Building a flexible trigger system for high-energy physics

In the 17/sup th/ century Sir Isaac Newton wrote "These are therefore the Agents in Nature able to make the Particles of Bodies ftick together by very ftrong Attractions, And it is the Bufinefs of Experimatal Philfophy to find them out." In the 21/sup st/ century the Large Hadron Collider (LHC) continues that business. Bunches of protons will be accelerated to an energy of 7TeV/c per proton around a 27 km circumference ring buried underneath French/Swiss countryside near Geneva. At a number of points around the ring counter-rotating bunches of protons will be passed through each other. Some of the protons will interact violently and the new particles generated will fly outwards into detectors surrounding the interaction region. Typically these detectors have millions of channels and data flows out of the front-end at about 400EBytes/year. Data can only be stored at about 1PByte/year, a factor of 4*10/sup 5/ less. Fortunately most interactions involve physical processes that are already understood and a multi-level trigger system is used to select data from interesting or unexpected events. This paper describes the Global Calorimeter Trigger (GCT), part of the trigger system for the Compact Muon Solenoid (CMS) detector at the LHC. (9 refs).