Performance of the ATLAS Trigger with Proton Collisions at the LHC

The ATLAS trigger has been used very successfully to collect collision data during 2009 and 2010 LHC running at centre-of-mass energies of 900 GeV, 2.36 TeV and 7 TeV. It is designed to reduce the event rate, from the design bunch crossing rate of 40 MHz, to a maximum recording rate of about 200 Hz. To achieve this it is composed of three levels. Level-1 uses custom electronics to reject most background collisions, in less than 2.5 micros, using information from the calorimeter, muon and minimum bias detectors. The upper two trigger levels, known collectively as the High Level Trigger (HLT), are software-based and run on farms of commodity processors. The Level-2 trigger uses mostly custom algorithms while the Event Filter is based on offline tools. In order to achieve average processing times of 40 ms at Level-2 and 4 s at the Event Filter, most HLT processing is performed on partial event data corresponding to Regions of Interest identified by the Level-1 trigger. The trigger selection is based on global event features, such as missing transverse energy, and identifies candidate muons, electrons, photons, tau mesons and jets. We present the results from detailed studies of trigger performance during collision running. We give distributions of key quantities calculated online, compared with the same quantities calculated offline and expectations from Monte Carlo simulation. These studies show that the trigger is performing wel l and is efficiently collecting the data needed for the ATLAS physics program.