Observation of B0→ΛΛ̄K0 and B0→Λ Λ̄K*0 at Belle

We study the charmless decays B→ΛΛ̄h, where h stands for π+, K+, K0,K*+, or K*0, using a 605fb-1 data sample collected at the Υ(4S) resonance with the Belle detector at the KEKB asymmetric energy e+e- collider. We observe B0→ΛΛ̄K0 and B0→ΛΛ̄K*0 with branching fractions of (4.76-0.68+0.84(stat)±0.61(syst))×10-6 and (2.46-0.72+0. 87±0.34)×10-6, respectively. The significances of these signals in the threshold-mass enhanced mass region, MΛΛ̄<2.85GeV/c2, are 12.4σ and 9.3σ, respectively. We also update the branching fraction B(B+→ΛΛ̄K+)=(3.38-0.36+0.41±0.41) ×10-6 with better accuracy, and report the following measurement or 90% confidence level upper limit in the threshold-mass-enhanced region: B(B+→ΛΛ̄K*+)=(2.19-0.88+1.13±0.33) ×10-6 with 3.7σ significance; B(B+→ΛΛ̄π+) <0.94×10-6. A related search for B0→ΛΛ̄D̄0 yields a branching fraction B(B0→ΛΛ̄D̄0)=(1.05-0. 44+0.57±0.14)×10-5. This may be compared with the large, ∼10-4, branching fraction observed for B0→pp̄D̄0. The MΛΛ̄ enhancements near threshold and related angular distributions for the observed modes are also reported.