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)×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 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/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.