First level trigger for HERA-B

HERA-B aims to measure CP violation in the B meson system. A high rate of B mesons is obtained from 820 GeV fixed target proton-nucleon interactions. 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 has been designed to acquire a sufficient amount of signal decays. It is able to find and reconstruct lepton and hadron tracks. The trigger decision is based on the momenta and masses of track pairs, and mainly aims at Jψ → ℓ+ℓ− signatures. A parallel and pipelined system of approximately 100 processors has been worked out to perform this job. It provides 3 orders of magnitude background suppression at an efficiency of 50% for the main physics channel.