The first-level (LVL1) trigger works on a subset of information from
the calorimeter and muon detectors. It requires about 2 micro-seconds
to reach its decision, including the propagation delays on cables between
the detector and the underground counting room where the trigger logic
is housed. All of the information from the detector must be stored in
pipeline memories until the LVL1 decision is available.
Level-2 Trigger
For events selected by the LVL1 trigger, the information from the
detector must be retained for further analysis. The data for such events
are transferred to readout buffers where they remain until the LVL2
decision is available. The data can be accessed selectively by the LVL2
trigger which uses regions of interest defined by the LVL1 trigger as
indicated in the figure.
The LVL2 trigger refines the selection of candidate objects compared
to LVL1, using full-granularity information from all detectors, including
the inner tracker which is not used at LVL1. In this way, the rate can
be reduced to ~1kHz. Many events are analysed concurrently by the LVL2
trigger system using processor farms, and an average latency of up to
~10ms is considered reasonable.
