Observation of \(\Xi _{c}(2930)^0\) and updated measurement of \(B^{-} \rightarrow K^{-} \Lambda _{c}^{+} \bar{\Lambda }_{c}^{-}\) at Belle
2018
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 fractionis \(\mathcal{B}(B^{-} \rightarrow \Xi _{c}(2930)^0 \bar{\
Lambda}_{c}^{-}) \mathcal{B}(\Xi _{c}(2930)^0 \rightarrow K^- \
Lambda_{c}^{+})=[1.73 \pm 0.45(\mathrm
stat.) \pm 0.21(\mathrm syst.)]\times 10^{-4}\). 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 massspectrum. 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
KEKBasymmetric energy electron–positron collider.
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