Global Polarization of $\mathrm{\Xi }$ and $\mathrm{\Omega }$ Hyperons in $\mathrm{Au}+\mathrm{Au}$ Collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$

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

Figure 1

Invariant mass distributions of ${\mathrm{\Xi }}^{-}$ (${\overline{\mathrm{\Xi }}}^{+}$) and ${\mathrm{\Omega }}^{-}$ (${\overline{\mathrm{\Omega }}}^{+}$) for 20%–80% centrality in $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$ taken in 2014. Vertical dashed lines indicate three standard deviations ($3\sigma$) from the peak positions, assuming a normal distribution.

Figure 2

The energy dependence of the hyperon global polarization measurements. The points corresponding to $\mathrm{\Lambda }$ and $\overline{\mathrm{\Lambda }}$ polarizations, as well as $\mathrm{\Xi }$ and $\mathrm{\Omega }$ points in $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{\mathrm{NN}}}=200\mathrm{GeV}$ are slightly shifted for clarity. Previous results from the STAR [4, 5, 41] and ALICE [9] experiments compared here are rescaled by new decay parameter indicated inside the figure. The data point for $\overline{\mathrm{\Lambda }}$ at 7.7 GeV is out of the axis range and indicated by an arrow with the value. The results of the AMPT model calculations [42] for 20%–50% centrality are shown by shaded bands where the band width corresponds to the uncertainty of the calculations.

Figure 3

The global polarization of $\mathrm{\Xi }$ hyperons obtained via measurements of the polarization of daughter $\mathrm{\Lambda }$ hyperons as a function of the collision centrality in $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$. Open boxes show the systematic uncertainties. Results for the inclusive $\mathrm{\Lambda }$ measurements [5] are shown for comparison.