New Methods for the Alignment of the ATLAS Muon Spectrometer with Tracks
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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 spectrome-
ter 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 µm. Muon tracks are used to
determine absolute position and in region of the detector where a high-precision
optical alignment system is missing.

In order to verify the correctness of the optical alignment and to cover is
gaps, new methods have been developed to measure the chamber positions with 
curved muon tracks which are recorded during the operation of the experiment. 
For this purposes, independent estimates of the muon momenta must be used. These
are not provided with sufficient accuracy by the track measurements in the inner detector because of 
energy loss fluctuations in the calorimeters. For muons of transverse momenta 
<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 µm in time intervals of one day during LHC operation.