BESIII上粲偶素到D(s)介子半轻衰变的寻找及缪子鉴别的改进

The Standard Model (SM) is the most successful theoretical model on particle physics and has been widely verified by experimental data. The semi-leptonic decay from Charmonium to D(s) meson and lepton is one kind of rare decays that is predicted by the SM, but has never been found experimentally. The measurement of these processes is not only a method of precision test of the SM, but also provides the possibility to search for the new physics beyond the SM. The BESIII experiment is a large scientific apparatus on BEPC and has been running at the tau-charm energy region for 10 years. In Feb 2019, BESIII has accumulated 10 Billion J/Psi events, which is the highest statistics J/Psi data sample produced near J/Psi threshold. With the large statistics and high quality J/Psi data, we propose to analyze the branching factor of the semi-leptonic decay from J/Psi to a D(s) meson and leptons, such as J/Psi->D(s)(*)-e+υ_e. Based on the statistics of the new J/Psi data sample, we expect to improve the current upper-limit measurement of these processes by about two orders of magnitude, to a precision close to the branching factor predicted by the SM. The applicants have developed the Muon-Counter track reconstruction and particle discrimination method during the construction stage of BESIII. With the experience, we expect to improve the efficiency of muon tracking and identification on BESIII, to measure the decay from Charmonium to D(s) meson and muon for the first time, and benefit other muon related analyses on BESIII.

粲偶素到D(s)介子和轻子末态的半轻衰变是标准模型预言可能存在的一类稀有衰变，但目前在实验上尚未观测到。通过对该类反应的测量可以精确检验标准模型，同时也提供了寻找超出标准模型新物理的机会。北京正负电子对撞机上的北京谱仪BESIII实验已在τ-粲能区稳定运行10年，并于2019年2月完成了100亿J/Psi事例的获取。这是世界上在阈值处获取的最高统计量J/Psi事例。申请人计划通过分析这批J/Psi样本，对J/Psi->D(s)(*)-e+υ_e等反应道的衰变分支比进行测量，预计能将现有上限测量的精度提高1~2个数量级，接近标准模型预言的上限区间。同时结合申请人在BESIII缪子径迹重建软件开发和粒子鉴别中的经验，采用多变量分析方法提高缪子的重建和鉴别效率，预计对粲偶素到D(s)介子和缪子半轻衰变的分支比实现首次测量，同时也对BESIII上其他缪子相关的测量提供帮助。

本项目基于北京谱仪BESIII实验采集的阈值处大统计量粲偶素样本，寻找粲偶素到D(s)介子和电子/缪子等轻子末态的半轻衰变。粲偶素半轻衰变是标准模型预言可能存在的一类稀有衰变，分支比小于10^-8量级，目前在实验上仍未观测到。通过对该类反应的测量既能够对标准模型进行精确检验，也可以寻找超出标准模型的新物理。.基于BESIII实验采集的100亿J/psi事例和27亿psi(3686)事例，我们开展和完成了一系列粲偶素半轻衰变的测量。其中D介子电子道末态J/psi->D-e+nv+c.c.的半轻衰变测量在90%的置信度下分支比上限达到7.1*10^-8，与之前世界上最精确的结果相比，精度提升了超过2个量级，进入了标准模型预言的弱衰变区域。同时，寻找Ds介子电子道末态和寻找D介子缪子道末态的两个分析工作也已进入合作组审核阶段。另外，基于本项目对粲偶素和缪子的研究，我们还开展了粲偶素带电轻子味破缺衰变J/psi->emu的寻找，90%置信度下的精度达到了4.5*10^-9，是目前重味夸克系统中带电轻子味破缺寻找世界上最精确的结果。.在本项目的支持下，我们共发表期刊论文3篇，国际会议报告4次，国内会议报告13次，已培养毕业博士2人，硕士1人、博士后出站2人，组织学术会议1次。

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