The use of silicon detectors for fast primary vertex reconstruction and pile-up rejection

97-016 The fraction of events with more than one interaction in LHCB is expected to be about 20 % at the nominal luminosity. In this note we describe a system of two backward Si­planes plus associated electronics for pile­up detection. A simple coincidence matrix technique is used for trigger processing at Level­0. The retention of single­events is 95 % for a pile­up rejection factor of 5. More B­events can be taken with a looser Level­0 trigger when rejecting the pile­up events. Moreover, double events can be reconstructed when the two interaction vertices are found to be more than a few cm apart. Additionally, the system can be used for luminosity monitoring.