Phenomenological Studies Of Some Semileptonic Decays Of B,B_s And B_c Mesons In The Perturbative QCD Factorization Approach

Although our universe is enormous,it's constituted of tiny particles.Those particles are known as�fundamental particles�.Particle physics is the very subject which focuses on studying the components and properties of the fundamental particles and the interactions among them.In the 1970s,Unified Electro-Weak theory and Quantum Chromodynamics?QCD?were established and combined into the Standard Model?SM?.Over decades,the Standard Model has been developed rapidly and becomes rather mature now.At the4th of July in 2012,the European Organization for Nuclear Research?CERN?announced that they had found the Higgs boson.This discovery represents a milestone in the history for the Standard Model of particle physics.Among all the fields of particle physics,the B meson physics is significantly important.There are a lot of interesting decay channels of the B meson?B/B_s/B_c?due to its huge mass and they are often used to test the Standard Model precisely,to study the CP violation of hadron and to search for the new physics beyond SM.At the first decade of this century,the two B meson experiment factories in Japan and America collected more than 1.7 billion pair production and decay events of B meson.They measured hundreds of decay modes of B/B_smeson and discovered the existence of CP violations in B meson system.At the present stage,the LHCb experiment in Europe and the recent Belle-II experiment in Japan provide more and more accurate experimental data.These experiments will strongly promote the development of the study in B meson physics.The study of the semi-leptonic B meson decays is of great importance in B meson physics which could be used to extract the CKM matrix elements,to examine Lepton Flavor Universality?LFU?and to search for the signal of new physics.In the study of semi-leptonic B meson decays,the most important and difficult part is extracting the relevant form factors through the hadron matrix elements of b quark transition.On the theory side,there are several methods could be used to calculate these form factors and the most popular ones are:Heavy Quark Effective Theory?HQET?,Light Cone Sum Rules?LCSR?,the perturbative QCD factorization approach?PQCD?and the lattice QCD?LQCD?.They all have their own advantages and disadvantages.In this thesis,based on the perturbative QCD factorization approach,the author cal-culated and analyzed the semi-leptonic decay process?B/B_s/B_c??(D^?*?,D_s^?*?,?_c,J/?)l?systematically.In the part of overview,the author gave a short review of the basic the-oretical knowledge of the Standard Model and B meson physics.The basic framework of PQCD is introduced then.The author also discussed the research ideas and calculation methods for the B meson decays under this particular framework.The main part of this dissertation is from chapter No.3 to chapter No.5 which contains the author's own work.Using the perturbative QCD factorization approach under the framework of SM,the author calculated hadron matrix elements of the B meson transition and abstracted the relevant form factors at the momentum transfer q²???0 region.To ob-tain the q²dependence of the relevant form factors,the PQCD predictions were combined with the LQCD results at the q²_maxarea and extrapolated to the whole region through the Bourrely-Caprini-Lellouch?BCL?parameterized function.After studying and analyzing the physical observable quantities?branching ratios,ratios of Brs,the longitude polar-ization and forward-backward asymmetry of?,the fraction of D^*longitude polarization?for the?B/B_s/B_c??(D^?*?,D_s^?*?,?_c,J/?)l?decays,the author concluded the following points:?1?The form factors calculated using PQCD for semi-leptonic B/B_s/B_cdecays vary smooth and steady at q²???0 region.However,they increase significantly with q² at large q²area.The PQCD results become less reliable near q²_maxregion since the non-perturbative effect gets stronger at there.Therefore,the author used the BCL parameterized function to extrapolate the reliable PQCD predictions for form factors from low q²region to high q²region,while taking the LQCD results at endpoints into account as inputs to increase the accuracy of the extrapolation.In order to distinguish from the traditional perturbative QCD factorization approach,we call this new method the�PQCD+Lattice�approach.The form factors obtained by the�PQCD+Lattice�approach are slightly smaller than the ones obtained by traditional PQCD approach at large q²area because of the suppression effect of LQCD results at endpoints.The difference between them is not much obvious at the entire range and it is acceptable considering the errors.?2?The theoretical prediction for each branching ratio obtained by the�PQCD+Latice�approach is about 15%smaller than the one obtained by the traditional PQCD approach.This difference is acceptable considering that the uncertainty of each decay mode is about 30%.The difference between the ratios of Brs:R(D^?*?),R(D_s^?*?),R??_c?,R?J/??from two methods is around 5%each while the uncertainty of each ratio R?X?is about 5%as well.?3?The theoretical predictions for the branching ratios and the ratios of Brs obtained by both methods agree with the experimental data and other theoretical results under SM.Besides,the author also calculated the longitude polarization of tau lepton P_?,the forward-backward asymmetry of tau lepton A_{F B}???and the fraction of D^* longitude polarization F_L?D^*?for part of these decay modes.Our numerical results from both methods are consistent with other SM theoretical predictions and the experimental data within errors.In chapter No.6,the author summarized the entire dissertation and discussed about the outlook of the B meson physics.The study of semi-leptonic B meson decays will remain a hot point and forward position field in the next few years because of its key role in testing the lepton flavor universality.The present theoretical methods of abstracting form factors are not mature yet,and the combination of different theories could be a simple and efficient way to solve this problem.With the LHCb and Belle-II experiments keeping on,there will be more and more better experimental data,and the development of the phenomenological study of the B meson physics will be promoted.