Study of J/ψ→p(p)over-bar,Λ(Λ)over-bar and observation of ηc→Λ(Λ)over-bar at Belle

We study the baryonic charmonium decays of B mesons B+->eta K-c(+) and B+-> J/psi K+, where the eta(c) and J/psi subsequently decay into a pp or Lambda Lambda pair. We measure the J/psi -> pp and Lambda Lambda anisotropy parameters alpha(B)=-0.60 +/- 0.13 +/- 0.14 (pp), -0.44 +/- 0.51 +/- 0.31 (Lambda Lambda) and compare to results from e(+)e(-)-> J/psi formation experiments. We also report the first observation of eta(c)->Lambda Lambda. The measured branching fraction is B(eta(c)->Lambda Lambda)=(0.87(-0.21)(+0.24)(stat)(-0.14)(+0.09)(syst)+/- 0.27(PDG))x10(-3). This study is based on a 357 fb(-1) data sample recorded on the Upsilon(4S) resonance with the Belle detector at the KEKB asymmetric-energy e(+)e(-) collider.