Decoding the structure near the π + π − mass threshold in ψ ( 3686 ) → J / ψ π + π − decays

In light of recent high-precision data taken by the BESIII Collaboration, we reconsider the dipion transition 𝜓⁡(3686) →𝐽/𝜓⁡𝜋+⁢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.