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

Abstract

Based on the $(2712.4\pm 14.4)\times 10^6$ $\psi (3686)$ events collected with the BESIII detector, we present a high-precision study of the ${\pi }^{+}{\pi }^{-}$ mass spectrum in $\psi (3686)\to {\pi }^{+}{\pi }^{-}J/\psi$ 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 $285.6\pm 2.5\mathrm{MeV}/c^2$ and a width of $16.3\pm 0.9\mathrm{MeV}$ 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.

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Cited by in corpus (1)

• Low-energy scattering of the $J/\psi \pi$ and $J/\psi K$ system