北京谱仪III上含轻子对末态的赝标量及矢量无味轻介子衰变研究

Hadron spectroscopy is a powerful tool to study the microscopic structure of matter. It has been a hot topic to study the decays with lepton-pair in the final states from the light unflavored mesons. Because of the clean background and suitable energy range, it is convenient to use the low-energy high-luminosity electron-positron collisions to precisely and systematically study the exclusive processes and provide the bases for background analysis for the heavy-ion collisions. The branching ratios are only known in a few cases of the decay processes with lepton-pair in the final states from the light unflavored mesons, especially the direct dilepton decay of pseudoscalar mesons, the three-body and four-body rare decays. The 1.3 billion J/ψ events collected by BESIII has been the largest and best J/ψ sample in the world which could be used to study the rare decay processes of the light unflavored mesons with lepton-pair in the final states. In this project, we would study the decays with lepton-pair in the final states from the light unflavored pseudoscalar and vector mesons, calculate the decay amplitude, develope BESIII dynamic generators. Then we would use the secondary particles of the J/ψ sample η，η'，ρ，ω，phi, (especially η and η') to precisely measure the branching ratios or the up limits, research the physics in and the new physics beyond the standard model, such as electromagnetic form factors, dark photons, C and CP violations.

强子谱是探寻物质微观结构的有效工具，含轻子对末态的无味轻介子衰变的研究是前沿热点课题。低能高亮度正负电子对撞实验由于本底干净、能区合适，有利于精确和系统的研究相关遍举过程，同时也能为高能重离子碰撞实验提供本底分析的依据。大多数含轻子对末态的无味轻介子衰变过程，尤其是赝标量介子的直接双轻子衰变以及三体和四体稀有衰变过程只有少量实验结果。BESIII已采集到十三亿J/ψ事例，是目前世界上最大最好的J/ψ样本，使得研究含轻子对末态的无味轻介子稀有衰变过程成为可能。本项目将研究含轻子对末态的赝标量及矢量无味轻介子衰变，计算衰变振幅，发展BESIII动力学产生子，然后利用BESIII的J/ψ样本衰变产生的次级粒子η，η'，ρ，ω，phi等（尤其是η，η'）精确测量其分支比或上限，探寻其中所包含的电磁形状因子、暗光子、C及CP破坏等标准模型物理及超出标准模型的新物理。

强子谱是探寻物质微观结构的有效工具，含轻子对末态的无味轻介子衰变的研究是前沿热点课题。低能高亮度正负电子对撞实验BEPCII/BESIII是研究这些过程的重要手段。本项目研究了含轻子对末态的赝标量及矢量无味轻介子衰变，计算衰变振幅，发展一系列BESIII动力学产生子。为探寻其中所包含的标准模型物理及超出标准模型的新物理，利用BESIII的J/ψ和ψ′样本精确测量赝标量η′及矢量J/ψ的部分含轻子对末态过程分支比或上限，同时为研究相同物理目标如CP破坏、非微扰QCD等标准模型物理或新物理以及科研遇到的实际问题拓展研究了稀有衰变道η′→Kπ过程、衰变道J/ψ→ϕη′过程以及hc→ππJ/ψ过程等。通过国际交流在同一领域互补手段拓展研究了赝标量无味轻介子的高能光产生过程，探索了其中动力学机制并将北京谱仪上的经验延伸到未来可能的正负电子对撞实验中辐射修正系统误差的研究。主要工作如下：.（1）整理总结了BESIII上轻的赝标量介子η∕η′的一系列衰变过程的振幅，创建和整理BESIII上相关过程动力学产生子，该结果已发表在Chinese Phys. C（vol. 42, 013001(2018)）上，并在致谢中注明本项目基金号。.（2）η′→ωe+e-分析于申请基金后获得基金号前完成并接收发表在Phys. Rev. D（vol. 92, 051101(R)(2015)）上，未来得及标注本基金号，未列入成果，但此研究目标已完成。.（3）为稀有衰变J/ψ/ψ(3686)→D0e+e-提供了产生子，论文已发表在该结果已发表在Phys. Rev. D （vol. 96, 111101(R) (2017)）上，并在致谢中注明本项目基金号。.（4）为稀有衰变J/ψ→φe+e-、η′→π0/ηe+e-创建和更新了产生子，并用于BESIII分析。.（5）稀有衰变η′→Kπ过程的寻找以及J/ψ→ϕη′过程分支比的精确测量及hc→π+π-J/ψ过程的研究。这两项研究分别发表在Phys. Rev. D上，并在致谢中注明本项目基金号。