Observation of B0 to Lambda Lambdabar K0 and B0 to Lambda Lambdabar K*0 at Belle

TL;DR

This paper reports the first observations of specific charmless B meson decays involving Lambda baryons, measuring their branching fractions and analyzing threshold enhancements and angular distributions using Belle data.

Contribution

It provides the first measurements of B0 to Lambda Lambdabar K0 and K*0 decays, with improved accuracy for related modes and detailed threshold and angular analyses.

Findings

Observation of B0 to Lambda Lambdabar K0 and K*0 with high significance.

Updated branching fraction for B+ to Lambda Lambdabar K+ with better precision.

Detection of threshold enhancements and angular distributions in the decay modes.

Abstract

We study the charmless decays B to Lambda Lambdabar h, where h stands for pi^+, K^+,K^0,K^{*+}, or K^{*0}, using a 605 fb^{-1} data sample collected at the Upsilon(4S)resonance with the Belle detector at the KEKB asymmetric energy e^+ e^- collider. We observe B0 to Lambda Lambdabar K0 and B0 to Lambda Lambdabar K*0 with branching fractions of (4.76^{+0.84}_{-0.68} (stat.) \pm 0.61 (syst.)) \times 10^{-6} and (2.46^{+0.87}_{-0.72} \pm 0.34) \times 10^{-6}, respectively. The significances of these signals in the threshold-mass enhanced mass region are 12.4 sigma and 9.3 sigma, respectively. We also update the branching fraction BF(B+ to Lambda Lambdabar K+) = (3.38^{+0.41}_{-0.36} \pm 0.41)\times 10^{-6} with better accuracy,and report the following measurement or 90% confidence level upper limit in the threshold-mass-enhanced region: BF(B+ to Lambda Lambdabar K*+) = (2.19^{+1.13}_{-0.89} \pm 0.33) \times 10^{-6} with 3.7 sigma significance; BF(B+ to Lambda Lambdabar pi+) < 0.94 \times 10^{-6}. A related search for B0 to Lambda Lambdabar D0bar yields a branching fraction BF(B0 to Lambda Lambdabar D0bar) = (1.05_{-0.44}^{+0.57} \pm 0.13)\times 10^{-5}. This may be compared with the large, ~10^{-4}, branching fraction observed for B0 to p pbar D0bar. The m_{Lambda Lambdabar} enhancements near threshold and related angular distributions for the observed modes are also reported. PACS:13.25.Hw, 14.40.Nd