Commissioning and performance of the ATLAS Trigger with Proton Collisions at the LHC

The ATLAS trigger has been used very successfully to collect cosmic‐ray and single‐beam events, and collision data during 2009 and 2010 LHC running at centre of mass energies of 900 GeV and 7 TeV. The trigger system reduces the event rate, from the design bunch‐crossing rate of 40 MHz to an average recording rate of 200Hz. The ATLAS trigger is composed of three levels. The first (Level 1) uses custom electronics to reject most background collisions, in less than 2.5μs, using information from the calorimeter and muon detectors. The upper two trigger levels, known collectively as the High Level Trigger (HLT), are software‐based triggers. The HLT software has been extensively exercised both in initial offline running and operating online. As well as triggers using global event features, such as missing transverse energy, there are selections based on identifying features in the event consistent with muons, electrons, photons, tau leptons or jets. We describe the strategy and tools used to commission these trigger selections using LHC data. We also describe the various metrics and validation procedures used to monitor and understand the trigger performance in the changing conditions of the LHC. The current status of each trigger selection is described, and examples of important applications will be given, such as the online determination of the LHC beam position. Finally, we discuss plans for the evolution of the Trigger d uring the forthcoming LHC running.