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

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