The CMS High Level Trigger: Commissioning and First Operation with LHC Beams

The CMS experiment will collect data from the proton-proton collisions delivered by the Large Hadron Collider (LHC) at a centre-of-mass energy up to 14 TeV. The CMS trigger system is designed to cope with unprecedented luminosities and LHC bunch-crossing rates up to 40 MHz. The unique CMS trigger architecture only employs two trigger levels. The Level-1 trigger is implemented using custom electronics. The High Level Trigger is implemented on a large cluster of commercial processors, the Filter Farm. Trigger menus have been developed for detector calibration and for fulfilment of the CMS physics program, at start-up of LHC operations, as well as for operations with higher luminosities. A complete multipurpose trigger menu developed for an early instantaneous luminosity of 10^{32}cm{-2}s{-1} has been tested in the HLT system under realistic online running conditions. The required computing power needed to process with no dead time a maximum HLT input rate of 50 kHz, as expected at startup, has been measured, using the most recent commercially available processors. The Filter Farm has been equipped with 720 such processors, providing a computing power at least a factor two larger than expected to be needed at startup. Results for the commissioning of the full-scale trigger and data acquisition system with cosmic muon runs are reported. The trigger performance during operations with LHC circulating proton beams, delivered in September 2008, is outlined and first results are shown.