The OPAL Trigger

As the electrons and positrons circulate around the LEP ring the OPAL detectors must be ready to detect events when they occur. The electrons and positrons circulate in several bunches which cross at a frequency of about 45 kHz. The OPAL detector is synchronised to become active when the bunches cross at its central point.

A large number of different events occur when the bunches cross, however many of them are of no interest for physics. The OPAL trigger system [31,32] decides whether the detector should read out the event or if the subdetectors should be reset and the event ignored. The trigger has two types of signal to decide whether the event is interesting. The $\theta-\phi$ signals are made by dividing the 4$\pi$ solid angle into 144 overlapping bins, 6 in $\theta$ and 24 in $\phi$. The trigger signals sent by detectors are matched to this spatial binning and then a decision can be made on the event. The second signals are stand alone signals, each subdetector has an energy and/or track count threshold, and if this threshold is breached then the event is read out.

The trigger system consists of a pre-trigger and trigger. Both combine stand alone and $\theta-\phi$ signals, however, since 1994, when LEP changed from $8+8$ bunch mode to $4+4$ bunch mode, the pre-trigger was no longer used, although it was still kept in place to minimise change. The time taken for a negative decision by the trigger is about 14.5 $\mu$s and it takes about 4.5 $\mu$s to reset the subdetectors. The trigger system reduces the 45 kHz frequency of bunch crossing to an event rate of about 10 - 15 Hz.