Abstract

Global polarization of Lambda hyperons has been measured to be of the order of a few tenths of a percentage in Au+Au collisions at root s(NN) = 200 GeV, with no significant difference between Lambda and (Lambda) over bar. These new results reveal the collision energy dependence of the global polarization together with the results previously observed at root s(NN) = 7.7-62.4 GeV and indicate noticeable vorticity of the medium created in noncentral heavy-ion collisions at the highest Relativistic Heavy Ion Collider collision energy. The signal is in rough quantitative agreement with the theoretical predictions from a hydrodynamic model and from a multi-phase transport model. The polarization is larger in more peripheral collisions, and depends weakly on the hyperon's transverse momentum and pseudorapidity eta(H) within vertical bar eta(H)vertical bar 1. An indication of the polarization dependence on the event-by-event charge asymmetry is observed at the 2 sigma level, suggesting a possible contribution to the polarization from the axial current induced by the initial magnetic field.