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STAR
Abstract:
Global polarization of Xi and Omega hyperons has been measured for the first time in Au+Au collisions at sqrt{s_NN} = 200 GeV. The measurements of the Xi- and barXi+ hyperon polarization have been performed by two independent methods, via analysis of the angular distribution of the daughter particles in the parity violating weak decay Xi\rightarrow\Lambda+\pi, as well as by measuring the polarization of the daughter Lambda-hyperon, polarized via polarization transfer from its parent. The polarization, obtained by combining the results from the two methods and averaged over Xi^- and bar{Xi}^+, is measured to be langle P_\Xi \rangle = 0.47\pm0.10~({\rm stat.})\pm0.23~({\rm syst.})\,\% for the collision centrality 20\%-80\%. The langle P_\Xi \rangle is found to be slightly larger than the inclusive Lambda polarization and in reasonable agreement with a multi-phase transport model (AMPT). The langle P_\Xi \rangle is found to follow the centrality dependence of the vorticity predicted in the model, increasing toward more peripheral collisions. The global polarization of Omega, langle P_\Omega \rangle = 1.11\pm0.87~({\rm stat.})\pm1.97~({\rm syst.})\,\% was obtained by measuring the polarization of daughter Lambda in the decay Omega \rightarrow \Lambda + K, assuming the polarization transfer factor C_{\Omega\Lambda}=1.
