Calibration and Performance of the precision chambers of the ATLAS muon spectrometer

The precision Monitored Drift Tube (MDT) [1] chambers of the ATLAS muon spec-trometer [2] measure the track sagitta up to a pseudo-rapidity of 2.7 with a 40µm uncertainty, yielding a design muon transverse momentum resolution of 10 % at 1TeV. To ensure a 80µm resolution on single hit, continuous MDT calibration [3] is neces-sary. It consists of computation of single tube drift time offset (t0), and determination of the space vs time (r-t) relation of each chamber [3]. High statistics is required: 2 · 108 muon tracks over the entire spectrometer in less than one week. To this pur-pose a dedicated Stream of muon data, the Muon Calibration Stream [4], is extracted from the second level trigger, requiring a track in the MDTs pointing to the colli-sion vertex (10kHz muon rate in 2010 obtained with an instantaneous luminosity of ∼ 1032cm−2s−1). Data are streamlined and processed in three Calibration Centres (sited in Ann Arbor, Munich and Rome). The full chain, from the data extraction to the assessment of the data quality (DQA) and the calibration computation, requires one day latency. The DQA at the Calibration Centers takes advantage of the large statistics o