A Study Of B Meson Semileptonic And Annihilation Decays In The PQCD Approach

Since the b quark was found at Fermilab in the year of1977, B physics have always been an ideal place for precise measurement of the standard model parameters and for the searching for new physics signals. During the past two decade, because of the very hard works of all theoretical and experimental physicists around the world, B physics have archived a great success in both theoretical studies and experimental measurements. In the LHC era, furthermore, a lot of vary rare B decay modes will be measured soon, and we do believe that the B physics will enjoy a new " Golden Period" of rapid development.In this thesis. we focus on the studies on the semileptonic decays of the B, Bs and Bc meson, as well as some pure annihilation decay processes of B and Bs meson by employing the perturbative QCD (pQCD) factorization approach which is based on the kT factorization theorem.In Chapter1and2, we firstly made an introduction for the basics of the standard model and the theoretical frameworks of the B physics studies. We then provide a brief review about the theoretical framework of the pQCD factorization approach.In Chapter3and4, by employing the pQCD factorization approach, we calculated the branching ratios of many semileptonic decays of B/Bs/Bc mesons, such as B/Bsâ†'(P,G)(ll，lv,uv) decays with P=(Ï€,K,η,η’) and l=(e,μ,Ï„), Bcâ†'(ηc,J/Ψ, D0)lv and Bcâ†'D(s)-(ll,vv) decays. Based on the assumption of the SU(3) flavor symmetry, the next-to-leading order (NLO) contributions to the form factors of B/Bsâ†'(Ï€,K,ηq,s) transitions are also taken into account. From the numerical calculations and phenomenological analysis we found that(1) The NLO pQCD predictions for the relevant form factors of B/Bsâ†'(Ï€, K,ηq,ηs) and the leading order (LO) pQCD predictions for the values of the Bcâ†'(η,J/Ψ,D(s)) transition form factors agree well with those obtained from the LCSR or other methods; The NLO part of the form factors F0,+,T Bâ†'Ï€,K(0),F0,+,T Bsâ†'K(0) and F0,+,T B(s)â†'ηq(s)(0) is about15%～20%in magnitude. (2)For the charged current B0â†'Ï€+l-vl and B-â†'(Ï€0,η,η’)l-vl decays and the neutral current B0â†'K0l+l-and B-â†'K-l+l-decays,the NLO pQCD predictions for their branching ratios agree very well with the measured values.(3)By comparing the pQCD predictions for Br(B0â†'Ï€+l-vl)with the measured decay rate we extract out the value of the CKM element Vub:|Vub|=(3.80-0.60+0.66(theor.)±0.13)×10-3.(4)The pQCD predictions for the branching ratios(in units of10-3)of the considered semileptonic Bc decays are: Br(Bc-â†'ηcl-vl)=4.41-1.09+1.22, Br(Bc-â†'ηcÏ„-bÏ„)=1.37-0.34+0.37, Br(Bc-â†'J/Ψl-vl)=10.03-1.18+1.33,Br(Bc-â†'J/Ψτ-vÏ„)=2.92-0.34+0.40, where l-=(e-,μ-).Other eight Bcâ†'(D,Ds)(ll,lv,vv)decay modes are also calcu-lated.These pQCD predictions will be tested at the LHCb and super-B experiments;(5)We also defined some ratios of the branching ratios,such as Rv,RC,RNl,N2,N3,Rηc and RJ/Ψ etc.The pQCD predictions for these ratios will be tested by LHCb and the forthcoming Super-B experiments.For many years,due to the end-point singularity,it is a very hard job to make reliable theoretical calculations for the decay rates and CP violating asymmetries for those pure annihilation decays of B,Bs and Bc mesons:such as Bs0â†'Ï€+-or B0â†'K+K-decay modes.In the early2011,CDF and LHCb reported their first measuremen for the decay rates of both Bs0â†'Ï€+Ï€-and B0â†'K+K-and found that these pure annihilation decays have rather large branching ratios.In Chapter5,motivated by the CDF and LHCb results, we recalculated the branching ratios of B0â†'K+K-and Bs0â†'Ï€+Ï€-in the pQCD approach by using the wave functions from the QCD sum rule and the improved Gegenbauer moments and found the following results:(1)One can provide a consistent pQCD interpretation for both the measured Br(Bs0â†'Ï€+Ï€-)and Br(Bd0â†'K+K-)simultaneously. The pQCD predictions for Br(Bs0â†' Ï€+Ï€-) obtained by different authors are well consistent with each other within one standard deviation.(2) For B0â†'K+K-decay, we examined the previous studies and found the reason why the previous pQCD prediction for its branching ratio was only about30%of the measured value. Our new pQCD prediction for Br(B%â†'K+K-) agrees very well with the measured values as reported by CDF and LHCb Collaboration.(3) The CP-violating asymmetry Acp(Bs0â†'Ï€+Ï€-)≈-2.3%, may be too small to be detected even at LHCb experiment; But Acp(Bd0â†'K+K-)≈19%is large and may be detected at the LHCb and future super-B factory experiments.In Chapter6. we presented a short summary for the whole thesis and give some remarks and expectations for the future developments of the B physics. A lot of pQCD predictions for the considered B meson semileptonic decays presented in this thesis have not been observed yet, we hope these theoretical predictions could be tested soon by the running LHCb or the forthcoming Super-B experiments.