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

TL;DR

This paper reports evidence for a new charged charmed-strange baryon, $oldsymbol{ ext{Xi}_c(2930)^+}$, in B meson decays, providing its mass, width, and branching fractions, and also measures the branching fraction of the decay with improved precision, while searching for related exotic states.

Contribution

First observation of the charged $ ext{Xi}_c(2930)^+$ baryon in B decays with detailed measurements, and an improved measurement of the $ar{B}^0$ decay branching fraction, along with a search for charmonium-like states.

Findings

Evidence for $ ext{Xi}_c(2930)^+$ with 3.9$\sigma$ significance.

Measured mass: 2942.3 MeV/$c^2$, width: 14.8 MeV.

Branching fraction $ar{B}^0 o ext{Xi}_c(2930)^+ ar{ ext{Lambda}}_c^-$ measured.

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.