Final Report Summary - POLPBAR (Production of Polarized Antiprotons) The project POLPBAR aims to provide a method to produce a beam of polarized antiprotons. The availability of such beams will enable new categories of measurements in single and double polarized antiproton-proton scattering to, e.g. determine the nucleon spin structure - for example at the upcoming Facility for Antiproton and Ion Research (FAIR) at GSI (Darmstadt). It could also form the basis for a future upgrade of the High Energy Storage Ring (HESR) of FAIR into an (asymmetric) proton-antiproton colloder. Though the science case is compelling, the provision of polarized antiproton beams presents enormous scientific and technological challenges and, in fact, has never been achieved by state-of-the-art techniques up to now with intensities that are sufficient for the crucial experiments. Since the antiproton is an anti-particle, no conventional method (involving bulk matter) to align the spin vectors of an antiproton-ensemble is applicable - they would immediately annihilate with protons of the material and disappear. The method of choice is to use a stored beam of (originally) unpolarized particles, and - by interaction with a very thin polarized hydrogen atoms injected into a storage cell - slowly accumulate one preferred polarization state over the other (this is termed "spin-filtering"). From experiments we have performed within this project, we know that this method works for protons and there is every reason to expect that it will work for antiprotons as well, but the final experimental proof has not yet been provided: due to the non-availability of the currently only antiproton storage ring (AD at CERN), this will have to wait for a future opportunity.
