【作者】 王文飞；

【导师】 肖振军；

【作者基本信息】 南京师范大学 ， 理论物理， 2013， 博士

【摘要】 自1977年费米实验室发现b夸克以来,B物理一直都被作为检验标准模型理论、精确测量标准模型参数以及寻找新物理信号的重要场所。得益于理论与实验两方面的辛勤努力,在过去的二十多年时间里,B物理在理论及实验方面都获得了长足的发展。尤其在LHC时代,许多稀有过程也必将很快被测量到。这将使得B物理再次迎来其快速发展的黄金时代。本论文在基于KT因子化的微扰QCD因子化方法下对B,Bs,Bc介子的半轻衰变过程以及B,Bs介子的一些纯湮灭衰变过程进行了研究。在本论文的综述部分,我们首先对标准模型基础理论以及用于B物理研究的理论方法进行介绍,并简要讨论本论文所采用的pQCD因子化方法的理论框架。在本论文的第三、四章中,作者采用pQCD因子化方法计算了B/Bs/Bc介子的一些半轻衰变过程的衰变分支比,如B/Bs→(P, G)(ιι,ιν,νν)(此处P=(π,K,η(1)),ι则为轻子e,μ,г),Bc→(ηc,J/Ψ,Do)ιν以及Bc→D(s)(ιι,νν)半轻衰变过程。对于B/Bs→(π、K,η9,8)跃迁过程,我们基于SU(3)味道对称性,给出了包含次领头阶修正的形状因子。通过数据计算以及唯象分析我们发现：(1)基于pQCD因子化方法我们得到了B/Bs→(π,K,ηq,ηs)跃迁过程的包含了次领头阶修正的形状因子和Bc→(ηc,J/Ψ,D,Ds)领头阶的形状因子。这些形状因子与由LCSR或其他方法所得到的结果一致。对Fo,+TB→π,K(0),Fo,+TB→π,K(0)和Fo,+TB(s)→ηq(s)(0)的次领头阶修正约为15%～20%.(2)对于通过带电流过程实现的半轻衰变过程Bo→π+ι-νι与B-→(π0,η,η1)ι-νι以及通过味道改变中性流实现的半轻衰变过程百BO→KOι+ι-与B-→K-ι+ι-,由pQCD因子化方法所预言的包含次领头阶修正的衰变分支比与现有实验结果符合的很好。(3)对于Br(BO→π+ι-νι)半轻衰变过程,通过对pQCD因子化方法所预言的衰变分支比与实验结果的比较抽出的CKM矩阵元Vub的大小为|Vub|=(3.801-0.60+0.66(theor.)±0.13)×10-3.这一结果与由其他方法抽取的|Vub|符合的很好。(4)pQCD因子化方法预言的Bc介子的部分半轻衰变分支比为(单位为10-3,ι为轻子e,μ)Br(Bc-→ηcl-vl)=1.41-1.09+1.22, Br(Bc-→ηυτ-vτ)=1.37-0.34+0.37, Br(Bc-→J/Ψl-vl)=10.03-1.18+1.33,Br(Bc-→J/Ψτ-vτ)=2.92-0.34+0.40,我们也给出了Bc→(D,Ds)(ll,lv,vv)半轻衰变过程的衰变分支比。这些分支比将在LHCb和未来的Super-B实验中得到验证。(5)在本论文中我们还定义了一些分支比的比值,如Rv,RC,RN1,N2,N3,Rηc以及RJ/Ψ等等。作者期待着这些比值能在LHCb和未来的Super-B实验中得以验证。由于端点奇异性的存在,对B/Bs/Bc介子的纯湮灭衰变过程,如Bo→K+K-和Bs|o→π+π-的衰变分支比以及CP破坏的计算,一直都难以给出可靠的理论计算结果。在2011年,CDF与LHCb实验组分别发布了他们首次测量到的纯湮灭衰变过程Bs|o→π+π-与BO→K+K-的衰变分支比,并且给出的结果都相当大。在本论文第五章中,受CDF与LHCb实验结果的启发,我们利用pQCD因子化方法,并使用由QCD求和规则所得到的介子波函数以及改进的Gegenbauer矩,重新计算了这两个衰变道的衰变分支比,我们发现：(1)对于Bs|o→π+π-与Bo→K+K-衰变分支比的实验结果可以由pQCD给出合理的解释。而对于Bs|o→π+π-衰变道,在pQCD方法下不同文献中的结果在一个标准偏差下是相一致的。(2)对于BO→K+Ⅳ-衰变道,我们仔细分析了以前的工作,并找出了早期文献中由pQCD所预言的结果仅为实验结果的30%的具体原因。我们给出的新结果与CDF以及LHCb实验组公布的实验测量结果符合的很好。(3)纯湮灭衰变过程Bs|o→π+π-的CP破坏仅约为-2.3%,这样一个较小的结果在LHCb上也未必能测量的到。但对于纯湮灭衰变过程BO→K+K-,pQCD预言的CP破坏约为19%,我们相信这一结果很可能在LHCb和将来super-B实验中得到验证。在第六章中,我们对全文进行了简短的总结,并对B物理未来的发展做出了的讨论。本论文中自pQCD因子化方法所预言的B介子的许多半轻衰变过程还都没有相关实验数据,我们希望这些理论预言能在LHCb或将来的Super-B实验中得到验证更多还原

【Abstract】 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/Bs→(P,G)(ll，lv,uv) decays with P=(π,K,η,η’) and l=(e,μ,τ), Bc→(ηc,J/Ψ, D0)lv and Bc→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/Bs→(π,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/Bs→(π, K,ηq,ηs) and the leading order (LO) pQCD predictions for the values of the Bc→(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 B→π,K(0),F0,+,T Bs→K(0) and F0,+,T B(s)→ηq(s)(0) is about15%～20%in magnitude. (2)For the charged current B0→π+l-vl and B-→(π0,η,η’)l-vl decays and the neutral current B0→K0l+l-and B-→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(B0→π+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-→ηcl-vl)=4.41-1.09+1.22, Br(Bc-→ηcτ-bτ)=1.37-0.34+0.37, Br(Bc-→J/Ψl-vl)=10.03-1.18+1.33,Br(Bc-→J/Ψτ-vτ)=2.92-0.34+0.40, where l-=(e-,μ-).Other eight Bc→(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 Bs0→π+-or B0→K+K-decay modes.In the early2011,CDF and LHCb reported their first measuremen for the decay rates of both Bs0→π+π-and B0→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 B0→K+K-and Bs0→π+π-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(Bs0→π+π-)and Br(Bd0→K+K-)simultaneously. The pQCD predictions for Br(Bs0→ π+π-) obtained by different authors are well consistent with each other within one standard deviation.(2) For B0→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%→K+K-) agrees very well with the measured values as reported by CDF and LHCb Collaboration.(3) The CP-violating asymmetry Acp(Bs0→π+π-)≈-2.3%, may be too small to be detected even at LHCb experiment; But Acp(Bd0→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.更多还原