Observation of Ξc(2930)0 and updated measurement of B−→K−Λ+cΛ¯−c at Belle

Abstract

We report the first observation of the Ξ c(2930) 0 charmed-strange baryon with a significance greater than 5σ. The Ξ c(2930) 0 is found in its decay to K-Λc+ in B-→K-Λc+Λ¯c- decays. The measured mass and width are [2928.9±3.0(stat.)-12.0+0.9(syst.)] MeV/c2 and [19.5±8.4(stat.)-7.9+5.9(syst.)] MeV, respectively, and the product branching fraction is B(B-→Ξc(2930)0Λ¯c-)B(Ξc(2930)0→K-Λc+)=[1.73±0.45(stat.)±0.21(syst.)]×10-4. We also measure B(B-→K-Λc+Λ¯c-)=[4.80±0.43(stat.)±0.60(syst.)]×10-4 with improved precision, and search for the charmonium-like state Y(4660) and its spin partner, Yη, in the Λc+Λ¯c- invariant mass spectrum. No clear signals of the Y(4660) nor its spin partner are observed and the 90% credibility level (C.L.) upper limits on their production rates are determined. These measurements are obtained from a sample of (772 ± 11) × 10 6BB¯ pairs collected at the Υ (4 S) resonance by the Belle detector at the KEKB asymmetric energy electron–positron collider.