Global polarization of $Λ$, $Ξ$, and $Ω$ hyperons in Au+Au collisions from the RHIC Beam Energy Scan-II

Abstract

Global polarization of $Λ$, $Ξ$, and $Ω$ hyperons is measured in Au+Au collisions at $\sqrt{s_{\mathrm{NN}}}=7.7$, 9.2, 11.5, 14.6, 17.3, 19.6, and 27~GeV using high-statistics data from the second phase of the RHIC Beam Energy Scan. No significant difference between the $Λ$ and $\barΛ$ polarizations is observed over the measured collision-energy range, providing improved constraints on magnetic-field-induced contributions. The $Λ$ polarization increases with the magnitude of rapidity, with the slope of the linear fit differing from zero by 4.8 standard deviations, providing the first evidence for a nonzero rapidity dependence of global polarization in this energy range. Global polarization of $Ξ^{-}+\barΞ^{+}$ is observed for the first time at these energies at a significance exceeding 5 standard deviations, with a magnitude comparable to that of $Λ+\barΛ$ and an increase toward lower collision energies. Measurements of $Ω^{-}+\barΩ^{+}$ are compatible with a similar energy dependence and favor the decay parameter $γ_Ω= +1$ with high probability under the assumption of the global polarization picture. These measurements provide new constraints on the energy evolution, rapidity dependence, and species dependence of global polarization in heavy-ion collisions.