Alignment of the ATLAS muon spectrometer with tracks and muon identification at high background rates

Abstract

The ATLAS muon spectrometer consists of three layers of precision drift-tube chambers in an average toroidal magnetic field of 0.4 T. Muon tracks are reconstructed with 97% efficiency and a momentum resolution of better than 10% for transverse momenta up to 1 TeV / c . The latter requires misalignment corrections of the track curvature with an accuracy of about 30 μ m which are provided by an optical alignment monitoring system. A method has been developed to measure relative chamber positions with curved muon tracks with comparable precision in order to increase the redundancy of the alignment system. During the operation of the LHC at its design luminosity of 10 34 cm - 2 s - 1 , the muon chambers will be exposed to a high flux of neutrons and γ rays which may lead to occupancies of up to 16%. Based on test-beam data taken at the Gamma-Irradiation Facility at CERN, we show that the anticipated reconstruction efficiency can be achieved with the precision chambers in this environment and how it can be sustained at even higher background rates which are expected for a possible luminosity upgrade of the LHC.