Dalitz Plot Study of the Charmless Decay B+ -> Ks pi + pi0 with the BABAR Detector

This thesis presents preliminary measurements of decays of B mesons in charmless final states, by using a data sample of 365 millions of B+B- pairs collected by the BABAR detector at the PEP-II Asymmetric B Factory, located at the Stanford Linear Accelerator Center. The three-body meson decays are more complicated than two-body decays as they receive resonant and non-resonant contributions. They are generally dominated by intermediate vector and scalar resonances, namely, they proceed via quasi-two-body decays containing a resonance state and a pseudoscalar meson. Indeed, most of the quasi-twobody B decays are extracted from the analysis of three-body B decays using the Dalitz plot technique, in order to study the properties of various resonances. The study of charmless hadronic B decays can make important contributions to the understanding of models of hadronic decays. The Dalitz plot analysis of three-body B decays provides a nice methodology for extracting information on the unitary triangle in the standard model and can help the understanding of CP violation. Studies of charmless three-body decays system combined with theoretical assumption, allow to put constraints on g angle of the Cabibbo-Kobayashi-Maskawa matrix. This is the first study of the charmless decay of the charged B meson into three-bodyfinal state Ks pi+ pi0. This thesis is organized as follows. The theory of the three-body charmless decays is reviewed in Chapter 1, together with a brief reminder of CP violation. Chapter 2 presents an overview of the BABAR detector used to collect the data studied in the analysis. The experimental techniques used to reconstruct events and identify particles are presented in Chapter 3. The studies performed on the Monte Carlo in order to discriminate the signal from the backgrounds are collected in Chapter 4. In Chapter 5 the steps done to validate the fitter are discussed, and the final fit on the data is presented. Chapter 6 presents the evaluation of systematic uncertainties. In Chapter 7 we show the results for branching ratios and CP asymmetries for the three-body decay under study. The results presented in this thesis are preliminary and show the potential capabilities that can be obtained with the BABAR dataset.