Commissioning the trigger of the Compact Muon Solenoid experiment at the CERN Large Hadron Collider

The Compact Muon Solenoid (CMS) experiment at the Large Hadron Collider (LHC) has taken cosmic-ray and colliding-beam data in 2009 and will be in continuous data-taking mode beginning at the end of February 2010. CMS detects the interaction products of proton beams colliding at a rate of 40 MHz. It uses a two-level trigger system to flag interesting events and reduce the event rate to a lower level, suitable for storage of full event data. The first-level trigger uses custom-built electronics to combine information from electromagnetic and hadronic calorimeters and muon detection systems to identify potential physics objects of interest with a target output rate of 100 kHz. The high-level trigger uses a computer farm to process the first-level information and more detailed detector readout including data from the pixels and tracker to reconstruct the event more fully and reduce the rate further to 100 Hz. For much of 2009, cosmic rays provided data for commissioning the trigger system. In addition, the 2009 LHC collision runs at 900 GeV and 2.36 TeV center-of-mass energy provided an opportunity to commission and study the performance of the trigger under colliding-beam conditions. This talk will report on the commissioning experience gained in the 2009 cosmic and collision runs and the 2010 collision run.