In-Situ Determination of the Performance of the ATLAS Muon Spectrometer

The muon spectrometer of the LHC detector ATLAS provides an independent and precise muon track measurement using three layers of precision drift tube chambers in a toroidal magnetic field with a bending power integral Bdl of 2Tm to 8 Tm. Muon tracks are reconstructed with 97% efficiency and a momentum resolution about 3% for most of the range and better than 10% for transverse momenta up to 1 TeV/c. The latter requires the knowledge of the magnetic field with 1% accuracy and misalignment corrections of the track curvature with an accuracy of about 30 mu m. The magnetic field is measured by Hall probes on each chamber with sufficient precision. The misalignment corrections are given by a system of optical sensors. We present methods which allow us to measure the performance of the muon spectrometer during the operation of the ATLAS detector. The alignment of the chambers can be verified with single muon tracks by making use of redundant momentum measurements in the spectrometer. The muon reconstruction efficiency and the momentum resolution can be determined with muons from Z decays.