Evidence of a structure in $\bar{K}^{0} \Lambda_{c}^{+}$ consistent with a charged $\Xi_c(2930)^{+}$, and updated measurement of $\bar{B}^{0} \to \bar{K}^{0} \Lambda_{c}^{+} \bar{\Lambda}_{c}^{-}$ at Belle

Abstract

We report evidence for the charged charmed-strange baryon $\Xi_{c}(2930)^+$ with a signal significance of 3.9$\sigma$ with systematic errors included. The charged $\Xi_{c}(2930)^+$ is found in its decay to $K_{S}^{0} \Lambda_{c}^+$ in the substructure of $\bar{B}^{0} \to K^{0}_{S} \Lambda_{c}^{+} \bar{\Lambda}_{c}^{-}$ decays. The measured mass and width are $[2942.3 \pm 4.4 (\rm stat.) \pm 1.5(\rm syst.)]$~MeV/$c^{2}$ and $[14.8 \pm 8.8(\rm stat.) \pm 2.5(\rm syst.)]$~MeV, respectively, and the product branching fraction is $\cal{B}(\bar{B}^{0} \to \Xi_c(2930)^{+} \bar{\Lambda}_{c}^{-}) \cal{B}(\Xi_c(2930)^{+}\to \bar{K}^{0} \Lambda_{c}^{+})=[2.37 \pm 0.51 (\rm stat.)\pm 0.31(\rm syst.)]\times 10^{-4}$. We also measure $\cal{B}(\bar{B}^{0} \to \bar{K}^{0} \Lambda_{c}^{+} \bar{\Lambda}_{c}^{-}) = [3.99 \pm 0.76(\rm stat.) \pm 0.51(\rm syst.)] \times 10^{-4}$ with greater precision than previous experiments, and present the results of a 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 $Y(4660)$ or $Y_{\eta}$ 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 $(772\pm11)\times 10^{6} B\bar{B}$ pairs collected at the $\Upsilon(4S)$ resonance by the Belle detector at the KEKB asymmetric energy electron-positron collider.