Observation of a threshold enhancement in the $\pi^+\pi^-$ spectrum in $\psi(3686) \rightarrow \pi^{+}\pi^{-}J/\psi$ decays

Based on the $(2712.4\pm14.4)\times 10^{6}$ $\psi(3686)$ events collected with the BESIII detector, we present a high-precision study of the $\pi^+\pi^-$ mass spectrum in $\psi(3686)\rightarrow\pi^{+}\pi^{-}J/\psi$ decays. A clear resonance like structure is observed near the $\pi^+\pi^-$ mass threshold for the first time. A fit with a Breit-Wigner function yields a mass of $285.6\pm 2.5~{\rm MeV}/c^2$ and a width of $16.3\pm 0.9~{\rm MeV}$ with a statistical significance exceeding 10$\sigma$. 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 $\psi(3686)$ 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.