT utilizes a pipelined architecture to provide trigger da ta for a Global First Level Trigger decision 5 mu s after each beam cr ossing, occurring every 96 ns. The charges from 13 K phototubes are su mmed into 1792 trigger tower pulseheights which are digitized by flash ADCs. The digital values are linearized, stored and used for sums and pattern tests. Summary data is forwarded to the Global First Level Tr igger for each crossing 2 mu s after the crossing occurred. The CFLT d etermines the total energy, the total transverse energy, the missing e nergy, and the energy and number of isolated electrons and muons. It a lso provides information on the electromagnetic and hadronic energy de posited in various regions of the calorimeter. The CFLT has kept the e xperimental trigger rate below approximate to 200 Hz at the highest lu minosity experienced at HERA. Performance studies suggest that the CFL T will keep the trigger rate below 1 kHz against a rate of proton-beam gas interactions on the order of the 100 KHz expected at design lumin osity.