TRIGGER CONCEPT OF THE HERA-B EXPERIMENT

At the HERA 800 GeV proton storage ring at DESY a fixed target experim ent to study CP violation in B meson decays has been proposed. It is b ased on the observation of the decay B-d(0) --> J/psi K-S(0), with the J/psi decaying into a lepton pail. With a time between proton bunches of 96 ns and interaction rates of 40 MHz the trigger has to reject mi nimum bias events by 5 orders of magnitude, leaving the signal essenti ally intact. The 3-level hierarchical trigger relies on the detection of lepton pairs from J/psi decays with the additional requirement of a finite flight path of the J/psi's parent particle. The design of such a trigger system is presented, with special emphasis on the most chal lenging level-1 trigger step.