Phenomenological Studies Of The Semileptonic Decays Of B_s Meson In PQCD Factorization Approach

The Standard Model（SM）of particle physics is one of the most important achieve-ment of high energy physics to date.This highly elegant theory sorts elementary particles according to their respective property and describes how they interact through the funda-mental interactions.The development of SM started in the 1960s and has so far withstood every experimental test.The latest success was the verification of the Brout-Englert-Higgs field by ATLAS and CMS at CERN’s Large Hadron Collider in 2012.Both experiments successfully detected the quantised excitation of the BEH filed-the so-called Higgs boson.Although SM provides the most accurate description of nature at the subatomic level,it is widely believed that this quantum field theory is just the low energy limit of a more fundamental theory.After all,SM describes only about 5%of the universe.Hence,any signs of irregularities between the predictions of SM and experimental results would spark tremendous excitement.An accurate phenomenological analysis of rare B meson decays is significant both for precise tests of the Standard Model and in the search for possible New physics in the heavy flavour sector.The primary subject of this thesis are exclusive semi-leptonic B decays of the flavour-changing neutral type process,mainly motivated by recent LHCb and Belle measurements that show tensions with respect to SM,in the flavour changing neutral currents（FCNCs）B→K*⁺-decay.Since such decays induced by FCNCs tran-sitions are heavily suppressed in SM,we attempt to verify whether the above mentioned sizeable deviation named anomaly is indeed the signal of Lepton Flavor Universality（L-FU）violation or possible New physics beyond SM.Therefore,the main focus is put on phenomenological analyses in the similar processes like B_s→K^*+-with b→d⁺-transitions and B_s→φ⁺-with the same b→s⁺-transitions at the quark level.With faithful determination of the form factors from the perturbative QCD（PQCD）approach,various observables in exclusive semileptonic decays of B meson are predicted.In this thesis,we study systematically the semileptonic decays B_s→K^*+-and B_s→φ⁺-with-=（e-,μ-,τ-）using PQCD approach based on the k_Tfactorization theorem.The first chapter contains a short introduction to the Standard Model of particle physics and flavor physics,especially its heavy flavour sector.The chapter 2 of the thesis extends to the overview for the SM,where the basic theoretical framework,the elementary particle content,CKM quark-mixing matrix,electroweak symmetry breaking mechanism and asymptotic freedom and quark confinement in QCD are introduced.After that,B meson physics are highlighted,including B meson creating and mixing,the classification of B meson decays,CP violation in B sector,effective weak Hamiltonians.In addition,several essential factorization methods to obtain the hadronic matrix element are also emphatically discussed.In section 2.5,we give an account of the detailed introduction to PQCD approach,involving the ways to eliminate the endpoint singularity and calculate the B→πtransition form factors.The main part of this dissertation is chapter 3-4,containing the primary results obtained by the author.The third chapter of the thesis is devoted to the phenomeno-logical analysis of the FCNC b→d transition observed in the form of the semileptonic B_s→K^*+-decays.In the latter decays,the contributions of the virtual resonances are systematically taken into account using the Breit-Wigner form.Together with the B_s→K^*form factors derived from PQCD approach and extrapolated to by Bourrely-Caprini-Lellouch（BCL）parametrization,the decay amplitude are used to predict various observables such as the differential decay rate（branching ratio）,direct CP-asymmetry,forward-backward asymmetry and so on.In addition,we develop a new method called“PQCD+Lattice”which combined the Lattice QCD results at the high-q2regions as theoretical and phenomenological constraints to enhance the reliability of the extrapola-tion.For the case of vector meson being the final state,the angular distribution of the4-body decay B_s→K*（→Kπ）+-are discussed and Form Factor Independent（FFI）observables which are free of form factor uncertainties are constructed.The research indi-cates that our theoretical predictions based on PQCD and“PQCD+Lattice”approaches accord well with each other.Furthermore,we compare our results with those using other theoretical models and make some discussions.The chapter 4 of the thesis is dedicated to the phenomenological analysis of the FCNC b→s observed in the form of the semileptonic B_s→φ+-decays.Among that,we extend our previous study to other semileptonic decays including B_s→φ+-with-=（e-,μ-,τ-）by using the PQCD and the“PQCD+Lattice”approach,respectively.Moreover,a new operators basis calledγ₅-free basis defined by Chetyrkin,Misiak and Munz（CMM）is adopted to avoid the technical difficulties that otherwise arise at higher orders when using dimensional regularization in conjunction with a fully anticommutingγ₅.We evaluate and predict the branching ratios,CP violation,the forward-backward asymmetry and some other physical observables related to the angular distribution of the semileptonic B_s→φ+-decays.For the channel B_s→φμ+μ-with some experimental results,we also calculate the q2-binned values for many physical observables including the branching ratios.Adopting the same choices of the q2bins as LHCb did,we find our theoretical predictions are consistent with the experimental results from LHCb considering the uncertainties.More theoretical results will be tested in the near future LHCb and Belle-II experiments.In the last chapter 5,we give a short summary for this thesis and make some prospects for B meson physics.In the appendix,we present the input parameters in the numerical calculations,besides,we provide the RG equation and explicit expressions of the Wilson Coefficients as well.</sup>