Performance of The ATLAS Muon Spectrometer During the Commissioning

The ATLAS detector has been operating for several months: in the fall 2008 the rst proton beam circulated in the LHC machine but no beam-beam collisions have been produced yet, so that the data collected so far are mainly cosmic rays. The very large ATLAS Muon Spectrometer (MS) includes four different technology chamber types. It has been designed to trigger on single muons of pseudo-rapidity up to 2.4 and transverse momentum down to a few GeV and track reconstruction with a standalone momentum resolution better than 10% for particles up to a few TeV. The Muon system, while still in the process of being completed, has undergone an intense program of commissioning, ranging from understanding the status of each single chamber to study the performance of the whole system for triggering, data acquisition, and reconstruction software. Several preliminary results relevant to the muon trigger and reconstruction performance are here presented.