Commissioning of the ATLAS Muon Trigger with Beam Collisions at the LHC

The ATLAS experiment at CERN's Large Hadron Collider (LHC) has taken its first data with colliding beams. The LHC aims to deliver an integrated luminosity of 1 fb-1 in the run period 2010/2011 at luminosities of up to 1032 cm-2 s-1, which requires active rejection of events in the trigger system. The muon system is the largest sub-detector of the ATLAS experiment and has the capability to reconstruct muons in standalone mode, as well as in combination with the Inner Detector tracking. It deploys different detector technologies, resistive plate chambers and thin gap chambers to provide fast trigger signals, and monitored drift tubes and cathode strip chambers for precision measurements. The L1 muon trigger gets its input from the fast muon trigger detectors. Fast sector logic boards select muon candidates, which are passed via an interface board to the central trigger processor and then to the High Level Trigger (HLT). The Muon HLT is purely software based and encompasses a level 2 trigger followed by an event filter for a staged trigger approach. It has access to the data of the precision muon detectors and other detector elements to refine the muon hypothesis. The muon trigger system has been extensively tested with cosmic data and collision data taken in 2009/2010 at centre of mass energies of 900 GeV and 7 TeV. Results will be presented for efficiency, resolution and general performance of the algorithms and detectors. The i ncreasing data set with colliding beams at 7 TeV will allow the final commissioning of the muon trigger system. The strategy towards active selection in the HLT with the increasing luminosity profile of the LHC will be described.