Observation of the Anomalous Shape of $X(1840)$ in $J/\ddot{I}ˆ\to {\hat{I}}^{3}3(\ddot{I}{€}^{+}\ddot{I}{€}^{-})$ Indicating a Second Resonance Near $p\bar{p}$ Threshold

Abstract

Using a sample of $(10087\pm 44)\times 10^6$ $J/\ddot{I}ˆ$ events, which is about 45 times larger than that was previously analyzed, a further investigation on the $J/\ddot{I}ˆ\to {\hat{I}}^{3}3(\ddot{I}{€}^{+}\ddot{I}{€}^{-})$ decay is performed. A significant distortion at 1.84 GeV/$c^2$ in the line-shape of the $3(\ddot{I}{€}^{+}\ddot{I}{€}^{-})$ 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 $10\ddot{I}ƒ$. 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.