Polarizing mechanisms for stored $p$ and $\bar p$ beams interacting with   a polarized target
  Kinetics of the polarization buildup at the interaction of stored protons (antiprotons) with a polarized target is considered. It is demonstrated that for small scattering angles, when a projectile remains in the beam, the polarization buildup is completely due to the spin-flip transitions. The corresponding cross sections turn out to be negligibly small for a hydrogen gas target as well as for a pure electron target. For the latter, the filtering mechanism also does not provide a noticeable beam polarization. 
A.I.Milstein and V.M.Strakhovenko