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October 20, 2025Open Access

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

Key Points

A clear resonance-like structure is observed near the π+π- mass threshold with a mass of 285.6 MeV/c².
The statistical significance of this observation exceeds 10σ, highlighting the robustness of the findings.
QCD multipole expansion effectively reproduces the observed mass spectrum, unlike chiral perturbation theory.
The pronounced dip near 0.3 GeV/c² provides new insights into the interplay between chiral dynamics and low-energy QCD.

Abstract

Based on the (2712. 414. 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 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 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/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.

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