Level-3 will allow event selection before event building, based on summary information extracted from the SVT and other detectors. The mechanism of this selection process will be a series of logical flags, which are set by the level-3 CPUs for each detector separately, which will vary according to the level-3 algorithm in place. A global level-3 decision will be based on some logical combination of all flags from all detectors involved.
At level-3, this will be the only way in which information from various detectors will be combined.
DAQ will make available, on a per-event basis, histograms of distributions of any quantities on which cuts are made in Level-3; this information will be included in the assembled (accepted) events.
Level-3 is an opportunity to select events before incurring the enormous overhead of moving large amounts of data. Well-designed algorithms to summarize the data from each detector will allow characterization of that detector. However, combining information from various detectors before event building requires a communication overhead among the detectors which will adversely affect system performance, as well as an unrealistic requirement for coordination of level-3 software among the detector systems.
Availability of histogrammed distributions of quantities on which cuts are placed is essential for determination of trigger bias.