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

Abstract

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 $B^{-}\to K^{-}{\Lambda }_c^{+}{\bar{\Lambda }}_c^{-}$ decays. The measured mass and width are $[2928.9\pm 3.0(stat.{)}_{-12.0}^{+0.9}(syst.)]$ MeV/$c^2$ and $[19.5\pm 8.4(stat.{)}_{-7.9}^{+5.9}(syst.)]$ MeV, respectively, and the product branching fraction is $\mathcal{B}(B^{-}\to {\Xi }_c(2930{)}^0{\bar{\Lambda }}_c^{-})\mathcal{B}({\Xi }_c(2930{)}^0\to K^{-}{\Lambda }_c^{+})=[1.73\pm 0.45(stat.)\pm 0.21(syst.)]\times 10^{-4}$. We also measure $\mathcal{B}(B^{-}\to K^{-}{\Lambda }_c^{+}{\bar{\Lambda }}_c^{-})=[4.80\pm 0.43(stat.)\pm 0.60(syst.)]\times 10^{-4}$ 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 ${\rm Y}(4S)$ resonance by the Belle detector at the KEKB asymmetric energy electron-positron collider.

Comments: 7 pages, 5 figures; accepted for publication in European Physical Journal C