Event Filter

Major Canadian Contributors

In the ATLAS detector there will be about a billion collision events per second, a data rate equivalent to twenty simultaneous telephone conversations by every person on the earth. When protons collide, some events are "interesting" and may tell us about exciting new particles or forces, whereas many others are "ordinary" collisions (often called "background"). Only 10 to 100 of the billion collisions that occur each second must be flagged as potentially interesting and recorded for further study, while all the others are rejected. That information must be obtained and analyzed very rapidly. Furthermore the information must be sufficiently detailed and precise to allow eventual recognition of certain "events" that may only occur at the rate of one in one million-million collisions (10-12), a very small fraction of the recorded events, which are a very small fraction of all events.

This influx of data is first filtered in real time by two levels of processing modules: The Level 1 Trigger makes an initial selection based on reduced granularity information from a subset of detectors. The Level 2 Trigger reduces the data flow to about 1 GB/s using full-precision data from most of the detectors, by examining only regions of the detector identified by the first trigger as containing interesting information. At the third trigger level, the Event Filter, the full event data are available to make the final selection of events that will be available for off-line analysis. The Event Filter must reduce the final data to storage rate to a level of 100 MB/s

The University of Alberta prototype Event Filter Farm (above-right) consists of seven dual Pentium 450MHz machines connected via a fast Ethernet Switch and will soon be upgraded to nine dual Pentium 450MHz machines connected in a 3x3 array using Scalable Coherent Interconnect (SCI).

McGill and the Université de Montréal are collaborating on the High Level Trigger testbed.
Trigger Happy: Steve Robertson during his shift in the ATLAS control room during the March HLT technical run

source: NSERC 1998 Project Research Description