Decoding the riddle of Y(4260) and $Z_c(3900)$

TL;DR

This paper proposes that the Y(4260) and Zc(3900) states are molecular states formed from D mesons, with evidence from decay patterns and threshold cusps, challenging traditional resonance interpretations.

Contribution

It introduces a molecular interpretation of Y(4260) and Zc(3900), supported by decay analysis and threshold cusp phenomena, offering a new perspective on these exotic states.

Findings

Zc(3900) is likely a D D* molecular state.

Y(4260) can be explained as a D D1(2420) molecular state.

Threshold cusps can mimic resonance signals in experimental data.

Abstract

The observation of $Z_c(3900)$ by the BESIII collaboration in the invariant mass spectrum of $J/\psi\pi^\pm$ in $e^+e^-\to J/\psi\pi^+\pi^-$ 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.