Observation of a resonance-like structure near the π^+π^- mass threshold in ψ (3686) π^+π^-J/ψ

Based on the (2712. 414. 4) 10^6 ψ (3686) events collected with the BESIII detector, we present a high-precision study of the π^+π^- mass spectrum in ψ (3686) π^+π^-J/ψ decays. A clear resonance-like structure is observed near the π^+π^- mass threshold for the first time. A fit with a Breit-Wigner function yields a mass of 285. 6 2. 5~ MeV/c² and a width of 16. 3 0. 9~ MeV with a statistical significance exceeding 10σ. To interpret the data, we incorporate final-state interactions (FSI) within two theoretical frameworks: chiral perturbation theory (ChPT) and QCD multipole expansion (QCDME). ChPT describes the spectrum above 0. 3 GeV/c² but fails to reproduce the threshold enhancement. In contrast, the QCDME model, assuming the ψ (3686) is an admixture of S- and D-wave charmonium, reproduces the data well. The pronounced dip near 0. 3 GeV/c² offers new insight into the interplay between chiral dynamics and low-energy QCD.