Decoding the structure near the ${\pi }^{+}{\pi }^{-}$ mass threshold in $\psi (3686)\to J\mathrm{/}\psi {\pi }^{+}{\pi }^{-}$ decays

In light of recent high-precision data taken by the BESIII Collaboration, we reconsider the dipion transition $\psi (3686)\to J\mathrm{/}\psi {\pi }^{+}{\pi }^{-}$. 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 ${\pi }^{+}{\pi }^{-}$ 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 $Z_c(3900)$ state improves the fit quality only slightly.