Observation of a Threshold Enhancement in the ${\pi }^0{\pi }^0$ Mass Spectrum in $\psi (3686)\to {\pi }^0{\pi }^{0}J\mathrm{/}\psi$ Decays

Using a data sample of $(2712.4\pm 14.4)\times {10}^6$ $\psi (3686)$ events collected with the BESIII detector, we report a high-precision study of the ${\pi }^0{\pi }^0$ mass spectrum from $\psi (3686)\to {\pi }^0{\pi }^{0}J\mathrm{/}\psi$ decays. The enhancement previously observed near the $\pi \pi$ mass threshold in the ${\pi }^{+}{\pi }^{-}$ channel is confirmed, with a statistical significance exceeding 10$\sigma$. A fit with a Breit-Wigner function yields a mass of $M=274.3\pm {2.1}_{\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{.}}\pm {0.6}_{\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{.}}\mathrm{M}\mathrm{e}\mathrm{V}\mathrm{/}{c}^2$ and a width of $\mathrm{\Gamma }=17.1\pm {1.5}_{\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{.}}\pm {0.5}_{\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{.}}\mathrm{M}\mathrm{e}\mathrm{V}$, consistent with those from the ${\pi }^{+}{\pi }^{-}$ 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 $\psi (3686)$, provides a better description of the full ${\pi }^0{\pi }^0$ mass spectrum.