Concept of the first level trigger for HERA-B

The HERA-B detector aims to measure CP violation in the B meson system. The B mesons under study are produced in interactions of 820 CeV/c protons with the fixed target. Together with the B mesons a 6 orders of magnitude larger background of minimum bias interactions is produced. A highly selective and efficient trigger system is required and has been designed to acquire a sufficient amount of signal decays. It is able to find lepton and hadron tracks and reconstruct their kinematics on the first level of the trigger chain. The final first level trigger decision is based on the properties of the reconstructed pair of tracks. This is mainly targeting at J//spl psi//spl rarr/l/sup +/l/sup -/ and B/sup 0//spl rarr//spl pi//sup +//spl pi//sup -/ signatures. A parallel and pipelined system of approximately 100 processors is designed to perform this job. It reduces a 10 MHz input rate by a factor of 200 with a latency of 5 to 10 /spl mu/s. A special detailed simulation package was designed to study the system performance and prove the algorithms used. Being a constituent part of the general HERA-B software, it provides not only the possibility of trigger studies but also selects FLT-passing events to be considered as input stream for the studies of the following levels of the HERA-B trigger system.