北京谱仪实验上寻找味道改变中性流D介子衰变研究

The standard model of elementary particle has achieved remarkably success in describing matter's most basic elements and the forces through which they interact. However, the questions of so-called fine-tunning, mass hierarchy, et. al., remain unexplained in standard model, until the theory beyond standard model is invented. In experiment, the sensitivities are still above the upper limit of standard model, and no evidence of the new physics has been established. The latest results from the LHCb collaboration indicate a hit of the new physics in D meson decays. The falvor-changing neutral current (FCNC) process in D meson is regarded as an unique tool to probe new physics. The Beijing Electron-Positron Collider (BEPCII) and the Beijng Spectrometer (BESIII) is the solar high-luminosity large-scale scientific facility running at the energy of charm region. The experiment BESIII has accumulated about 16 millions D meson pairs at the energy of psi(3770) resonance, which is the largest data sample of D meson pairs in the world. The threshold generated D meson pairs have the advantage of low background contamination, which make the sample capable of all the study on the D meson FCNC decays. In the future, BESIII will continue taking data at the psi(3770) resonance, and the promising amount of D meson pairs will reach to at least 65 millions. Taking advantage of BESIII data, we are going to focus our study on the D meson FCNC decays, mainly including the channels of hadrons plus two neutrinos or two leptons final states, to test the standard model and search for new physics.

超标准模型能够解决标准模型中的一些困难，如精细调节和质量等级等问题，但目前实验上并没有证据表明新物理的存在。LHCb合作组最新的实验结果表明，在D介子衰变中可能存在新物理的迹象。D介子的味道改变中性流衰变(FCNC)被公认为是实验上寻找新物理的一种重要手段。北京正负电子对撞机BEPCII和北京谱仪BESIII实验是目前国际上运行在粲介子能区唯一的高亮度大型高能实验装置，运行至今已经在psi(3770)共振峰上采集了约1600万D介子对，为世界上的最大阈值产生D介子对样本。阈值产生的D介子具有本底低的优点，可以测量所有的D介子FCNC过程。今后BESIII将继续采集更多D介子对样本，预期总数至少可达6500万。本项目利用BESIII实验数据，将集中研究强子加两个中微子或两个轻子等的D介子FCNC衰变模式，检验标准模型和寻找新物理。

本项目利用BESIII实验的高统计量阈值产生的介子对数据，研究了D介子的味道改变中性流衰变，顺利执行了项目的研究计划，完成了相应的科学目标，同时在粲重子衰变和奇异粲强子领域得到了新的扩展，获得了“出人意料”的成果。项目研究内容主要包括如下几个方面：BESIII探测器的模拟优化、数据重建性能改善、以及数据信号提取方法等相关方面开展大量研究工作，发表多篇相关论文成果，有效保障项目内容的执行；分析了D0π0νν味道改变中性流的衰变过程，利用盲分析和双标记技术，得到了分支比上限为10-5量级的结果，这是世界上相关过程的首次测量；基于盲分析技术完成了D0π0e+e-、ηe+e-、ωe+e-、Kse+e-、π+π-e+e-、K+π-e+e-、K+K-e+e-等味道改变中性流过程的分支比测量，给出了世界上最精确的测量；基于盲分析技术完成了D+π+π0e+e-, K+π0e+e-, Ksπ+e+e-, K+Kse+e-等过程的研究，给出了世界上最精确的测量；推动粲重子阈值取数，国际上首次绝对测量Λc的强子衰变和半轻衰变的分支比；利用D介子衰变末态，在实验室发现三个四夸克候选粒子。