Observation of a Threshold Enhancement in the $π^0π^0$ Mass Spectrum in $ψ(3686) \rightarrow π^{0}π^{0}J/ψ$ Decays

Abstract

Using a data sample of $(2712.4\pm14.4)\times 10^{6}$ $ψ(3686)$ events collected with the BESIII detector, we report a high-precision study of the $π^{0}π^{0}$ mass spectrum from $ψ(3686)\rightarrowπ^{0}π^{0}J/ψ$ decays. The enhancement previously observed near the $ππ$ mass threshold in the $π^+π^-$ channel is confirmed, with a statistical significance exceeding 10$σ$. A fit with a Breit-Wigner function yields a mass of $M = 274.3\pm 2.1_{\rm stat.}\pm0.6_{\rm syst.}~{\rm MeV}/c^2$ and a width of $Γ= 17.1\pm 1.5_{\rm stat.}\pm0.5_{\rm syst.}~{\rm MeV}$, consistent with those from the $π^{+}π^{-}$ channel. By including final-state interactions, we analyze the data using both chiral perturbation theory (ChPT) and QCD multipole expansion (QCDME). While the ChPT model describes the higher-mass region above 0.3 GeV/$c^2$ well, it fails to reproduce the significant threshold enhancement. In contrast, the QCDME model, which incorporates the $S$- and $D$-wave admixture structure of $ψ(3686)$, provides a better description of the full $π^{0}π^{0}$ mass spectrum.