The ATLAS trigger performance and evolution

During the data taking period from 2009 until 2012, the ATLAS trigger has been very successfully used to collect proton-proton data at LHC centre-of-mass energies between 900 Ge V and 8 Te V at record breaking luminosities. The three level trigger system reduces the event rate from the design collisions per bunch crossing of up to 40 MHz to an average recording rate of about 300 Hz. Using custom electronics with input from the calorimeter and muon detectors, the first level rejects most background collisions in less than 2.5 μs. Then follow two levels of software based triggers. The trigger system is designed to select events by identifying muons, electrons, γ, τ, jets, and B hadron candidates, as well as using global event signatures, such as missing transverse energy. The high trigger efficiencies illustrate that we have achieved a very good level of understanding of both the detector and trigger performance and successfully selected suitable streamed data samples for analysis.