Mesure du temps de vie de la composante CP paire du $B_{s}^{0}$ dans LHCb ; Qualification des phototubes du preshower

This manuscript discusses a twofold research: an instrumental work and a prospective physics study both conducted in the framewor k of the LHCb experiment (Cern, Geneva), foreseen to start at the Spring 2008. TheLHC machine is a proton-proton collider clocked at 40MHz and delivering a center-of-mass energy of 14 TeV. At nominal luminosity, about 1012 pairs of b-particles will be produced in LHCb. After the remarkable achievements of the B factories (BaBar and Belle), it is established that the Standard Mode l(the CK Mmechanism) is the dominant source of CP violation in the B system. Moreover , no significant deviation to the Standard Model predictions is observed in the whole Flavour Physics data up to now. In this scope, the large statistics which will be available at LHCb should allow to reach rare decays potentially sensitive to New Physics contribution. It will also allow to perform precision measurements and deeply test the global consistency of the CKM predictions, in particular with the B particles. This document describes a prospective study about the measurement of the lifetime of the short component of the $B{^0} _{s}$ meson, reconstructed in the final state $B{^0} _{s} to D ^{+}_{s} D^{-}_{s}$, where the Ds meson decays in KK. It has been shown that LHCb should reach a 2% precision on this observable for a nominal year of data taking, yielding interesting constraints on $ \Delta \Gamma_{s} /\Gamma_{s}$. On the ins trumentation side, the team I belong to has the charge of the entire readout of the PreShower (PS) detector of the LHCb spectrometer (design, construction and qualification). My contribution was devoted to the operation of a dedicated test bench aimed at qualifying the phototubes reading the PS detector. The photodetectors which have been retained are the 64anodes PMTs produced by the Hamamatsu company. The main physics specifications for these devices were concerning the uniformity of theresponse of the 64 channels, their linearity, the control of their drift along time and eventually thecross-talk between the channels. I contributed to the analysis of all the data recorded with the test bench and singularly put on evidence a critical problem of cross-talk for a large fraction of the production and made sure all the tubes eventually selected for the experiment were satisfactorily within the requirements.