THE ONLINE EVENT FILTER OF THE OPAL EXPERIMENT AT LEP

The on-line event filter of the OPAL experiment at LEP is described. It acts as the second-level software trigger, following the first-level hardware trigger and is implemented on a workstation fully integrated into the data acquisition system. In addition to background rejection and event classification it provides detailed monitoring of the detector status and data quality, event visualisation and data compression. The partial event reconstruction and the selections based on physics quantities used for the event classification are described, and the performance of the system as used for data taking in 1991 is discussed.