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 new window to physics uniquely accessible at the High Energy Storage Ring (HESR) at the Facility of Antiproton and Ion Research (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 GE,M of the proton. In polarized and unpolarized p pbar 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.

The proposed detector consists of a large-angle apparatus optimized for the detection of DY electron pairs and a forward dipole spectrometer with excellent particle identifcation. The design and performance of the new components, required for the polarized antiproton program, are outlined. A low-energy Antiproton Polarizer Ring (APR) yields an antiproton beam polarization of Ppbar = 0.3 to 0.4 after about two beam life times, which is of the order of 5-10 h. By using an internal H polarized target and a detector installed in a 3.5 GeV/c Cooler Synchrotron Ring (CSR), the Phase-I experimental pbar p (polarized) program could start in 2014, completely independent of the operation of the HESR. In Phase-II, the CSR serves as an injector for the polarized antiprotons into the HESR. A chicane system inside the HESR is proposed to guide the high-energy pbar (polarized) beam to the PAX detector, located inside the CSR straight section. In Phase-II, fixed-target or collider pbar p (polarized) experiments over a broad energy range become possible. In the collider mode, polarized protons stored in the CSR up to momenta of 3.5 GeV/c are bombarded head-on with 15 GeV/c polarized antiprotons stored in the HESR. This asymmetric double-polarized antiproton-proton collider is ideally suited to map e.g. the transversity distribution in the proton.

Antiproton-Proton Scattering Experiments with Polarization (PAX Collaboration)