The CMS trigger system

This paper describes the CMS trigger system and its performance during Run 1 of the LHC.Thetriggersystemconsistsoftwolevelsdesignedtoselecteventsofpotentialphysicsinterest from a GHz (MHz) interaction rate of proton-proton (heavy ion) collisions. The first level of the trigger is implemented in hardware, and selects events containing detector signals consistent with an electron, photon, muon, τ lepton, jet, or missing transverse energy. A programmable menu of up to 128 object-based algorithms is used to select events for subsequent processing. The trigger thresholdsareadjustedtotheLHCinstantaneousluminosityduringdatatakinginordertorestrict the output rate to 100kHz, the upper limit imposed by the CMS readout electronics. The second level,implementedinsoftware,furtherrefinesthepurityoftheoutputstream,selectinganaverage rate of 400Hz for offline event storage. The objectives, strategy and performance of the trigger systemduringtheLHCRun1aredescribed.