Observation of $$\Xi _{c}(2930)^0$$ and updated measurement of $$B^{-} \rightarrow K^{-} \Lambda _{c}^{+} \bar{\Lambda }_{c}^{-}$$B-→K-Λc+Λ¯c- at Belle

We report the first observation of the \(\Xi _{c}(2930)^0\) charmed-strange baryon with a significance greater than 5\(\sigma \). The \(\Xi _{c}(2930)^0\) is found in its decay to \(K^- \Lambda _{c}^+\) in \(B^{-} \rightarrow K^{-} \Lambda _{c}^{+} \bar{\Lambda }_{c}^{-}\) decays. The measured mass and width are \([2928.9 \pm 3.0(\mathrm stat.)^{+0.9}_{-12.0}(\mathrm syst.)]\) MeV/\(c^{2}\) and \([19.5 \pm 8.4(\mathrm stat.) ^{+5.9}_{-7.9}(\mathrm syst.)]\) MeV, respectively, and the product branching fraction is \(\mathcal{B}(B^{-} \rightarrow \Xi _{c}(2930)^0 \bar{\Lambda }_{c}^{-}) \mathcal{B}(\Xi _{c}(2930)^0 \rightarrow K^- \Lambda _{c}^{+})=[1.73 \pm 0.45(\mathrm stat.) \pm 0.21(\mathrm syst.)]\times 10^{-4}\). We also measure \(\mathcal{B}(B^{-} \rightarrow K^{-} \Lambda _{c}^{+} \bar{\Lambda }_{c}^{-}) = [4.80 \pm 0.43(\mathrm stat.) \pm 0.60(\mathrm syst.)] \times 10^{-4}\) with improved precision, and search for the charmonium-like state Y(4660) and its spin partner, \(Y_{\eta }\), in the \(\Lambda _{c}^{+}\bar{\Lambda }_{c}^{-}\) 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 \((772\pm 11)\times 10^{6} B\bar{B}\) pairs collected at the \(\Upsilon (4S)\) resonance by the Belle detector at the KEKB asymmetric energy electron–positron collider.