Observation of a Threshold Enhancement in the π + π − Spectrum in ψ ( 3686 ) → π + π − J / ψ Decays

Based on the (2712.4±14.4)×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 resonancelike 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/c^{2} 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/c^{2} 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/c^{2} offers new insight into the interplay between chiral dynamics and low-energy QCD.