Study of the effect of the misalignment of the muon detectors at the ATLAS experiment on the discovery of the Higgs particle H->4mu

The ATLAS (A Toroidal LHC ApparatuS) detector is one of the four detectors which will be installed in the new accelerator, at LHC, CERN. One of the experiments' research aims is the discovery of the Higgs particle and thus, the confirmation or not of the Standard Model. The Muon Spectrometer of the detector has special importance, because the decay channels of the Higgs particle which have muons at the final state are clear signatures of the existence of the Higgs particle. This project focuses on the Higgs decay through the channel: H->ZZ->4mu. The alignment of the muon detectors has to be very accurate, so that its contribution on the measurement of the muons' momentum, to be low compared to the intrinsic resolution of the detectors themselves. Although the alignment of the muons detectors at the barrel and the end-caps regions is well controlled, the relative alignment of the end-caps with respect to barrel is not controlled with the same accuracy. In this project, we study the influence of such misalignments (translations and rotations) on the search of the Higgs particle (width and discovery potential), which decays through the channel mentioned above. The first part of the project refers to the Physics of the ATLAS detector, to its specific construction features and finally to the alignment scheme of the muon detectors at the barrel and forward region. In the second part, there is a detailed reference to the data acquisition and analysis, using the muon spectrometer alone. The method, the mathematical formulation and the C++ programming application concerning the fitting of the muons' tracks under the constraint of the parent's Z invariant mass (Constraint Fitting) are presented. This method results in diminishing the reconstructed Higgs width,therefore in improvement of the Signal/Background ratio. Finally, the applied misalignments (translations and rotations) of the one end-cap with respect to the barrel are presented and the effects on the determination of the mass and the width of the Higgs and Z particles are studied. At the conclusions, we mention the usage of the well-known Z width in order to isolate the kind and the amount of the probable misalignments.