Introduction

The HERA accelerator at the DESY laboratory in Hamburg, Germany collides 26.7 GeV electrons with 820 GeV protons. Interesting physics data occurs in the ZEUS detector and is written to tape at the rate of a few Hz. However, background from proton-beam-gas interactions has a much higher rate, on the order of 100 kHz. The time between beam crossings at HERA is 96 nsec. This is too short a time for the ZEUS detector to read out the 200 kBytes of data for each event and provide a trigger decision. The data are therefore kept in a pipeline and the first level trigger decision is postponed until 5 $\mu$sec after the crossing. In order to avoid deadtime, the trigger electronics itself is pipelined: every process in the trigger must be repeated every 96 nsec[1]. The total first level trigger output rate from all components is limited to less than 1 kHz by the front end electronics readout and the second level trigger input capacity[2]. Therefore, the ZEUS calorimeter first level trigger (CFLT) rate from beam-gas interactions must be reduced to a fraction of a kHz.

The ZEUS CFLT detects charged and neutral current processes. In these events the current jet(s) and lepton emerge on opposite sides of the beam axis, balancing each other in transverse momentum. The debris of the proton is emitted forward in a narrow cone ( 10 mrad). Neutral current events are characterized by an electron and jet(s) with balanced transverse momentum $p_T$. Charged current events contain jets and missing $p_T$. In addition, exotic processes are distinguished by the presence of lepton(s) and jet(s) with missing $p_T$.

The CFLT identifies charged and neutral current, photoproduction, and exotic physics events while rejecting beam gas background using three different approaches. They are: (i) detection of isolated electrons and muons using pattern analysis logic, (ii) identification of patterns of energy deposits obtained from local energy sums, and (iii) recognition of characteristic deposits of total transverse and missing transverse energy.

The ZEUS calorimeter consists of depleted uranium plates interleaved with plastic scintillator. The scintillator plates form towers which are read out on two sides with wavelength shifter bars, light guides and photomultipliers. The calculations required for the calorimeter trigger include summing all of the pulseheights recorded in the photomultipliers every 96 ns. In addition, calculation of the transverse energy and missing $p_T$  requires algebraically summing pulseheights multiplied by geometric factors. The detection of an electron requires evidence of electromagnetic energy. This is done by comparing energy deposited in the first interaction length of the calorimeter with that deposited in subsequent interactions lengths on a tower by tower basis.