Global $Λ$ polarization in heavy-ion collisions from a transport model

Abstract

The polarizations of $Λ$ and $\barΛ$ hyperons are important quantities in extracting the fluid vorticity of the strongly coupled quark gluon plasma and the magnitude of the magnetic field created in off-central heavy-ion collisions, through the spin-vorticity and spin-magnetic coupling. We computed the energy dependence of the global $Λ$ polarization in off-central Au+Au collisions in the energy range $\sqrt{s_{NN}}=7.7-200$ GeV using a multiphase transport model. The observed polarizations with two different impact parameters agree quantitatively with recent STAR measurements. The energy dependence of the global $Λ$ polarization is decomposed as energy dependence of the $Λ$ distribution at hadronization and the space-time distribution of the fluid-vorticity field. The visualization of both the $Λ$ distribution and the fluid-vorticity field show a smaller tilt at higher collisional energies, which indicates that the smaller global polarization at higher collisional energies is caused by a smaller angular momentum deposition at midrapidity.