Polarizing stored beams by interaction with polarized electrons.

A polarized, internal electron target gradually polarizes a proton beam in a storage ring. Here, we derive the spin-transfer cross section for e (p, p)e scattering. A recent measurement of the polarizing effect of a polarized atomic hydrogen target is explained when the effect of the atomic electrons is included. We also consider the interaction of a stored beam with a pure electron target which can be realized either by a comoving electron beam or by trapping of electrons in a potential well. In the future, this could provide a practical way to polarize antiprotons.