Global polarization of Λ hyperons in Au+Au collisions at ^√s_NN = 200 GeV

Global polarization of Λ hyperons has been measured to be of the order of a few tenths of a percent in Au+Au collisions at ^√s_NN = 200 GeV, with no significant difference between Λ and Λ. ^¯ These new results reveal the collision energy dependence of the global polarization together with the results previously observed at ^√s_NN = 7.7 - 62.4 GeV and indicate noticeable vorticity of the medium created in non-central heavy-ion collisions at the highest RHIC collision energy. The signal is in rough quantitative agreement with the theoretical predictions from a hydrodynamic model and from the AMPT (A Multi-Phase Transport) model. The polarization is larger in more peripheral collisions, and depends weakly on the hyperon's transverse momentum and pseudorapidity η^H within |η^H| < 1. An indication of the polarization dependence on the event-by-event charge asymmetry is observed at the 2σ level, suggesting a possible contribution to the polarization from the axial current induced by the initial magnetic field.