  We propose a microscopic description for the polarization from the first
principle through the spin-orbit coupling in particle collisions. The model is
different from previous ones based on local equilibrium assumptions for the
spin degree of freedom. It is based on scatterings of particles as wave
packets, an effective method to deal with particle scatterings at specified
impact parameters. The polarization is then the consequence of particle
collisions in a non-equilibrium state of spins. The spin-vorticity coupling
naturally emerges from the spin-orbit one encoded in polarized scattering
amplitudes of collisional integrals when one assumes local equilibrium in
momentum but not in spin.
