Observation of the Anomalous Shape of $X(1840)$ in $J\mathrm{/}\psi \to \gamma 3({\pi }^{+}{\pi }^{-})$ Indicating a Second Resonance Near $p\bar{p}$ Threshold

Using a sample of $(10087\pm 44)\times {10}^6$ $J\mathrm{/}\psi$ events, which is about 45 times larger than that was previously analyzed, a further investigation on the $J\mathrm{/}\psi \to \gamma 3({\pi }^{+}{\pi }^{-})$ decay is performed. A significant distortion at 1.84 GeV/$c^2$ in the line-shape of the $3({\pi }^{+}{\pi }^{-})$ invariant mass spectrum is observed for the first time, which could be resolved by two overlapping resonant structures, $X(1840)$ and $X(1880)$. The new state $X(1880)$ is observed with a statistical significance larger than $10\sigma$. The mass and width of $X(1880)$ are determined to be $1882.1\pm 1.7\pm 0.7$ MeV/$c^2$ and $30.7\pm 5.5\pm 2.4$ MeV, respectively, which indicates the existence of a $p\bar{p}$ bound state.