Antiproton polarization induced by lepton interactions

We present expressions for electromagnetic helicity amplitudes and spin observables for any elastic spin 1/2 - spin 1/2 scattering to first order in QED. In particular all electromagnetic helicity amplitudes and spin observables for elastic antiproton-electron and antiproton-proton scattering via single t-channel photon exchange are presented. Spin observables are required to describe the rate of increase of polarization in spin filtering. The PAX collaboration at GSI Darmstadt is interested in the buildup of polarization of an antiproton beam by repeated interaction with a hydrogen gas target in a storage ring. In order for the beam particles to remain in the ring after scattering it is important to consider small angle scattering, hence small momentum transfer t. In the low momentum transfer region electromagnetic effects dominate the hadronic effects. Of immediate importance is whether the polarization of an antiproton beam can be built up by spin filtering off polarized electrons either in a target or in a beam. We present the theoretical background for this discussion.