Global polarization of $\Xi$ and $\Omega$ hyperons in Au+Au collisions at $\sqrt{s_{{}_{NN}}}$ = 200 GeV

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 ${\bar{\Xi }}^{+}$ 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 $\Xi \to \Lambda +\pi$, 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 $⟨P_{\Xi }⟩=0.47\pm 0.10(stat.)\pm 0.23(syst.)\%$ for the collision centrality 20\%-80\%. The $⟨P_{\Xi }⟩$ is found to be slightly larger than the inclusive $\Lambda$ polarization and in reasonable agreement with a multi-phase transport model (AMPT). The $⟨P_{\Xi }⟩$ is found to follow the centrality dependence of the vorticity predicted in the model, increasing toward more peripheral collisions. The global polarization of $\Omega$, $⟨P_{\Omega }⟩=1.11\pm 0.87(stat.)\pm 1.97(syst.)\%$ was obtained by measuring the polarization of daughter $\Lambda$ in the decay $\Omega \to \Lambda +K$, assuming the polarization transfer factor $C_{\Omega \Lambda }=1$.

7 pages, 3 figures, Accepted for publication in Physical Review Letters

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