非对称末态双标记分析方法研究及测量Ds+介子衰变常数

Ds+ meson is the lightest strange charm meson which consists of charm quark c and strange anti-quark s-bar. Study of Ds+ decay is a chance to get many important information about particle interaction. Cross section for Ds*+Ds- pair production is about 3 times than it for Ds+Ds- pair production near the peak respectively, so that the Ds+ sample has better been collected near the cross section peak of Ds*+D- pair production. Due to charm particles produced in pairs, double tag method is usually applied in the analysis for the charm particle pair sample. First we reconstruct one particle of the charm particle pair, than look for the signal charm particle decay event on its recoil-side. It is because that beam energy constrained condition is applied in the double tag analysis, so that it is usually used in the analysis of symmetrical final states pair event which with the same mass. Due to the Ds*+Ds- pair is an asymmetrical final states event and the Ds*+ mass is different than the Ds+ mass, so that beam energy constrained condition cannot be applied in the analysis.We improve the double tag analysis method by building a virtual particle on the recoil-side, and applying center-of-mass energy-momentum constrain condition in the analysis for the asymmetrical final states Ds*+Ds- events. By this way we can reduce the systematic error in the analysis obviously. Apply the improved double tag method we can measure Ds+ pure lepton decay constant by analysis of the Ds+ pure leptonic decay events, and measure pseudo-scalar mixing angle by analysis of Ds+ semileptonic decay events which included eta and eta’.

Ds+介子是粲夸克与奇异反夸克组成的最轻奇异粲介子，对它的衰变研究可获得关于粒子相互作用的许多重要信息。在各自产生截面峰附近Ds*+Ds-介子对截面是Ds+Ds-对的约三倍，在Ds*+Ds-截面峰附近采集Ds+数据更有利。在粲粒子对数据分析中常用双标记分析方法，即首先完整重建一个粲粒子，然后在其反冲侧重建信号粲粒子。在双标记分析方法中常用到束流能量约束条件，比较适用于质量相同的对称末态事例分析。而Ds*+Ds-介子对是非对称的，Ds*+与Ds+的质量不同，束流约束条件已不适用。本选题通过设置虚拟反冲粒子，利用事例质心系能动量约束条件改进双标记方法以用于非对称末态Ds*+Ds-事例分析。新的双标记分析方法能明显减小Ds*+Ds-数据分析中的系统误差。利用改进的双标记方法对Ds+纯轻子衰变测量可获得Ds+的纯轻衰变常数fDs，其次通过Ds+含eta和eta’半轻子衰变测量可以研究赝标混合参数。

利用改进了的双标记方法，我们研究了奇异粲介子Ds的纯轻子衰变和半轻子衰变，同时还研究了D+介子的半轻子衰变，测量了Ds介子的纯轻衰变常数和CKM矩阵元|Vcs|。.. 通过分析在正负电子对撞机上工作的北京谱仪BESIII在质心系能量点Ecm=4.18 GeV采集的的数据，我们精密测量了Ds+→μ+v纯轻子衰变过程分支比为(5.49±0.16±0.15)×10-3。在此基础上，结合Ds介子和μ轻子的质量以及Ds的寿命，我们测定了Ds+纯轻衰变常数和夸克混合矩阵元|Vcs|的乘积为fDs|Vcs|=246.2±3.6±3.5 MeV。输入基于标准模型的CKMfitter给出的|Vcs|，我们计算得到fDs=252.9±3.7±3.6 MeV。输入最新的基于格点量子色动力学理论计算得到的fDs，我们计算得到CKM矩阵元|Vcs|=0.985±0.014±0.014。这些结果都是目前世界上精度最高的实验测量值。.. 我们分析了BESIII在质心系能量点4.009GeV采集的积分亮度482 pb-1的数据，测定Ds+的半轻子(Фeν和Ф/η/η’ μ ν)衰变过程的绝对分支比分别为：B(Ds→Φeν)=(2.26±0.45±0.09)%，"B" ("Ds→Φμν" )"=（1.94±0.53±0.09)%" , B(Ds→ημν)=(2.42±±0.46±±0.11)%和B(Ds→η^' μν)=(1.06±0.54±0.07)%，这里第一个和第二个不确定值分别是统计误差和系统误差。在这些测量值中，Ds介子含μ的半轻子衰变过程是世界上首次测量，另外一个Ds含电子的半轻子衰变(Фeν)分支比测量值与其他实验测量结果的世界平均值在测量误差范围内一致。.. 我们通过分析BESIII在质心系能量点3.773 GeV采集的2.93 fb-1积分亮度的数据，在国际上首次观测到卡比玻压制衰变D+→ωμ+v，测定D+→ωμ+v绝对分支比为(0.1770.0180.011)%，我们还初步测定了分支比之比B[D+→ωμ+v]/B[D+→ωe+v]=1.05±0.14。在现有精度下，该比值在一个标准误差范围内排除了在D+→ωl+v衰变中轻子普适性破坏的可能性。