First measurements of $J/\psi$ decays into ${\Sigma }^{+}{\overline{\Sigma }}^{-}$ and ${\Xi }^0{\overline{\Xi }}^0$

Based on $58\times {10}^6$ $J/\psi$ events collected with the BESII detector at the Beijing Electron-Positron Collider, the baryon pair processes $J/\psi \to {\Sigma }^{+}{\overline{\Sigma }}^{-}$ and $J/\psi \to {\Xi }^0{\overline{\Xi }}^0$ are observed for the first time. The branching fractions are measured to be $\mathcal{B}(J/\psi \to {\Sigma }^{+}{\overline{\Sigma }}^{-})=(1.50\pm 0.10\pm 0.22)\times {10}^{-3}$ and $\mathcal{B}(J/\psi \to {\Xi }^0{\overline{\Xi }}^0)=(1.20\pm 0.12\pm 0.21)\times {10}^{-3}$, where the first errors are statistical and the second ones are systematic.

Figure 1

Invariant mass distributions of two photons for $J/\psi \to {\Sigma }^{+}{\overline{\Sigma }}^{-}$ candidate events. (a) Scatter plot of $M(\gamma \gamma {)}_1$ versus $M(\gamma \gamma {)}_2$ for the combination with the minimum $R({\pi }^0)$, (b) distribution of $M(\gamma \gamma {)}_1$, (c) distribution of $M(\gamma \gamma {)}_2$ in the data sample, and (d), (e), and (f) are the corresponding plots for MC simulated $J/\psi \to {\Sigma }^{+}{\overline{\Sigma }}^{-}$ events.

Figure 2

Distributions for $J/\psi \to {\Sigma }^{+}{\overline{\Sigma }}^{-}$ candidate events. (a) Scatter plot of $M({\pi }_{(1)}^{0}p)$ versus $M({\pi }_{(2)}^0\overline{p})$; (b) $M({\pi }_{(1)}^{0}p)$ distribution by requiring $|M({\pi }_{(2)}^0\overline{p})-M({\Sigma }^{+})|<0.03\mathrm{GeV}/c^2$, (c) $M({\pi }_{(2)}^0\overline{p})$ distribution by requiring $|M({\pi }_{(1)}^{0}p)-M({\Sigma }^{+})|<0.03\mathrm{GeV}/c^2$, and (d) is the sum of (b) and (c) scaled by a factor of 0.5. (e), (f), (g), and (h) are the corresponding plots of (a), (b), (c), and (d) for MC simulated $J/\psi \to {\Sigma }^{+}{\overline{\Sigma }}^{-}$ events.

Figure 3

Fit to $\pi p$ invariant mass of $J/\psi \to {\Sigma }^{+}{\overline{\Sigma }}^{-}$ candidate events with MC simulated signal shape and a second order polynomial as background shape. The shaded histogram is background from MC simulated $J/\psi \to p\overline{p}{\pi }^0$, $J/\psi \to p\overline{p}\eta$, and $J/\psi \to p\overline{p}\omega$, normalized according to the branching fractions in PDG [19]. The dashed histogram shows the shape of MC simulated $J/\psi \to p\overline{p}{\pi }^0{\pi }^0$ normalized using an assigned branching fraction of 0.003.

Figure 4

Plots for $J/\psi \to {\Xi }^0{\overline{\Xi }}^0$ candidate events. (a) is the scatter plot of $M({\pi }_{(1)}^0\Lambda )$ versus $M({\pi }_{(2)}^0\overline{\Lambda })$, (b) is the $M({\pi }_{(1)}^0\Lambda )$ distribution recoiling against ${\overline{\Xi }}^0$, selected by requiring $|M({\pi }_{(2)}^0\overline{\Lambda })-M({\Xi }^0)|<0.03\mathrm{GeV}/c^2$, (c) is the $M({\pi }_{(2)}^0\overline{\Lambda })$ distribution recoiling against ${\Xi }^0$, selected by requiring $|M({\pi }_{(1)}^0\Lambda )-M({\Xi }^0)|<0.03\mathrm{GeV}/c^2$, (d)  is the sum of (b) and (c) scaled by a factor of 0.5. (e), (f), (g), and (h) are the corresponding plots of (a), (b), (c), and (d) for MC simulated $J/\psi \to {\Xi }^0{\overline{\Xi }}^0$ events.

Figure 5

Fit to $\pi \Lambda$ invariant mass spectrum of $J/\psi \to {\Xi }^0{\overline{\Xi }}^0$ candidate events. Dots with error bars are data, the hatched histogram is the normalized background from all the channels considered in the text, and the solid histogram is the fit to data using a histogram of the signal shape from MC simulation plus a second order polynomial for background.