Search for strange four-quark states in the charmonium sector with BESIII

寻找奇异的强子，即不符合重子的三夸克态或介子的夸克-反夸克态的常规分配，是强子光谱学中一个高度热门的问题。我们建议利用BESIII实验的新数据和现有数据来寻找已建立的奇异四夸克Z_c态缺失的奇异伴侣Z_cs。在最近的一篇综述中，寻找这些Z_cs态被命名为未来几年实验强子光谱学中最重要的课题之一。用QCD求和规则对J^P=1^+的最轻Z_cs的四个衰变通道的质量和衰变宽度进行了理论预测。其中两个通道包含开放粲介子，另外两个通道包含隐藏粲介子。我们打算通过识别这四种衰变模式来寻找在正电子湮灭中与一个带电的介子结合的Z_cs的产生。所提出的复杂分析计划由两名博士生进行。第一个将分析包含两个d介子的最终态，而第二个将集中于具有调和态的最终态。因此，这些分析将是相互补充的，在最后的解释中将需要密切合作。在BESIII实验中，我们已经在4.0 GeV到4.6 GeV的质心能量范围内收集了约17.1 /fb的e+e-碰撞数据，专门用于粲子区奇异态的光谱研究。在对已经完成的北京电子正电子对撞机II （BEPCII）进行升级后，将在即将到来的2019年10月至2020年6月期间收集与本文提出的研究最相关的4.6至4.7 GeV质心区域的新数据。假设假设的Z_cs对e+e-→J/ψ K+K-通道的贡献的相对强度与e+e-→J/ψ π+π-中的Z_c（3900）的贡献的相对强度相同，则估计了Z_cs衰变为J/ψ和带电K子的观测结果。这将标志着Z_cs的首次观测，这一发现与Z_c（3900）的原始发现同样具有开创性，Z_c（3900）被选为2013年APS物理学的杰出发现之一，并且仍然是被引用最多的BESIII出版物。此外，其他三个通道同样重要，因为它们不仅对Z_cs具有重大的发现潜力，而且还允许搜索在初始e+e-湮灭中形成的新矢量共振。

The search for exotic hadrons, states that do not fit the conventional assignment as either three quark states for baryons or quark-antiquark states for mesons, is a highly topical issue in hadron spectroscopy. We propose to search for the Z_cs, the missing strange partner of the established exotic four-quark Z_c states, using new and existing data of the BESIII experiment. In a recent review, the search for these Z_cs states was named as one of the most important topics in experimental hadron spectroscopy in the coming years. Both mass and decay widths for four decay channels of the lightest Z_cs with J^P=1^+ have been predicted by theory using QCD sum rules. Two of these decays include open charm mesons while the other two channels include hidden charm mesons. We intend to search for the production of the Z_cs in combination with one charged kaon in electron-positron annihilations via the identification of these four decay modes. The proposed complex analyses are planned to be performed by two PhD students. While the first one will analyze final states including two D-mesons, the second one will focus on final states with charmonia. Therefore, the analyses will be complementary to each other and will require a close cooperation in the final interpretation. With the BESIII experiment, we already collected about 17 1/fb of e+e- collision data in the center-of-mass energy range of 4.0 GeV to 4.6 GeV thus far, dedicated to the spectroscopy of exotic states in the charmonium sector. After an upgrade of the Beijing Electron Positron Collider II (BEPCII), that has already been finished, new data in the center-of-mass region 4.6 GeV to 4.7 GeV most relevant for the studies proposed here will be gathered in the upcoming measurement period between October 2019 and June 2020.Assuming that the relative strength of a hypothetical Z_cs contribution to the e+e- → J/ψ K+K- channel is of the same magnitude as for the Z_c(3900) in e+e- → J/ψ π+π-, an observation of the Z_cs via its decay into a J/ψ and a charged kaon is estimated. This would mark the first ever observation of the Z_cs and a discovery that is equally groundbreaking as the original discovery of the Z_c(3900), which was selected as one of the standout discoveries of 2013 by APS Physics and is still the most cited BESIII publication. Moreover, the other three channels are equally important, as they do not only pose significant discovery potential concerning the Z_cs, but also allow to search for new vector resonances formed in the initial e+e- annihilation.