Performance Validation of the ATLAS Muon Spectrometer

ATLAS (A Toroidal LHC ApparatuS) is a general-purpose experiment for the future Large Hadron Collider (LHC) at CERN, which is scheduled to begin operation in the year 2007, providing experiments with proton-proton collisions. The center-of-mass energy of 14TeV and the design luminosity of 1034 cm−2s−1 will allow to explore many new aspects of fundamental physics. The ATLAS Muon Spectrometer aims at a momentum resolution better than 10% for transverse momentum values ranging from pT = 6 GeV to pT = 1TeV. Precision tracking will be performed by Ar-CO2-gas filled Monitored Drift Tube chambers (MDTs), with a single wire resolution of < 100 μm. In total, about 1 200 chambers, arranged in a large structure, will allow muon track measurements over distances up to 15m in a magnetic field of 0.5 T. Given the large size of the spectrometer it is impossible to keep the shape of the muon chambers and their positions stable within the requested tracking accuracy of 50 μm. Therefore the concept of an optical alignment system was chosen. An alignment accuracy of 30–40 μm must be achieved in order to meet the design resolution. This work is dedicated to the validation of the required precision of the optical alignment system with straight muon tracks in a test beam environment. A 1:1 test stand of the ATLAS muon spectrometer has been setup in the CERN north area on the H8 beam line. The setup consists of two stands, including both tracking and trigger chambers, which emulate in detail projective towers of the barrel and of the end-cap spectrometers, allowing an overall system test. An important aspect of the system test is the study of the offline reconstruction software, and the development of the code necessary for the application of the ATLAS software framework to real detector data. A C++ package, developed to fulfill this task, is described. The outcome of the first test on the track fit performance of the ATLAS reconstruction software with real detector data is presented, followed by detailed studies of MDT chamber performance in the test beam, involving analysis of data taken at different beam energies and with different amount of material placed along the beam. A detailed comparison of chamber position obtained by tracking and obtained by the optical alignment system allows the validation of the general performance of the alignment system of the ATLAS muon spectrometer. Additionally, the concept of a final test on the overall performance of MDT chambers, coupled with RPC chambers, before their final installation in ATLAS is demonstrated on a cosmic ray test stand.