Paper detail

Measurement of the cross section for $e^{+}e^{-}\rightarrowΞ^{-}\barΞ^{+}$ and observation of an excited $Ξ$ baryon

Using a total of 11.0 fb$^{-1}$ of $e^{+}e^{-}$ collision data with center-of-mass energies between 4.009 GeV 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 $e^{+}e^{-}\rightarrowΞ^{-}\barΞ^{+}$ by means of a single baryon-tag method. After performing a fit to the dressed cross section of $e^{+}e^{-}\rightarrowΞ^{-}\barΞ^{+}$, no significant $ψ(4230)$ or $ψ(4260)$ resonance is observed in the $Ξ^{-}\barΞ^{+}$ final states, and upper limits at the 90\% confidence level on $Γ_{ee}\mathcal{B}$ for the processes $ψ(4230)$/$ψ(4260)\rightarrowΞ^{-}\barΞ^{+}$ are determined. In addition, an excited $Ξ$ baryon at 1820 MeV/$c^{2}$ is observed with a statistical significance of 6.2 $\sim$ 6.5$σ$ by including the systematic uncertainty, and the mass and width are measured to be $M = (1825.5 \pm 4.7 \pm 4.7)$~MeV/$c^{2}$ and $Γ= (17.0 \pm 15.0 \pm 7.9)$~MeV, which confirms the existence of the $J^{P}=\frac{3}{2}^{-}$ state $Ξ(1820)$.