The performance of the ATLAS innermost MDT muon precision tracker in cosmic rays and in positron and muon beams

The ATLAS muon spectrometer has been designed to measure final state muons from proton-proton interactions at the LHC with good momentum resolution (from 3% up to 10% for muons of momentum of 1 TeV). Monitored drift tube (MDT) chambers are used for precision tracking covering the area |eta| > 2.7. The MDTs are built from aluminum tubes filled with Ar/CO₂ at a pressure of 3 bar, operated at 3080 V (gas gain 2 times 10⁴). Each tube measures charged particle tracks with an average spatial resolution better than 80 mum and with high efficiency (>95%). Extensive studies of the performance of the barrel inner small chambers have been accomplished both with cosmic rays and positron and muon test beams. A complete study, of the effect of fluctuations in the chamber operating conditions (HV, pressure, gas composition, and analogue pulse height threshold) on the resolution and efficiency of individual tubes, with cosmic muons, is presented in this work. The track reconstruction quality for muon and positron beams in a dedicated setup at the H8 ATLAS test beam at CERN, of various beam incident angles, are also reported.