Hydrodynamic study of hyperon spin polarization in relativistic heavy ion collisions

We perform a systematic study of the spin polarization of hyperons in heavy ion collisions using the music hydrodynamic model with a multiphase transport preequilibrium dynamics. Our model calculations nicely describe the measured collision energy, centrality, rapidity, and pₓ dependence of the polarization. We also study and predict the global spin polarization of ^- and ^- as a function of collision energy, which provides a baseline for the studies of the magnetic moment, spin, and mass dependence of the spin polarization. For the local spin polarization, we calculate the radial and azimuthal components of the transverse polarization and find specific modulating behavior which could reflect the circular vortical structure. However, our model fails to describe the azimuthal-angle dependence of the longitudinal and transverse polarizations, which indicates that the hydrodynamic framework with the spin Cooper-Frye formula under the assumption of thermal equilibrium of the spin degree of freedom needs to be improved.