Measurement of the Cross Section for e+e−→Ξ−¯Ξ+ 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 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−→Ξ−¯Ξ+ by means of a single baryon-tag method. After performing a fit to the dressed cross section of e+e−→Ξ−¯Ξ+, no significant ψ(4230) or ψ(4260) resonance is observed in the Ξ−¯Ξ+ final states, and upper limits at the 90% confidence level on ΓeeB for the processes ψ(4230)/ψ(4260)→Ξ−¯Ξ+ are determined. In addition, an excited Ξ baryon at 1820 MeV/c2 is observed with a statistical significance of 6.2–6.5σ by including the systematic uncertainty, and the mass and width are measured to be M=(1825.5±4.7±4.7) MeV/c2 and Γ=(17.0±15.0±7.9) MeV, which confirms the existence of the JP=32− state Ξ(1820)