Observation of ${\mathrm{\Xi }}_c(2930{)}^0$ and updated measurement of $B^{-}\to K^{-}{\mathrm{\Lambda }}_c^{+}{\bar{\mathrm{\Lambda }}}_c^{-}$ at Belle

We report the first observation of the ${\mathrm{\Xi }}_c(2930{)}^0$ charmed-strange baryon with a significance greater than 5$\sigma$. The ${\mathrm{\Xi }}_c(2930{)}^0$ is found in its decay to $K^{-}{\mathrm{\Lambda }}_c^{+}$ in $B^{-} \to K^{-} \Lambda_{c}^{+} \bar{\Lambda}{c}^{-}$decays\mathrm{.}Themeasuredmassandwidthare$[2928.9 \pm 3.0(\rm stat.)^{+0.9}{-12.0}(\rm syst.)]$MeV\mathrm{/}$c^{2}$and$[19.5 \pm 8.4(\rm stat.) ^{+5.9}{-7.9}(\rm syst.)]$ MeV, respectively, and the product branching fraction is ${\cal B}(B^{-} \to \Xi{c}(2930)^0 \bar{\Lambda}{c}^{-}) {\cal B}(\Xi{c}(2930)^0 \to K^- \Lambda_{c}^{+})=[1.73 \pm 0.45(\rm stat.) \pm 0.21(\rm syst.)]\times 10^{-4}$\mathrm{.}Wealsomeasure${\cal B}(B^{-} \to K^{-} \Lambda_{c}^{+} \bar{\Lambda}_{c}^{-}) = [4.80 \pm 0.43(\rm stat.) \pm 0.60(\rm syst.)] \times 10^{-4}$ with improved precision, and search for the charmonium-like state $Y(4660)$ and its spin partner, $Y_{\eta }$, in the ${\mathrm{\Lambda }}_c^{+}{\bar{\mathrm{\Lambda }}}_c^{-}$ 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 $(772\pm11)\times 10^{6} B\bar{B}$pairscollectedatthe$\Upsilon(4S)$ resonance by the Belle detector at the KEKB asymmetric energy electron-positron collider.