Measurement of the Cross Section for ${e}^{+}{e}^{-}\to {\Xi }^{-}{\overline{\Xi }}^{+}$ and Observation of an Excited $\Xi $ Baryon

Using a total of $11.0\text{}\text{}{\mathrm{fb}}^{-1}$ of ${e}^{+}{e}^{-}$ collision data with center-of-mass energies between 4.009 and 4.6 GeV and collected with the BESIII detector at BEPCII, we measure fifteen exclusive cross sections and effective form factors for the process ${e}^{+}{e}^{-}\to {\Xi }^{-}{\overline{\Xi }}^{+}$ by means of a single baryon-tag method. After performing a fit to the dressed cross section of ${e}^{+}{e}^{-}\to {\Xi }^{-}{\overline{\Xi }}^{+}$, no significant $\psi \left(4230\right)$ or $\psi \left(4260\right)$ resonance is observed in the ${\Xi }^{-}{\overline{\Xi }}^{+}$ final states, and upper limits at the 90% confidence level on ${\Gamma }_{ee}B$ for the processes $\psi \left(4230\right)/\psi \left(4260\right)\to {\Xi }^{-}{\overline{\Xi }}^{+}$ are determined. In addition, an excited $\Xi $ baryon at $1820\text{}\text{}\mathrm{MeV}/{c}^{2}$ is observed with a statistical significance of $6.2–6.5\sigma $ by including the systematic uncertainty, and the mass and width are measured to be $M=\left(1825.5±4.7±4.7\right)\text{}\text{}\mathrm{MeV}/{c}^{2}$ and $\Gamma =\left(17.0±15.0±7.9\right)\text{}\text{}\mathrm{MeV}$, which confirms the existence of the ${J}^{P}={\frac{3}{2}}^{-}$ state $\Xi \left(1820\right)$.