The ATLAS trigger system: Performance and evolution

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During the data taking period from 2009 until 2012, the ATLAS trigger has been used very successfully to collect proton–proton data at LHC center-of-mass energies between 900GeV and 8TeV. Using custom electronics with input from the calorimeter and muon detectors, the first level rejects most background collisions in less than 2.5μs. Then follow two levels of software-based triggers. The trigger system is designed to select events by identifying muons, electrons, photons, taus, jets, and B hadron candidates, as well as using global event signatures, such as missing transverse energy.We give an overview of the performance of the different trigger selections mainly based on the experience during the 2011 LHC run. Furthermore, we describe how the trigger selections have evolved with increasing LHC luminosity to cope with the increasing pileup conditions.