Observation of the Anomalous Shape of $X(1840)$ in $J/\psi \to \gamma 3({\pi }^{+}{\pi }^{-})$ 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 $J/\psi \to \gamma 3({\pi }^{+}{\pi }^{-})$ decay is performed. A significant distortion at $1.84\mathrm{GeV}/c^2$ 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 $1882.1\pm 1.7\pm 0.7\mathrm{MeV}/c^2$ and $30.7\pm 5.5\pm 2.4\mathrm{MeV}$, respectively, which indicates the existence of a $p\overline{p}$ bound state.

Figure 1

$M(6\pi )$ distribution from $J/\psi \to \gamma 3({\pi }^{+}{\pi }^{-})$ events. The dots with error bars are data. The inset shows the data between 1.75 and $1.95\mathrm{GeV}/c^2$ and the vertical dotted line represents the $p\overline{p}$ mass threshold.

Figure 2

Fit result of the $M(6\pi )$ distribution with a simple BW function. The dots with error bars are data, the solid curve in red is the total fit result, the dashed line in blue is the $X(1840)$ signal, the dash-dotted line in green is the background events from $J/\psi \to {\pi }^{0}3({\pi }^{+}{\pi }^{-})$, and the dotted line in magenta is the sum of background.

Figure 3

Fit result of the $M(6\pi )$ distribution with model I (a), and solution I for the model II (b). The dashed line in blue is the $X(1840)$ signal for (a), and the sum of $X(1840)$ and $X(1880)$ for (b).

Figure 4

The acceptance-corrected angular distribution with respect to $|\cos \theta |$ for the $X(1840)$ and $X(1880)$, respectively. The curves represent the fit results with the function of $1+{\cos }^2\theta$ to the above two components: the solid curve in blue is for the $X(1840)$; the dashed one in red is for the $X(1880)$.