JRA12-SPINFORFAIR: Spin for FAIR

The physics potential for studies with polarized antiprotons is enormous. The flag-ship experiment, Drell- Yan production, in double polarized proton-antiproton annihilation, will provide direct access to the transverse spin structure of the nucleon. In this perspective the implementation of a double polarized proton-antiproton collider at the coming FAIR facility, as a long-term upgrade, would open new and unique research opportunities for spin-physics with polarized antiprotons.

So far no antiproton beams with sizeable polarization could be produced. It is the aim of the PAX Collaboration to develop an efficient method for polarizing antiproton beams by in-situ build-up in a storage ring. The only viable way to do this effectively is by “spin-filtering” by the repeated interaction of an antiproton beam with a polarized hydrogen gas target in a cooler storage ring, selectively discarding more particles in one of the two spin states.

In the framework of the I3HP3 project, the PAX collaboration has successfully performed a spin-filtering measurement with protons at the COSY-ring by using a transverse polarized hydrogen target. The COSY measurement, actually a determination of the transverse spin-dependent polarization build-up cross section, proves that spin-filtering can be considered as a method to polarize a stored beam and that the interpretation of the polarization build-up mechanism in terms of the proton-proton interaction is valid. With the present JRA, PAX intends to make use of the unique environment offered by the COSY ring to transfer this method to longitudinal polarization. This point is motivated by the fact that all the different models for spin-filtering predict a significantly higher degree of polarization for the longitudinal case than for the transverse one. For the optimization of the polarization buildup it is therefore necessary to study both cases in conjunction. In addition, this approach represents the only way to obtain the relevant spin-dependent cross-sections for producing polarized antiprotons. Afterwards the PAX collaboration is ready to perform the corresponding experiments with antiprotons.