Y（4260）与$Z_c(3900)$之谜的破解
Decoding the riddle of Y(4260) and $Z_c(3900)$
摘要：
在质心4.260 GeV的$e^+e^-\to J/ψπ^+π^-$中，通过BESIII合作对$J/ψπ^\pm$的不变质谱进行了观测，表明存在带电荷的$\bar D D^*+D\bar D^*$分子态和$I(J^P)=1(1^+)$，这可能是著名的X（3872）的等矢量兄弟，也是Belle合作组织声称的$Z_b(10610)$的类似物。我们证明，这一观察提供了强有力的证据，神秘的Y（4260）是一个$\bar{D}D_1(2420) +D\bar{D}_1(2420)$分子状态。特别是，我们表明，这种分子的衰变自然填充低动量$\bar DD^*$对，并导致在$\bar DD^*$阈值的尖点。我们讨论的签名，区分这样一个$\bar DD^*$尖从一个真正的共振的存在。
英文摘要：
The observation of $Z_c(3900)$ by the BESIII collaboration in the invariant mass spectrum of $J/ψπ^\pm$ in $e^+e^-\to J/ψπ^+π^-$ at the center of mass 4.260 GeV suggests the existence of a charged $\bar D D^*+D\bar D^*$ molecular state with $I(J^P)=1(1^+)$, which could be an isovector brother of the famous X(3872) and an analogue of $Z_b(10610)$ claimed by the Belle Collaboration. We demonstrate that this observation provides strong evidence that the mysterious Y(4260) is a $\bar{D}D_1(2420) +D\bar{D}_1(2420)$ molecular state. Especially, we show that the decay of this molecule naturally populates low momentum $\bar DD^*$ pairs and leads unavoidably to a cusp at the $\bar DD^*$ threshold. We discuss the signatures that distinguish such a $\bar DD^*$ cusp from the presence of a true resonance.
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