Polarization dynamics from moment equations

We derive an expression for the local transverse polarization of a boost-invariant expanding system of massive particles, which involves a set of dynamical spin moments. Starting from spin kinetic theory, we obtain a closed set of equations of motion for these spin moments. These equations are valid during the full evolution of the system, from free streaming to local equilibrium, and can be used to study polarization phenomena in relativistic heavy-ion collisions.