Prospects on Higgs boson discovery and physics beyond the Standard Model at the LHC and development of the Transition Radiation Tracker of the ATLAS experiment

ATLAS, a general purpose proton-proton detector for the Large Hadron Collider (LHC), will be capable of exploring the new energy regime of 14 TeV which will become accessible. LHC is designed to be built in the existing 27-km circumference LEP (Large Electron Positron collider) tunnel at CERN. It is expected to start operating in 2006. The Transition Radiation Tracker (TRT) is a combined tracking and electron identification device, which will be part of the inner tracking detector of ATLAS. Tracking is carried out by gas-filled flexible straw tubes, while the interleaving radiators produce detectable X-rays when electrons traverse them. In this thesis, the design of the end-cap TRT is finalized and validated through detailed studies performed on a prototype module. These include full-scale measurements of the anode wire tension and determination of the straw straightness, in order to assure a uniform response along the straws during normal operation. In both tests, the performance of the module was found satisfactory and according to predefined specifications. The gas flow uniformity is also studied for various gas supply configurations, by exploiting the sensitivity of gas gain on gas composition. It leads to a final gas supply/exhaust scheme for the TRT end-cap wheels. An analysis of the potential of the ATLAS detector to discover a supersymmetric Higgs boson in the theoretical framework of supergravity has been performed. It is shown that in this model, it will be possible to discover the light Higgs particle across a substantial fraction of the parameter space. For the specific channel studied, h->bb, the b-tagging capabilities of the ATLAS inner detector are of great importance. A comparison of the potential for detecting a Standard Model Higgs between ATLAS and the detectors running at Tevatron is discussed. The expected performance by Tevatron for Run II in the WH, H->bb channel is found to be rather optimistic. The precision of the mass measurement of the Higgs bosons of the Minimal Supersymmetric Standard Model (MSSM) is also investigated. By exploiting the variety of signatures accessible at the LHC, a precision of ~0.1% to ~3% in a large part of the MSSM parameter space can be accomplished.