Muon identification

 

Muons are essential in most of the physics processes and can be used either for study of signal events, for example Higgs decaying to 4 leptons, or as tag for certain processes, for example processes that require b-quark identification(b-tagging). Therefore muon identification presents an important issue in all the physics topics we will concentrate on in the present RTN. Indeed, muons in a momentum range between 5 GeV to 1 TeV are present in most of the channels of interest presented in WP's 2-4.

In ATLAS the MuonSpectrometer (MS) is composed of Tracking and Trigger detectors which span a total area of 5600 m2, located inside the volume of an air core toroid magnet, to minimize the effect of energy losses and multiple scattering. The designed momentum resolution varies between 2Ð10 % for a momentum range between 5 GeV to 1 TeV. To achieve this momentum resolution in the spectrometer the 1200 individual tracking chambers -composed of drift tubes- were constructed with a mechanical precision of better than 20 microns (the position of the drift tubes in the chamber) while the position and deformation of individual chambers is monitored to better than 30 microns during data taking.

To maintain the designed momentum resolution the following ingredients are needed :

á         Alignment constants of individual chambers extracted from alignment data recorded during data taking

á         Calibration constants for the precise calculation of the distance of minimum approach (r-t relation) of the muon tracks from the individual detectors (drift tubes)

á         Energy corrections due to energy losses of the muon in the 'dead' material before the muon spectrometer and energy correction using the measured energy loss in the calorimeters

á         Evaluation of the performance of the muon spectrometer and optimization of the track reconstruction efficiency. This evaluation is especially important for BR measurements of rare processes and searches for new particles

A first evaluation of the performance of the spectrometer is already accomplished using muon test beam data at the CERN H8 test area where an octant of the ATLAS muon spectrometer was installed together with samples of the inner tracker and calorimeter modules of the final detector. With the first LHC data the calibration and validation of the muon spectrometer will be done using dedicated event samples from W and Z boson decays. The reconstruction of the invariant mass of the Z-boson decaying to a pair of muons, will provide the absolute scale calibration and a validation of the alignment and calibration constants.

 

Participating institutes in the proposed RTN have played a central role in the design, construction and test of the muon tracking and alignment systems and in the development of the muon track reconstruction algorithm.

Therefore are expected to take in hand the muon identification issues, which are crucial for the physics goals of this network.