Study of $J/\psi \to p\overline{p},\Lambda \overline{\Lambda }$ and Observation of ${\eta }_c\to \Lambda \overline{\Lambda }$ at Belle

We study the baryonic charmonium decays of $B$ mesons $B^{+}\to {\eta }_c{K}^{+}$ and $B^{+}\to J/\psi K^{+}$, where the ${\eta }_c$ and $J/\psi$ subsequently decay into a $p\overline{p}$ or $\Lambda \overline{\Lambda }$ pair. We measure the $J/\psi \to p\overline{p}$ and $\Lambda \overline{\Lambda }$ anisotropy parameters ${\alpha }_B=-0.60\pm 0.13\pm 0.14$ ($p\overline{p}$), $-0.44\pm 0.51\pm 0.31$ ($\Lambda \overline{\Lambda }$) and compare to results from $e^{+}{e}^{-}\to J/\psi$ formation experiments. We also report the first observation of ${\eta }_c\to \Lambda \overline{\Lambda }$. The measured branching fraction is $\mathcal{B}({\eta }_c\to \Lambda \overline{\Lambda })=({0.87}_{-0.21}^{+0.24}(\mathrm{stat}{)}_{-0.14}^{+0.09}(\mathrm{syst})\pm 0.27(\mathrm{PDG}))\times {10}^{-3}$. This study is based on a $357{\mathrm{fb}}^{-1}$ data sample recorded on the ${\rm Y}(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider.

Figure 1

(a) $B$ signal yield versus $M_{p\overline{p}}$ and (b) $B$ signal yield versus $M_{\Lambda \overline{\Lambda }}$. The inset shows the ${\eta }_c\mathrm{\text{−}}J/\psi$ mass region. The curves represent the unbinned likelihood fits to the data.

Figure 2

(a) Likelihood fit and (b) ${\chi }^2$ fit results of the $J/\psi \to p\overline{p}$ helicity angle distribution. In the maximum likelihood fit plot, the solid, dotted solid, and dashed lines represent the combined fit result, fitted signal, and fitted background, respectively. In the ${\chi }^2$ fit plot, the inset shows the fit result for $B$ signal yield of $B^{+}\to J/\psi K^{+}$, $J/\psi \to {\mu }^{+}{\mu }^{-}$.