BESIII上电磁Dalitz衰变过程J/ψ -> e+e-X(1835)的实验研究

北京谱仪实验是寻找和研究新型轻强子态的理想场所。BESII首次发现了质子-反质子阈值附近的反常增强结构X(ppbar)和新共振态X(1835)，BESIII对这两个态进行了确认。实验结果表明这两个态不但有着相近的质量，而且具有相同的自旋宇称内禀属性，然而对其内部结构以及其之间关系所知很为有限，因而需要通过理论和实验方面更多的研究来确定X(1835)和X(ppbar)是否为同一个态，以及X(1835)究竟是ppbar束缚态，还是胶球候选者，亦或是其他的结构。另外，研究J/ψ介子的电磁Dalitz衰变是理解其形成机制和内部结构的黄金过程。本项目利用BESIII采集的世界上最大统计量的13.1亿J/ψ数据样本，首次在实验上观测到电磁Dalitz衰变过程J/ψ->eeX(1835)，并通过该过程探寻X(1835)粒子和X(ppbar)的内在关联；还将首次在实验上测量该过程的跃迁形状因子。

The BESIII experiment is an ideal place to search for and study new forms of light hadrons. An anomalously strong enhancement at the proton-antiproton(ppbar) mass threshold, dubbed X(ppbar), and another state X(1835) were first observed by BESII in J/ψ radiative decays. These two states were confirmed by the upgraded BESIII experiment through the same decay processes. The BESIII results indicate that these two states have overlapped masses and the same 0^{-+} quantum number, but we still know very few about their internal structures and properties, and also the connections between them. We need more inputs from theory and experiment researches, in order to determine whether X(1835) and X(ppbar) are the same state and whether X(1835) is a ppbar bound state, or glueball candidate, or other physics structures. Further more, the electromagnetic Dalitz decay of J/ψ meson, i.e., J/ψ->P l+l- (l=e,mu, P is pseudo-scalar particle), are the golden processes to study J/ψ production mechanism and its internal structure. Based on the total 1.31 billion J/ψ data sample, we first observe the J/ψ->e+e-X(1835) decay, from which we will study the property and structure of X(1835) and explore the connection between X(1835) and X(ppbar); we will also first determine the transition form factor of this process, and test the predictions of the Standard Model.