The muon pretrigger system of the HERA-B experiment

One of the main goals of the HERA-B experiment at DESY in Hamburg, Germany, is to study the properties of B-mesons with the emphasis on CP violation. B-mesons are produced in hadronic interactions of a 920-GeV proton beam wit h an internal wire target. An effective bunch crossing rate of about 8.5 MH z leads to about 200 charged tracks per event. Therefore, a highly selectiv e and efficient trigger system providing high suppression of background eve nts is required. The HERA-B trigger system consists of four levels. A rate reduction factor of 200 is aimed at by the first-level trigger (FLT). The u nion pretrigger system, as a part of the FLT, is a modular system consistin g of about 100 large-size VME modules of three different types: the pretrig ger link board (PLB), the pretrigger coincidence unit (PCU), and the pretri gger message generator (PMG). The data rate processed by the pretrigger sys tem is about 19.5 GByte/s. The PLBs process digitized hit information in ei ght independent electronic channels in parallel. Every electronic channel h andles 32 bits of hit information received from the front-end driver buffer system. Optical links operating at 800 Mb/s transmit the data after serial ization to PCUs, which calculate coincidences using complex programmable lo gic devices [I]. The PMGs transform this coincidence information into messa ges for the FLT processors. The concept and design as well as results of th e union pretrigger running at HERA-B are presented.