SPIN Physics at GSI

Polarized antiprotons produced by spin filtering with an internal polarized gas target provide access to a wealth of single-- and double--spin observables, thereby opening a window to physics uniquely accessible with the HESR at FAIR. This includes a first measurement of the transversity distribution of the valence quarks in the proton, a test of the predicted opposite sign of the Sivers--function, related to the quark distribution inside a transversely polarized nucleon, in Drell--Yan (DY) as compared to semi--inclusive DIS, and a first measurement of the moduli and the relative phase of the time--like electric and magnetic form factors G_{E,M} of the proton. In polarized and unpolarized proton--antiproton elastic scattering open questions like the contribution from the odd charge--symmetry Landshoff--mechanism at large |t| and spin--effects in the extraction of the forward scattering amplitude at low |t| can be addressed.