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 $\left(10087±44\right)×{10}^{6}\text{}\text{}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 $1.84\text{}\text{}\mathrm{GeV}/{c}^{2}$ 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 $1882.1±1.7±0.7\text{}\text{}\mathrm{MeV}/{c}^{2}$ and $30.7±5.5±2.4\text{}\text{}\mathrm{MeV}$, respectively, which indicates the existence of a $p\overline{p}$ bound state.