Performance of the ATLAS muon trigger in 2011

Events with muons in the final state are an important signature for many physics analyses in the harsh environment produced by collisions of high energy protons at high luminosity. The ATLAS experiment employs a multi-level trigger architecture that selects events in three sequential steps of increasing complexity and accuracy. The Level~1 trigger is implemented with custom built hardware to reduce the event rate from 40~MHz to 75~kHz. The software based higher level triggers refine the trigger decisions reducing the output rate to about 400~Hz. This note presents the performance of the muon trigger evaluated with proton-proton collision data collected in 2011 at a centre-of-mass energy of 7 TeV.