An accurate phenomenological analysis of rare B-meson decays is important both for precise tests of the Standard Model and in the search for possible New Physics in the flavour sector. In the framework of this thesis, several semileptonic B-meson decays induced by flavour-changing quark currents are studied. The main focus is put on the analysis of the underlying hadronic input including the relevant form factors and nonlocal hadronic amplitudes. With accurate determination of the hadronic input from the QCD-based methods, various observables in inclusive and exclusive semileptonic decays of B mesons are predicted.
This dissertation is written in the form of a cumulative work based on our four articles published in international peer reviewed journals. The first chapter contains a short introduction to the Standard Model and a brief discussion of theoretical methods.
The chapter 2 of the thesis is dedicated to the analysis of the flavour-changing neutral current (FCNC) b --> d observed in the form of the semileptonic B --> pi ell ell decays. In the latter decays, the underlying hadronic nonlocal contributions are systematically taken into account using the operator-product expansion, QCD factorization and light-cone sum rules. Together with the B --> pi form factors derived from light-cone sum rules, these nonlocal hadronic amplitudes are used to predict various observables such as the differential rate, direct CP-asymmetry and isospin asymmetry in B --> pi ell ell decays. In addition, the total width of B --> pi nu nu decay is estimated.
The third chapter of the thesis is devoted to the analysis of the higher-twist effects in the QCD light-cone sum rule for the heavy-to-light transition form factors. To this end, the light-cone expansion of the massive quark propagator in the external gluonic field is extended to include new terms containing the derivatives of gluon-field strength. The resulting analytical expressions for the twist-5 and twist-6 contributions to the correlation function are obtained in a factorized approximation, expressed via the product of the lower-twist pion distribution amplitudes and the quark-condensate density. The numerical analysis reveals a smallness of the higher twist effects justifying the conventional truncation of the operator product expansion in the light-cone sum rules up to twist 4.
The chapter 4 of the thesis extends the analysis of hadronic input to other semileptonic decays including B --> K ell ell and Bs --> K ell ell. To this end, the light-cone sum rule results for the relevant form factors are updated taking into account the estimate of the higher twist effects. In addition, the corresponding nonlocal hadronic matrix elements are extracted in a systematic way. Moreover, a new way to determine the Wolfenstein parameters of the Cabibbo-Kobayashi-Maskawa matrix is suggested based on the observables in semileptonic FCNC B --> pi ell ell and B --> K ell ell decays. The prediction for the Bs --> K ell nu partial decay width provides an additional source for the determination of the CKM parameter |Vub|.
The last chapter 5 of the thesis deals with the inclusive semileptonic B --> Xc tau nu decays. Using the standard techniques of heavy quark expansion, the decay width and moments of tau-lepton energy distribution are calculated including power corrections up to order Lambda_QCD^3/m_b^3. The result is compared with the sum of the predictions for the branching fractions of the exclusive semileptonic B --> (D, D*, D**) tau nu decays as well as with the relevant experimental data. In addition, the impact from physics beyond the Standard Model to the inclusive B --> Xc tau nu rate is discussed.