The upgrade of the L3 third level trigger for high luminosity runs at LEP

The third level trigger of the L3 experiment has been upgraded to cope with the high luminosity currently reached at LEP (∼ 1.4 · 10 31 cm −2 s −1 ). The new hardware implementation is based on a farm of four VAXstations 4000/90, which receive data through FASTBUS to SCSI interfaces and send accepted events to the main online computer via FDDI. A new software architecture, which makes extensive use of VMS interprocess communication tools, has been built. The current filter algorithms, briefly described here, ensure an output rate of less than 1.5 Hz (in addition to the events used to measure the luminosity) with a very low background contamination. The trigger efficiency, studied with the leptonic Z decays, is 100%.