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

Based on the (2712.4±14.4)× 10⁶ $ψ(3686)$ 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 10$σ$. 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.