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)
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