Calibration of the wire positions of the ATLAS Muon Chambers and studies of Neutralino decays at LHC

ATLAS is a general-purpose detector that is being built for the Large Hadron Collider (LHC) at CERN. The precision tracking devices of the ATLAS Muon Spectrometer are high pressure Monitored Drift Tube (MDT) chambers. The LHC physics discov- ery potential sets stringent requirements on the performance of these chambers. They should be able to achieve a momentum resolution of 46587:9 for muons with a trans- verse momentum of ;<>=?5 TeV. In order to accomplish this goal, the MDT chambers should be constructed with 20 @ m mechanical accuracy (r.m.s of the relative distance between the wires of the drift tubes). The present work describes a wire calibration method for the MDT chambers, with- out the use of any external reference system. The success of the method in finding wire displacements in the direction perpendicular to the muon tracks was verified dur- ing two test beam periods: first the displacements of 12 tubes were measured while the chamber was filled with the old baseline gas A.BDC!EGFDC'HJI.K (91/4/5), and later while the new baseline gas was used ( AGBCLHNMOF (93/7)), wire displacements of 52 tubes were calculated. The results of the method were compared with those given by the X-ray tomograph at CERN, revealing an accuracy of the method better than 10 @ m. Simulation studies of the application of the method along the full width of a cham- ber, were performed and were focussed on its systematic and statistical effects. It turned out that even when Gaussian distributed time offsets of 200 and 300 ps are added to the drift time spectra of the tubes, the local uncertainty remains in the order of 10 @ m. By combining the local information for the wire displacements the absolute wire positions within the whole chamber with respect to the first central wires within each multilayer are acquired. In this case, the global accuracy of the method is in the order of 20 @ m. Moreover, considering random and uncorrelated wire displacements this accuracy improves to 10 @ m. This work also presents studies relevant to the ATLAS Supersymmetry (SUSY) discovery potential. SUSY is considered as a very motivated extension of the Standard Model by many theorists. If it exists it will lead to discoveries at LHC over most of its parameter space. Within the Minimal Supergravity Model (mSUGRA) of SUSY, five parameters are used in order to determine the masses and couplings of the particles. Six combinations of these parameters, selected by the LHC Committee and the AT- LAS Collaboration for detailed studies, define representative points of the mSUGRA parameter space. The decay channel P RQFNS P RQTUWVRU%X through a virtual Z Y was studied at the fourth point of mSUGRA. Two versions of PYTHIA were compared by studying the differences in the invariant mass distributions of opposite-sign same-flavour lepton pairs. The newest version of the two, includes the spin-averaged matrix element in sparticle decays, thus taking into account the spin of the neutralinos and the propaga- tor. The effect of this inclusion is profound in most of the cases while studying this specific channel in different points of the mSUGRA parameter space.