Physics with Polarized Antiprotons

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, and a first measurement of the moduli and the relative phase of the time–like electric and magnetic form factors GE,M of the proton. In polarized and unpolarized 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.