The CMS Muon Trigger System

The CMS experiment will operate at the LHC, a high-energy, high-luminosity proton-proton collider that is under construction at CERN. A very powerful trigger system is needed to allow on-line selection of interesting events in the demanding LHC environment (high luminosity, pile-up of several events in the same bunch crossing, high bunch crossing frequency, high background rates). The CMS experiment will have an innovative, highly selective multi-level trigger system, in order to reduce the total interaction rate of O(1) GHz to about 100 Hz of interesting events written to mass storage. The design, implementation and performance of the CMS muon trigger system are presented. This system should be able to select events with final-state muons, indicating interesting physics. It consists of a "Level-1" trigger implemented on custom-built programmable hardware and of a "High Level Trigger" system, where trigger algorithms are implemented in software running on a farm of commercial processors.