Exotic Meson Spectroscopy (Project XYZ)

在过去的20年里，大量含有重夸克（c-或b-夸克）的新态被发现接近或超过开放的重味阈值。这些态是奇异的，因为它们不能被解释为夸克-反夸克态，而是基于QCD双夸克、QCD杂化、强子分子或强子-粲偶素态的四夸克态。XYZ项目旨在提取现有和新的奇异状态的属性，这将导致对其性质的更详细的理解。该项目分为三个部分，涵盖XYZ状态的不同方面。 首先，通过BESIII协作组和RU内的理论家之间的合作努力，将对粲偶素扇区中奇异矢量XYZ态到两介子末态的隐味衰变模式进行分波分析。这些过程与轻介子系统中的逐光融合过程有直接联系，RU的理论家已经发展了色散形式。作为一个额外的工具，模型无关的光-光求和规则将被用来研究奇异态的性质，通过辐射跃迁的矢量charmonium和bottomonium状态。最后，在BESIII上通过双光子产生测量电子-正电子湮灭中的非矢量粲偶素态的实验计划有可能成为粲偶素态的发现渠道，并将允许人们区分描述其内在性质的各种模型。

Over the past two decades a plethora of new states containing heavy quarks (c- or b-quarks) have been discovered near or above open heavy-flavor thresholds. These states are exotic, in the sense that they cannot be interpreted as quark-antiquark states but rather as tetraquark states based on QCD diquarks, QCD hybrids, hadronic molecules, or hadro-charmonium states. The project XYZ aims to extract the properties of existing and new exotic states, which will lead to a more detailed understanding of their nature. The project is split into three parts that encompass different aspects of XYZ states. Firstly, through a collaborative effort between the BESIII Collaboration and theorists within the RU, a partial-wave analysis will be performed of the hidden-flavor decay modes of exotic vector XYZ states in the charmonium sector into two-meson final states. These processes have a direct connection to the light-by-light fusion processes in light meson systems, for which the dispersive formalism has already been developed by the theorists within the RU. As an additional tool, model-independent light-by-light sum rules will be used to investigate the nature of exotic states through the radiative transitions of vector charmonium and bottomonium states. Finally, the proposed experimental program on measuring non-vector charmonium states in the electron-positron annihilation via two-photon production at BESIII has the potential to be a discovery channel of charmonium states and will allow one to distinguish between the various models describing their intrinsic nature.