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Wednesday, May 16, 2007

Complex Event Processing at CERN

Posted by Giles Nelson

This week I visited CERN in Switzerland, the European Organisation for Nuclear Research, who is a customer of Progress. It was an astonishing and inspiring visit. CERN is in the final stages of building the Large Hadron Collider (LHC) which is due to go into production late this year. The LHC consists of a 27km loop in which protons will be accelerated and collided at unprecedented power levels to give us new insights into the building blocks of matter. In particular the search is on for the Higg's Boson, predicted originally in a paper dating from the 1960s. Finding this will fill a gap in the Standard Model of elementary particles and forces, and will help in furthering a "theory of everything". A particular highlight was to go down nearly 100m underneath the ground to look at the ATLAS experiment - a truly massive particle detector. Its enormous size consists of a number of different elements which detect different types of particles - muons, gluons and many others. The huge magnets which form part of the detector are cooled with liquid helium down to -269 degrees C to make them superconducting (and therefore more powerful). Viewing all this brought home what a remarkable engineering effort it all is.

Anyway, what has all this got to do with events? Well, through a number of presentations that CERN staff were kind enough to give us throughout the day it became apparent that their whole world is to do with events and the processing of them. The term "event" is one which they used often, to describe the information gathered by the detectors which sit around the collider. Every time a set of protons collides sets of events are created which need to be analysed and filtered to determine which are of real interest. For example, there are two ways in which a proton can decay to produce two Z particles (check). One is predicted to involve a Higg's Boson so the set of events to look for is something like "proton collision followed by a Higg's Boson followed by two Z particles". To identify such sets of temporally correlated events the raw events are propagated up through three levels of filter to be finally sent through to a central computing resource for further research and analysis. Up to 40 million collisions per second take place. These are firstly analysed in FPGA hardware reducing the 40 million collisions to a few thousand of interest. These are further filtered in software to produce finally a few hundred. These few hundred are then sent to other computing systems for further analysis.

It's not only collider events that CERN needs to handle. CERN also has a newly built central control centre, part of which is used to monitor CERN's technical infrastructure. About 35,000 separate sensors exist to monitor everything from fire, to electricity substations, to coolant plants. All these sensors are currently producing about 1.6M events per day all of which have to propagated to a central point for analysis. In turn these 1.6M are reduced to 600K events which are overviewed by human operators. Most are inconsequential (for example the 18KeV power supply is still producing 18KeV) but some will require attention. By appropriately analysing these CERN can ensure that the colliders are running as smoothly and as safely as possible. With billions of euros invested so far in the LHC, keeping the collider up and running as continually as possible is a top priority.

The visit proved a fascinating insight into the world of particle physics and the data processing challenges it produces. It really showed event processing at its most extreme.

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