Drift-time calibration of the ATLAS Monitored Drift Tubes with a precision mini-chamber

Muon track reconstruction in the ATLAS Muon Precision (MDT) Chambers relies on many thousands of calibration parameters: T-zeros offsets and on time-to-space (RT) functions that convert drift time to drift radius. A data intensive computing program has been created to obtain compute these constants daily for each of thousands of muon-spectrometer subregions. For early LHC running a simplified calibration employing a single universal RT function (URT) provided by a dedicated gas-monitor MDT chamber can be invoked. Constructed with standard MDT's, the monitor chamber records cosmic ray drift-time spectra derived from gas flowing in the input and output arteries of the main ATLAS MDT gas supply. These spectra are used to compute the URT bi-hourly, referenced to a standard operating pressure and temperature.