A RICH multiplicity veto for the HERA-B experiment

We present the design and commissioning of a new multiplicity veto for the HERA-B detector, a fixed-target spectrometer originally designed to study the physics of B mesons in proton-nucleus interactions. The HERA-B trigger is a message-driven multilevel track trigger. The first level trigger (FLT) consists of custom-made electronics, and the higher trigger levels are implemented as PC farms. The multiplicity veto has been designed to reject high-multiplicity events before they enter the trigger chain. A veto signal is generated based on the comparison of the number of photons in part of the HERA-B ring-imaging Cerenkov counter (RICH) with a programmable threshold. The RICH multiplicity veto is a modular system. First the hits in 256 detector channels are summed by base sum cards (BSC), then FED sum cards (FSC) sum the subtotals of up to eight BSCs. Finally, the veto board (VB) takes the veto decision based on the sum of up to 14 FSCs. The RICH multiplicity veto has been successfully installed and commissioned in HERA-B. The. measured veto efficiency is (99.9991 +/- 0.0001)%, and the system is used in the routine data-taking of the HERA-13 experiment.