Global polarization of $\hat{I}ž$ and $\hat{I}\text{c◯}$ hyperons in Au+Au collisions at $\sqrt{s_{{}_{NN}}}$ = 200 GeV

Abstract

Global polarization of $\hat{I}ž$ and $\hat{I}\text{c◯}$ hyperons has been measured for the first time in Au+Au collisions at $\sqrt{s_{{}_{NN}}}$ = 200 GeV. The measurements of the $\hat{I}{ž}^{-}$ and $\bar{\hat{I}}{ž}^{+}$ 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 $\hat{I}ž\to \hat{I}›+\ddot{I}€$, as well as by measuring the polarization of the daughter $\hat{I}›$-hyperon, polarized via polarization transfer from its parent. The polarization, obtained by combining the results from the two methods and averaged over $\hat{I}{ž}^{-}$ and $\bar{\hat{I}}{ž}^{+}$, is measured to be $⟨P_{\hat{I}}ž⟩=0.47\pm 0.10(stat.)\pm 0.23(syst.)\%$ for the collision centrality 20\%-80\%. The $⟨P_{\hat{I}}ž⟩$ is found to be slightly larger than the inclusive $\hat{I}›$ polarization and in reasonable agreement with a multi-phase transport model (AMPT). The $⟨P_{\hat{I}}ž⟩$ is found to follow the centrality dependence of the vorticity predicted in the model, increasing toward more peripheral collisions. The global polarization of $\hat{I}\text{c◯}$, $⟨P_{\hat{I}}\text{c◯}⟩=1.11\pm 0.87(stat.)\pm 1.97(syst.)\%$ was obtained by measuring the polarization of daughter $\hat{I}›$ in the decay $\hat{I}\text{c◯}\to \hat{I}›+K$, assuming the polarization transfer factor $C_{\hat{I}\text{c◯}\hat{I}›}=1$.

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