Measurement of the Branching Ratio of the charmless decay Bs → φφ at CDFII

We present a study of the charmless Bs → φφ decay performed with the CDFII detector at the Fermilab Tevatron Collider. Charmless B0 decays currently can be studied only at the Tevatron and represent a field still to be fully explored that offers additional ways to test our present theoretical understanding. The Bs → φφ belongs to a particular class of these decays: the Bs meson decays into a pair of vector particle and the fi al state is self- conjugate. It can be used to measure the Bs decay width difference (∆Γs), to improve our understanding about the Cabibbo Kobayashi Maskawa matrix, and to perform tests of decay polarization predictions. The Bs → φφ decay proceeds through a b → sss transition and in the Standard Model the dominant process is the b → s “penguin” diagram. The same penguin amplitude is involved in several processes which have shown several discrepancies with the Standard Model predictions, raising considerable attention on the theoretical side and new physics interpretations have been considered to explain the experimental data. To shed light on this experimental and theoretical rather complex scenario, new and more precise measurements are clearly needed in as many interesting channels as possible. The study of Bs → φφ channel is an important player in this experimental effort. The Bs → φφ decay has been observed for the first time by CDF in 2005 [1] in a data sample of 180 pb−1; eight events have been seen and a first measurement of the Branching Ratio (BR) has been performed. In this thesis we present an update of this measurement with an integrated luminosity of 2.9fb−1. An improvement of a factor 5 compared to the previous Branching Ratio measurement is achieved. In Chap.2 a short introduction on the Standard Model with special emphasis on the electroweak sector is presented. In Chap.3 the Fermilab accelerator complex and the CDFII detector is described. The trigger strategy to select events relevant to B physics is explained in some detail. In Chap.4 we present the data sample used for this analysis and some informations concerning the offline reconstruction program. In Chap.5 the Monte Carlo program and its validation procedure are explained. In Chap.6 the strategy of the Branching Ratio measurement and the optimization procedure used to define the analysis selection criteria both for the Bs → φφ and the Bs → J/ψφ decays are presented. In Chap.7 all the steps needed to perform the measurement of the BR are reported. Signal yields of the Bs → φφ and the Bs → J/ψφ decays are presented. The trigger and selection efficiencies and the muon efficiency are evaluated. The ratio of the BR(Bs → φφ) to the BR(Bs → J/ψφ) is measured and is used to compute the Branching Ratio of Bs → φφ. The systematic errors on the BR are estimated. In Chap.8 we present the summary of our results.