Infoscience

Thesis

Exclusive trigger selections and sensitivity to the Bs-Bs 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 dedicated 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 dedicated 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 Bs–Bs system plays a predominant role. The Bs–Bs mixing phase, denoted by φs, has not yet been measured, and represents a crucial probe of New Physics. This electroweak phase may be tested through theoretically clean b → ccs quark-level transitions to CP eigenstates, by performing a time-dependent measurement of mixing-induced CP violation. Among the Bs decays to pure CP eigenstates mediated by the b → ccs transitions, the channel Bs → ηcφ yields one of the best sensitivities to φs. We present the reconstruction and selection of Bs → ηcφ 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 Bs–Bs 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 Bs → J/ψφ decay to an admixture of CP eigenstates requiring an angular analysis to disentangle the different CP components, as well as the Bs → ηcφ, Bs → DsDs, and Bs → J/ψη decays to pure CP eigenstates. Each of these signal samples are simultaneously fitted with a flavor-specific control sample, Bs → Dsπ, allowing the extraction of the Bs–Bs mixing frequency ΔMs. We demonstrate that the sensitivity to φs is dominated by Bs → J/ψφ 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 φs, although small compared to Bs → J/ψφ, is non negligible.

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