Exclusive Trigger Selections and Sensitivity to the $B_s-\bar{B}_s$ Mixing Phase at LHCb

The LHCb experiment is one of the main experiments that will be hosted at the Large Hadron Collider (LHC) at CERN, in the area of Geneva, and is scheduled to start after the Summer 2007. The LHCb detector is a single-arm forward spectrometer to precise measurements of CP violation and rare decays in the b sector. The primary goal is to test the Standard Model description of flavor physics, and possibly to look for New Physics beyond it. The event rate resulting from the LHC proton collisions will be tremendous, and the production of b hadrons will be copious, thus providing the required statistics for the study of flavor physics. However, not all the events are relevant to LHCb physics program, and a trigger is required. In particular, the High-Level Trigger will take the final decision whether to accept or discard an event before sending it to permanent storage, in a limited period of time. We present in this dissertation the implementation and design of the High-Level Trigger exclusive selections, and assess their performance in selecting the b and c decays of interest, based on a full Monte Carlo simulation. The physics program of LHCb is vast, with the study of a large collection of b decays offering the possibility to investigate CP violation, and any deviation from the Standard Model. In this quest, the neutral B_s--B_sbar system plays a predominant role. The B_s--B_sbar mixing phase, denoted by phi_s, has not yet been measured, and represents a crucial probe of New Physics. This electroweak phase may be tested through theoretically clean bbar -> cbar c sbar quark-level transitions to CP eigenstates, by performing a time-dependent measurement of mixing-induced CP violation. Among the B_s decays to pure CP eigenstates mediated by the bbar -> cbar c sbar transitions, the channel B_s -> eta_c phi yields one of the best sensitivities to phi_s. We present the reconstruction and selection of B_s -> eta_c phi events using a full Monte Carlo simulation, and we determine the characteristics relevant to a CP violation measurement with this channel. The sensitivity of LHCb to the B_s--B_sbar mixing parameters is studied, using a fast parameterized Monte Carlo simulation. This simulation uses the outputs of the realistic full simulation to determine the statistical precision to the mixing observables. We consider the B_s -> J/psi phi decay to an admixture of CP eigenstates requiring an angular analysis to disentangle the different CP components, as well as the B_s -> eta_c phi, B_s -> D_s D_s, and B_s -> J/psi eta decays to pure CP eigenstates. Each of these signal samples are simultaneously fitted with a flavor-specific control sample, B_s -> D_s pi, allowing the extraction of the B_s--B_sbar mixing frequency Delta M_s. We demonstrate that the sensitivity to phi_s is dominated by B_s -> J/psi phi events, with the statistical precision required to uncover New Physics effects. In comparison, the decays to pure CP eigenstates have an order of magnitude smaller event yields, but their contribution to the determination of phi_s, although small compared to B_s -> J/psi phi, is non negligible.