B→ρρ衰变过程次领头阶修正研究

【作者】 张晓君；

【导师】 肖振军；

【作者基本信息】 南京师范大学 ， 理论物理， 2015， 硕士

【摘要】 B物理一直是粒子物理的热门领域之一,为了配合高能物理实验的发展,要求理论工作者首先就应该对衰变道进行分析研究,得出理论预测,以期对实验有所帮助,此外,还应结合新的实验数据进行全面分析来检验标准模型。目前,如何处理衰变过程中的QCD效应是热门的话题,在很多方面也有许多成功的理论和模型,其中微扰QCD因子化方法是比较成功的方法之一,它预言了衰变过程的很多实验,该方案需要利用B介子电弱衰变的有效哈密顿量和横动量的因子化理论。B介子物理研究的主要困难在于强子矩阵元的计算,尽管有效哈密顿量方法已经成熟,但由于强子化过程的非微扰QCD性质,B介子跃迁形状因子计算和强子矩阵元计算-直存在较大的理论误差。而在过去的十年里,大量的关于B→M2M3两体衰变为人们所学习,但都只停留在加上领头阶和部分次领头阶贡献的水平。本文主要是利用pQCD因子化方法对B介子到两个矢量介子衰变过程B→ρρ的研究：首先,对B介子物理的基础理论知识(包含衰变分枝比和CP破坏)做了简单的综述,重点介绍了微扰QCD因子化方法；其次,我们给出了B→ρρ三个道分别在领头阶和次领头阶修正的解析计算,得到衰变分支比和CP破坏的理论值；然后进行数值分析。结果表明：①对于B0→ρ+ρ-衰变道,若只考虑领头阶贡献,它的pQCD理论预言值与Babar,Belle实验测量数据之间存在一定的偏差,但在考虑了次领头阶之后,理论值和实验值符合得很好。而其他两个衰变道在加了次领头阶修正后的数值还有待改善,即并不能和实验数据很好地符合。②对于B0→ρ+ρ-和B+→ρ+ρ0两个衰变道,它们的纵向极化分数大约在95～98%,和最新实验值符合得很好。这两个衰变道的过程并不存在极化反常的现象。③对于B0→ρ0ρ0衰变道,它的纵向极化分数大约为50%,而两个横向分量“N”和“T”占到了一定的比例。④关于B→ρρ三个衰变道的CP破坏分析,目前实验现有的数据误差太大,不能严谨地进行比对。最后,我们对全文做了总结,并对未来B介子物理的发展做了展望。B介子的非轻衰变是粒子物理中很重要的研究课题,在过去的几年里,许多研究B介子衰变的方法已经带来了很多的突破。微扰QCD因子化方案本身也在不断地改善之中,我们期待着未来实验可以给出重要的结果。更多还原

【Abstract】 B meson physics has been one of the hot topics of the high energy particle physics. It is necessary for us to provide the theoretical predictions for the physical observables about the B meson decays and compare these theoretical results with those measured values, in order to test the standard model (SM) and to find the signal of new physics beyond the SM. The QCD dynamics involved in the B meson decays is complex in nature. In order to calculate the hadronic matrix elements of B meson decays reliably, several factorization approaches have been constructed in recent years:the perturbative QCD (pQCD) factorization approach is one of such approaches. By employing the effective Hamiltonian method and the pQCD factorization approach, one can deal with the hadronic two-body B meson decays B→M2M3 reliably.As is well-know, the main difficulty lies in B meson physics is the calculation of hadron matrix elements. Although the effective Hamiltonian has been improved greatly, the evalua-tions for the B meson transition form factors do have large theoretical uncertainties, due to the existence of the non-perturbative QCD contributions in such processes. During the past decades, many two-body hadronic B →M2M3 decays have been studied by employing the pQCD factorization approach at the leading order (LO) level or the partial next-to-leading (NLO) order.In this paper, we present our studies on the hadronic B→ρ+ρ-,ρ+ρ0 and ρ0ρ0 decays by using the pQCD approach. We firstly give a brief review about the decays and the CP vio-lation of the B meson system, and present a short introduction about the basics of the pQCD factorization approach; We then calculate the polarization fractions, the branching ratios and the CP-violating asymmetries of the three considered decay modes:B→ (ρ+ρ-.ρ+ρ0,ρ0ρ0) in the pQCD approach at the full leading order and with the inclusion of currently known NLO contributions; Based on the numerical results and the phenomenological analysis, we found that ① For branching ratio B0→ρ+ρ-, there exist a clear difference between the LO pQCD prediction and the measured value from the BaBar and Belle experiments. But the pQCD results become consistent with the data after the inclusion of the NLO contri-butions. For other two decays, the agreement between the theory and the data also improved to some extent by the inclusion of the NLO pieces.② For the longitudinal polarization fractions of B0→ρ+ρ- and B+ →ρ+ρ0 decays, the pQCD predictions are 95～98%, agree well with those measured values. That means that there is no polarization anomaly for these two decays.③ For B0→ρ0ρ0 decay, the longitudinal polarization fraction is around 50%, while the "normal" and "transverse" fraction share the remaining part.④ We show the pQCD predictions for the CP vilating asymmetries of the considered three decay modes, but can not compare them with the data because the relevant measurements still have large uncertainties.A short summary of this thesis and some remarks on future development of B physics are given in the last chapter.Studying the non-leptonic decays of B meson is an important work in the particle physics. During the past years, many good researches have been done in this area. The pQCD fac-torization approach itself are also improved rapidly. We expect that the future experiments can provide a great help to test the pQCD approach.更多还原