Commissioning of the ATLAS Muon High Level 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 10^32 cm^-2s^-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. The L1 muon trigger gets its input from 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 HLT 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. The increasing data set with colliding beams at 7 TeV will allow the final commissioning of the HLT algorithms. This talk describes the strategies at L1 and the HLT to detect muons and presents first performance results with collision da ta. Emphasis is given on the physics performance and the commissioning strategy towards active selection in the HLT.