A RICH multiplicity veto for the HERA-B experiment

We present the design and commissioning of a new multiplicity veto for the HERA-B experiment, a fixed-target spectrometer originally designed to study B meson physics in proton-nucleon interactions. The HERA-B trigger is a message-driven multi-level track trigger. The first level trigger (FLT) consists of custom-made electronics, and the higher trigger levels are implemented as PC farms. The RICH 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 parts of the HERA-B ring-imaging Cerenkov hodoscope (RICH) with a programmable threshold.