Observation of a Doubly-strange Hyperon $\Xi(1720)$ in $J/\psi\rightarrow{}K^{-}\Sigma^0\bar{\Xi}^{+}+c.c.$

Abstract

Based on a sample of $(10087 \pm 44) \times 10^6$ $J/\psi$ events collected with the BESIII detector at BEPCII, we report the first observation of the decay $J/\psi \rightarrow K^- \Sigma^0 \bar{\Xi}^++c.c.$. A partial wave analysis is performed to investigate the involved excited states. In addition to the well-established $\Xi(1690)$, a new doubly-strange hyperon $\Xi(1720) $ is observed decaying to $K^- \Sigma^0$ with a mass of $1721.0 \pm 5.2_{\rm stat.} \pm 3.4_{\rm syst.} ~{\rm MeV}/c^2$ and a width of $31.3 \pm 18.3_{\rm stat.} \pm 15.4_{\rm syst.} ~{\rm MeV}$, with a statistical significance exceeding $10\sigma$. The spin-parity hypothesis testing across various quantum number configurations reveals that the spin-parity of $\Xi(1720)$ favors $J^P = {\frac{3}{2}}^+$. Furthermore, the branching fraction of $J/\psi \rightarrow K^- \Sigma^0 \bar{\Xi}^++c.c.$ is determined to be $(2.68 \pm 0.04_{\rm stat.} \pm 0.17_{\rm syst.}) \times 10^{-5}$.