Study of Charmonium Decays into Baryon-Antibaryon Pairs

We study the baryonic charmonium decays of $B$ mesons, $B^+ \to \eta_c K^+$ and $B^+ \to J/\psi K^+$, where $\eta_c$ and $J/\psi$ subsequently decay into a $p\bar p$ or $\Lambda\bar\Lambda$ pair. The charmonium produced in the above $B$ meson decays is fully polarized. The polar angular distributions of the baryon-antibaryon pairs are presented, along with fit results to a $1 + \alpha_B\cos^2\theta$ parametrization. Comparisons are made with the results from $e^+e^- \to J/\psi$ formation experiments. We also report the firstobservation of $\eta_c \to \Lambda\bar\Lambdar$. The measured branching fraction is ${\mathcal B}(\eta_c \to \Lambda\bar\Lambda) = (0.87^{+0.24}_{-0.21} \pm 0.14 \pm 0.27) \times 10^{-3}$. This study is based on a $357~fb^{-1}$ data sample recorded on the $\Upsilon({\rm 4S})$ resonance with the Belle detector at the KEKB asymmetric-energy $e^+e^-$ collider.