Observation of B0 → ΛvK0 and B0 → ΛvK*0 at Belle

We study the charmless decays B → ΛΛh, where h stands for π + , K + , K 0 ,K* + , or K* 0 , using a 605 fb -1 data sample collected at the Y(4S) resonance with the Belle detector at the KEKB asymmetric energy e + e - collider. We observe B 0 → ΛΛK 0 and B 0 → ΛΛK* 0 with branching fractions of (4.76 +0.86 -0.68 (stat) ± 0.61(syst)) × 10 -6 and (2.46 +0.87 -0.72 ± 0.34) × 10 -6 , respectively. The significances of these signals in the threshold-mass enhanced mass region, M ΛΛ < 2.85 GeV/c 2 , are 12.4σ and 9.3σ, respectively. We also update the branching fraction Β(B + → ΛΛK + ) = (3.38 +0.41 -0.36 ± 0.41) X 10 -6 with better accuracy, and report the following measurement or 90% confidence level upper limit in the threshold-mass-enhanced region: B(B + → ΛΛK* + ) = (2.19 +1.13 -0.88 ± 0.33) X 10 -6 with 3.7σ significance; Β(B + → ΛΛπ + ) < 0.94 X 10 -6 . A related search for B 0 → AAD 0 yields a branching fraction B(B 0 → ΛΛD 0 ) = (1.05 +0.57 -0.44 ± 0.14) × 10 -5 . This may be compared with the large, ∼10 -4 , branching fraction observed for B 0 → pPD 0 . The M ΛΛ enhancements near threshold and related angular distributions for the observed modes are also reported.