THE 1ST LEVEL ENERGY TRIGGER OF THE L3 EXPERIMENT - DESCRIPTION OF THE HARDWARE

This paper describes the design and the performance of the first level energy trigger, as used in the L3 experiment at LEP. The trigger is based on energy measurements in the electromagnetic and the hadronic calorimeters and the luminosity monitors. The bunch crossing time at LEP is 22.2-mu-s; the information from these detectors is digitized and analyzed in 20-mu-s. The trigger is a fast processor, which makes use of about 350 CAMAC modules. Each of them performs a dedicated function and is connected with the other modules via front-panel buses. The CAMAC dataway is used only for initialization and checking purposes. The whole processor operates synchronously with a clock period of 60 ns. The trigger decision is mainly based on the total and electromagnetic energy in the event, on the cluster finding algorithm, which searches localized energy deposits in the calorimeters, and on the presence of a single photon, where the most of the electromagnetic energy is confined to a few crystals in the electromagnetic calorimeter. The system also includes the luminosity trigger, to detect small angle Bhabha events for the measurements of the luminosity.