Systematic study of the calibration of the drift tubes for muon tracking in the ATLAS experiment at the LHC and possible use of fast gas mixtures for SLHC

The Muon Spectrometer of the ATLAS experiment is based on regular arrays of 3 cm diameter high pressure drift tubes (MDT) in a toroidal magnetic field with an associated bending power of about 3 and 5 Tm (barrel and endcap). The gas composition (93%Ar -7%CO₂) and gain (2 ×10⁴), chosen in order to limit aging effects, make the drift response function slow and highly non linear. A test beam setup, with a small MDT chamber and a high resolution silicon strip telescope, was organised at the CERN H8 high energy muon beam with the goal of measuring the drift properties of both the standard ATLAS gas mixture and of alternative, faster gas mixtures, typically based on CF₄, in view of a possible use at SLHC or elsewhere. We describe the analysis of the test beam data and show the first, preliminary, results and their comparison with the expectations of the simulation.