$\Lambda$ and $\overline{\Lambda}$ Freeze-Out Distributions and Global Polarizations in Au+Au Collisions

Abstract

The gold--gold collisions at $\sqrt{s_{NN}}=7.7$ and $11.5$GeV are simulated within the PHSD transport model. In each collision event, the spectator nucleons are separated and the fluidization procedure for the participants is performed. The local velocities are determined in the Landau frame and the kinematic and thermal vorticity fields are evaluated. We analyze the thermodynamic properties of the cells where $\Lambda$s and $\overline{\Lambda}$s were born or had their last interaction. Such cells contribute to the formation of the observed global polarization of hyperons induced by the thermal vorticity of the medium. The $\overline{\Lambda}$ polarization signal is found to be mainly determined by hot, dense, and highly vortical cells at the earlier stage of the collision, whereas the $\Lambda$ polarization signal is accumulated over the longer time and includes cells with lower vorticity. The calculated global polarizations for both $\Lambda$s and $\overline{\Lambda}$s agree well with the experimental finding by the STAR collaboration at energy $\sqrt{s_{NN}}=11.5$GeV. For collisions at $\sqrt{s_{NN}}=7.7$GeV, we can reproduce the STAR data for $\Lambda$ hyperons, but significantly underpredict the observed global polarization of $\overline{\Lambda}$. Furthermore, we consider the centrality dependence of the hyperon polarization in collisions at 7.7GeV. It increases with an increase of centrality, reaches a maximum at 65--75%, and then starts decreasing rapidly for peripheral collisions.