Concept of the First Level Global Trigger for the CMS experiment at LHC

The requirements, the principles and the design of the First Level Global Trigger Processor of the CMS experiment under construction at the CERN Large Hadron Collider are described. A trigger decision based on trigger objects delivered by the detector subsystems is provided for each beam crossing at a rate of 40 MHz. The Global Trigger comprises a novel concept where event selection is not only based on objects exceeding energy or momentum thresholds but also on complex event topology, which is made possible by the availability of space, charge and quality information in the algorithm calculations. The trigger logic is largely programmable in order to satisfy all possible physics requirements.