Dependence of the space-to-drift-time relationship of monitored drift-tube chambers on the magnetic field in the ATLAS muon spectrometer

The ATLAS detector is a multi-purpose detector which is built for the search for the standard model Higgs particle and new physics at the Large Hadron Collider (LHC). A striking feature of its design is the muon spectrometer, which is able to measure muon momenta with an accuracy of 3% over a wide momentum range; at 1 TeV/c an accuracy of 10% will be achieved. In the muon spectrometer, the muon trajectories will be measured by three stations of monitored drift-tube (MDT) chambers. The MDT chambers are operated in an average magnetic field of 0.4 T which is generated by 8 superconducting air-core toroid coils. The MDT chambers of the inner and outer layers in the barrel region are mounted outside the magnet coils and therefore experience a highly non-uniform magnetic field with field variations of up to 0.4 T. Due to the bending of the paths of the drift electrons in the magnetic field, the space-to-drift-time relationship r(t) depends on the magnetic field inside a tube. The maximum drift-time of 700 ns in absence of a magnetic field is extended by about 70 ns/T. The dependence of r(t) must be corrected for with an accuracy of 1 ns in order to achieve a chamber resolution of 50 /mum. The dependence of r(t) on the magnitude and the orientation of the magnetic field with respect to the anode wires of the tubes and the muon incident angle was measured in a test-beam. These measurements allow for a parameterization of the magnetic-field dependence of r(t) with the required accuracy of 1 ns