Calibration of the ATLAS Muon Spectrometer Precision Chambers for Initial LHC Beam Collisions

ATLAS is a general purpose experiment at the Large Hadron Collider (LHC) at CERN which began operation in September, 2009. ATLAS has a large muon spectrometer which is designed to measure muon momentum with a resolution ranging from 3% at 100 GeV/c to 10% at 1 TeV for a pseudorapidity range of 0-2.7. In this paper, we describe the derivation and validation of calibration constants for the Monitored Drift Tubes (MDTs) in the ATLAS muon spectrometer for use with initial LHC collisions. At LHC start-up, the luminosity was not sufficient for ordinary calibrations with collision muons, and so alternative methods were developed. The time-to-space (RT) functions were obtained from the University of Michigan gas monitor chamber and the ($T0$s) were derived from LHC beamsplash events from 2009 and 2010. We have validated these calibration constants by performing muon track reconstruction using 900 GeV collision data from December, 2009 and 7 TeV data from 2010. We find that these calibration constants yield an MDT chamber hit resolution of 142~$mu$m and a residual width of 91~$mu$m.