The upgrade of RICH detectors for the LHCb experiment and a study of charmless B decays

This thesis reports the first evidence of B0s→ηcπ+π− decay using an integrated luminosity of 3.0 fb−1, recorded by the LHCb experiment during 2011 and 2012. The branching fraction is measured to be: B(B0s→ηcπ+π−) = (1.76±0.59±0.12±0.29)×10−4 where the two first uncertainties are statistical and systematic, respectively, and the third uncertainty is due to the limited knowledge of the external branching fractions. This re sult is in good agreement with the naive prediction. The statistical significance of this decay, including systematic uncertainties, is 4.6σ. Moreover there is an indication that this decay proceeds via the f0(980) resonance. The size of the data sample recorded by the LHCb experiment in Run-I of LHC is not sufficient to perform a measurement of the CP violating phase φs from time-dependent analysis of B0s→ηcπ+π− decays. In the future with significative improvement of the hadronic trigger efficiencies, this decay mode might become of interest to add sensitivity to the measurement of φs, which precision is expected to be limited by the statistical uncertainty until the 2030.