New methods for the alignment of the ATLAS muon spectrometer

The ATLAS muon spectrometer consists of three layers of precision drift-tube chambers in an air-core toroid magnet system with an average field of 0.4 T. The muon momenta are determined with high accuracy from the measurement of the sagitta of the muon tracks in the three chamber layers. In order to achieve the required momentum resolution of the muon spectrometer of better than 4% for transverse momenta below 400 GeV/c and of 10% at 1 TeV/c, the relative positions of the muon chambers are measured by a system of optical sensors with an accuracy of 30 mum. In order to verify the correctness of the optical alignment, a method has been developed to measure the relative chamber positions with muon tracks which are recorded during the operation of the experiment. For this purpose, an independent estimate of the muon momenta is needed. This is not provided with sufficient accuracy by the track measurement in the inner detector because of energy loss fluctuations in the calorimeters. For muons of _PT < 40 GeV/c, however, the momenta can be determined with high-enough precision independently of the relative misalignment of the chambers from comparison of the local track direction measurements in the individual chamber layers. This method allows for monitoring of the chamber positions with an accuracy of about 30 mum in time intervals of a few hours during LHC operation.