In light of recent high-precision data taken by the BESIII Collaboration, we reconsider the dipion transition ψ(3686)→J/ψπ+π−. The strong pion-pion final-state interactions are taken into account model-independently by using dispersion theory. We find that we can reproduce the substructure near the π+π− threshold observed experimentally without introducing an extra resonance state. While a helicity-flip amplitude plays an important role for the formation of the dip in the invariant-mass distribution, the virtual exchange of the charmoniumlike exotic Zc(3900) state improves the fit quality only slightly.
@article{arxiv.2512.01679,
title = {Decoding the structure near the $\pi^+\pi^-$ mass threshold in $\psi(3686) \rightarrow J/\psi \pi^+\pi^-$ decays},
author = {Yun-Hua Chen and Xiang-Kun Dong and Feng-Kun Guo and Christoph Hanhart and Bastian Kubis},
journal= {arXiv preprint arXiv:2512.01679},
year = {2026}
}