三角奇点与XYZ类粲偶素结构

对强子谱的研究有助于加深量子色动力学中色禁闭现象的理解。强子谱中存在诸多疑难问题，特别是2003年以来各高能物理实验上发现了很多难以理解的新的强子共振结构，其中大量是在重夸克偶素质量区间发现的，它们被统称为$XYZ$结构。强子共振态对应于$S$矩阵的极点。除此之外，$S$矩阵还具有其它类型的奇点，比如由于三个中间态粒子同时在 壳导致的三角奇点。本文中，我们将讨论三角奇点在$XYZ$物理中的实现，注重于构造可以用来分析 其对$e^+e^-\to\psi\pi^+\pi^-$反应的$\psi\pi$不变质量谱中的带电$Z_c$结构的影响的振幅。

Triangle Singularities and Charmonium-like XYZ States

The spectrum of hadrons is important for understanding the confinement of quantum chromodynamics. Many new puzzles arose since 2003 due to the abundance of experimental discoveries with the $XYZ$ structures in the heavy quarkonium mass region being the outstanding examples. Hadronic resonances correspond to poles of the $S$-matrix, which has other types of singularities such as the triangle singularity due to the simultaneous on-shellness of three intermediate particles. Here we briefly discuss a few possible manifestations of triangle singularities in the $XYZ$ physics, paying particular attention to the formalism that can be used to analyze the data for charged $Z_c$ structures in the $\psi\pi$ distributions of the reaction $e^+e^-\to \psi\pi^+\pi^-$.