Spin in Heavy Ion Collisions: Theoretical Progress

Non-central heavy-ion collisions induce a huge global orbital angular momentum (OAM) perpendicular to the reaction plane. Due to the spin-orbit coupling, the initial OAM is partially converted to the spin angular momentum of particles in the quark-gluon plasma. This in turn results in phenomena such as polarizations of Λ hyperons and vector mesons, which have been measured in recent experiments. In this talk, we will review recent theoretical progresses on these phenomena. Microscopically, the generation of quark polarization is induced by nonlocal parton scatterings, which can be described by kinetic theories based on the Wigner function approach. Polarized hyperons and vector mesons are formed at the hadronization stage, described by quark coalescence model with spin degrees of freedom. Based on theoretical studies and numerical simulations, people found that the hyperons' global polarization is dominated by vorticity field, while their local polarization is dominated by the shear-stress tensor. On the other hand, the vector meson's spin alignment may be related to the anisotropic fluctuation of the strong field. Therefore measuring the spin phenomena in heavy-ion collisions provides an oppotunity to study properties of these fields in the quark-gluon plasma.