BESⅢ上Jψ总数的确定和Jψ→μltgtμltgt分支比的测量

1、广西师范大学硕士学位论文BES上J/总数的确定和J/分支比的测量姓名：廖小涛申请学位级别：硕士专业：理论物理指导教师：杨永栩;杨洪勋20090401iBESIII 上上 J/总数的确定和总数的确定和 J/+ - 分支比的测量分支比的测量 研究生姓名：廖小涛 导师姓名：杨永栩教授、杨洪勋副研究员、沈肖雁研究员 学科专业：理论物理 研究方向：原子核物理 年级：2006 级 摘摘 要要 BESIII 探测器是工作在北京正负电子对撞机 BEPCII 上的大型、通用、高精度谱仪，主要开展-粲物理能区的正负电子对撞物理实验、进行弱电相互作用和强相互作用研究以及新物理的寻找。BESIII 自 2008 年夏天开始运行至今，各项性能不断提高，运行状态良好，并且先后从 2008 年 5 月到 11 月获取超过 1000 万的(2S)数据；2009 年 3 月获取了超过 6000 万的(2S)数据。 J/粒子是粲夸克偶素家族中极其重要的一员，对 J/粒子的各项基本参数的测定是进行其它各项研究的基础。由于在测量 J/衰变的各种反应道的绝对分支比中，都需要用到J/总数这一基本的参量，所以对 J/事例的总数测量

2、工作显得尤为重要，其测量精度直接影响到 J/衰变物理的各种物理分析的系统误差大小。在 BESII 上，J/总数的测量精度为4.7%，而 BESIII 各项性能指标优于 BESII，因此人们期望在 BESIII 上对 J/总数的确定有更小的误差。考虑到对 J/+-的测量精度已经达到了 1%以内，同时经过改造后的BESIII，子探测器有较大改进，对子的探测、分辨能力大幅提高，因而本文通过 J/衰变到+-来确定 BESIII 上 J/事例的总数。基于 BESIII 的离线软件系统 BOSS634 版本，详细地研究了事例的选择条件，并进行了最优化选取。蒙特卡罗分析表明，我们的做法能有效地排除本底事例，精确地确定 J/事例总数。本方法将会用来确定 BESIII 上 J/总数。 由于在实验上，J/介子通常通过它的两个最大最干净的遍举衰变道 J/e+e-和J/+-来作标记，二者的分支比也被用来确定强相互作用的耦合常数 s；并且它们的比例 Be/B也用来检验轻子的普适性；另外，J/衰变到轻子对的分支比也是 J/粒子的基本参数之一。本文利用 BESIII 于 2008 年 9 月至 10 月试运行阶段获

3、取的部分(2S)数据，通过(2S)+-J/ (J/anything 和 J/+-) 间接地测量 J/衰变到+-的分支比。先通过对事例末态特征的分析和计算，对所需要的挑选条件进行了仔细研究；然后基于 BESIII的离线软件系统 BOSS641 版本，确定事例的挑选条件并进行事例挑选工作；对挑选出来ii的事例用 Roofit 作拟合，得到真正的好事例。利用 Monte Carlo 模拟，得到单举道和遍举道的效率，于是可以计算出 J/+-的分支比。在拟合的过程中，我们采用双高斯拟合和用遍举道的+-反冲质量谱形状去拟合单举道的+-反冲质量谱形状的方法， 并将二者得到的结果取平均，得到 J/+-的分支比为 5.70%，与 PDG 给出的值 5.93%基本一致。 关键词：关键词：BESIII， J/总数，总数， 子，子， 分支比分支比 iiiJ/ Total Number Determination and J/+ - Branching Ratio Measurement at BESIII Graduate Student: Liao Xiao-tao Supervisor: Prof. Ya

4、ng Yong-yu, Prof. Yang Hong-xun, Prof. Shen Xiao-yan Major: Theoretical Physics Field: Nuclear Physics Grade: 2006 Abstract The BEijing Spectrometer III(BESIII) operating at Beijing Electron Positron Collider II (BEPCII) is a high quality magnetic detector built with modern techniques. It is used to do research on electron positron colliding physics at the energy range of tau-charm. Its main physics goal is to study the electro-weak interaction, strong interaction and search for new physics. BES

5、III began operating in last summer. It operates well, and gained more than 10 million (2S) data from May to November last year. And it even acquired 60 million (2S) data during March 2009. J/ particle is an important member in charm quark family. It is a primary work to measure the parameters of J/ particle. J/ total number is one of the most important parameters, for it is the inevitable parameter to measure the branching ratio of any J/ decay. So precise J/ total number measurement is necessar

6、y to get a smaller systematic uncertainty. The uncertainty of J/ total number measurement at BESII is 4.7%. Because the BESIII detector is better than BESII in many aspects, a precise J/ total number is expected at BESIII. By 2008, the branching ratio precise of J/ decaying to +- is less than 1%, and BESIII has much better muon detector. In this article, we use J/ decaying to +- to measure the total number of J/. The BESIII offline software system(BOSS) version used is BOSS634. The events select

7、ion conditions are studied and optimized in the article. Monte Carlo study shows that our method can exclude background events and can gain precise J/ total number. And it will be used to determine the total number of J/ for the coming J/ data taking. J/ particle is usually tagged by its two largest and cleanest decaying channels: J/e+e-, J/+-. The branching ratios of the two decaying channels can be used to determine the coupling constant of strong interaction and to check the universal of lept

8、ons by Be/B. The branching ratios of leptons are basic parameters. The (2S) data, which acquired by BESIII from September to October, is used in our analysis. The method we used to measure the branching ratio of J/+- is to select (2S)+-J/ (J/anything and J/+-) events. This ivmethod is an indirect way to measure the branching ratio. First, after analysis and calculation of the character of the final particles, proper conditions can be determined to select the events we want. Then, we use our prog

9、ram to select good events and record them based on BOSS641. Finally , we fit the recorded good events with Roofit tool, to exclude background events and get the real events we want, and the selection efficiencies. Double guassian fitting method and fitting the +- recoiling mass spectrum of inclusive process with exclusive process, and each fitting method gives a branching ratio value. We get the final result by averaging the two values. The result is Br(J/+-)=5.70%, which agrees the PDG value 5.93% on the whole. Key words: BESIII, J/ total number, particle, branching ratio 论文独创性声明 论文使用授权声明 论文独创性声明论文独创性声明 本人郑重声明： 所提交的学位论文是本人在导师的指导下进行的研究工作及取得的成果。除文中已经注明引用的内容外，本论文不含其他个人或其他机构已经发表或撰写过的研究成果。对本文的研究作出重要贡献的个人和集体，均已在文中以明确方式标明。本人承担本声明的法律责任。 研究生签名： 日期： 论文使用授权声明论文使用授权声明 本人完

《BESⅢ上Jψ总数的确定和Jψ→μltgtμltgt分支比的测量》由会员lizhe****0920分享，可在线阅读，更多相关《BESⅢ上Jψ总数的确定和Jψ→μltgtμltgt分支比的测量》请在金锄头文库上搜索。