First results from the ATLAS muon spectrometer optical alignment system

The muon spectrometer of the ATLAS detector at the Large Hadron Collider (LHC) at CERN consists of 1182 muon chambers for precision track measurements, arranged in three concentric cylinders of up to 25 m length in the central (barrel) region, and in four wheels of up to 25 m diameter in each of the two endcaps. They are located between 5 and 22 m from the interaction point, and each muon track is detected in three equally spaced chambers. The muon chambers are equipped with a complex high-precision optical alignment system to monitor their positions and deformations during ATLAS data-taking to an accuracy of 30 – 40 μ m , which is ultimately required for reconstructing high-momentum final-state muons with the desired momentum resolution of 10% at 1 TeV. The alignment system, the sensors, and the readout and reconstruction software are described. The installation and commissioning of the more than 12 000 sensors in the ATLAS cavern were completed in 2008, and first data from the full system are presented.