Characterization of Silica Aerogel for the LHCb RICH Detector and Measurement of the Oscillation Parameter $\Delta m_{s}$

LHCb is the Large Hadron Collider experiment to precise measurements of CP violation and rare decays in the B meson sector. It is presently under construction at CERN, and it will start operations in 2007. In the Standard Model picture, CP violation naturally arises by the complex phase in the unitary 3x3 Cabibbo-Kobayashi-Maskawa (CKM) matrix which accounts for the quark mixing. Thanks both to the high bb cross section and to the high luminosity, the LHC collider will be by far the most copious source of B mesons. A large amount of data will be available and the consistency of the Standard Model will be definitively tested by measuring in several ways all the angles and all the sides of the Unitarity Triangle. These measurements will over-constrain the model and look for inconsistencies due to New Physics. The design of the detector has been optimized to match the kinematical structure of bb events produced in a proton-proton collision. One of the key aspects of LHCb is the identification of the particles produced in b-hadron decays. This tool is used to enhance the signal to background ratio in the selection channels and to provide an efficient kaon tag; it will be achieved using Ring Imaging CHerenkov (RICH) detectors. In particular, LHCb will use two RICH detectors, one covering the charged particle momentum range 1-65 GeV/c using both solid silica aerogel and gaseous C4F10 radiators, and the other covering up to 100 GeV/c using gaseous CF4. Pixel Hybrid Photon Detectors (HPDs) have been developed to detect Cherenkov light in the wavelength range 200-600 nm. The thesis is structured into six chapters: - 1) is a general introduction to the ``CP violation'' problem and to the ``Beauty Physics''. The experimental status of the CKM matrix will be given; - 2) describes the Large Hadron Collider machine and the general lay-out of the LHCb experiment; - 3) is devoted to RICH detectors. Some basic operating principles are given and the experimental challenges together with the factors limiting the particle identification power are discussed. In the second part of the chapter, the design and the performances of the LHCb RICH detectors will be presented; - 4) introduces solid silica aerogel. This chapter describes the characterization of the optical properties of silica aerogel and all the tests performed during the R&D activity. Results of these tests are discussed; - 5) shows a possible approach to extract the unmeasured oscillation frequency \Delta m_{s} from fully reconstructed B0s->Dspi events. The measurement of this parameter is part of the wide physics program of the LHCb collaboration. In the Standard Model picture, the value of \Delta m_{s} is expected to be 20 ps^-1, and its measurement is quite challenging. Deviation from the expected value would suggest New Physics beyond the Standard Model; - 6) presents the Level-0 Muon Trigger. A fast and efficient trigger system is required by the detector to reduce the large amounts of uninteresting processes, such as minimum bias events. Muons with high transverse momentum p_{T} will be used by the trigger system. An introduction to the stand-alone application currently used to optimize and to test the performances of the Level-0 Muon Trigger will be given.