Alignment of the ATLAS muon spectrometer with tracks

The muon spectrometer of the ATLAS experiment at the Large Hadron Collider is designed to measure the muon momenta of up to 1 TeV/c with a resolution of better than 10% in a toroidal magnetic field of superconducting air–core magnets. The muon track sagitta is measured in three layers of pressurized drift-tube chambers. The precision muon chambers have to be aligned with an accuracy of better than 30 μ m in the track bending plane. An optical alignment system monitors movements of the muon chambers with a precision of few microns. In order to determine the chamber positions in the spectrometer, the initial chamber positions have to be measured with 30 μ m accuracy using straight muon tracks from proton–proton collisions in a dedicated run of the ATLAS detector with the toroid magnets turned off. A Least Square algorithm has been developed which determines the misalignment parameters of a complete azimuthal sector of the barrel part of the muon spectrometer. It has been tested with straight cosmic muon tracks during the commissioning of the ATLAS experiment. Simulations show that the required alignment accuracy is reached with 100,000 muons per sector originating from the interaction point with transverse momentum greater than 10 GeV. About half of the barrel chambers have already been aligned with 30 μ m accuracy using cosmic muons.