Measurement of the Spin–Dependence of the $\overline{p}-p$ Interaction at the AD–Ring

We propose to use an internal polarized hydrogen storage cell gas target in the AD ring to determine for the first time the two total spin–dependent pbar-p cross sections σ1 and σ2 at antiproton beam energies in the range from 50 to 450 MeV. The data obtained are of interest by themselves for the general theory of pbar-p interactions since they will provide a first experimental constraint of the spin–spin dependence of the nucleon–antinucleon potential in the energy range of interest. In addition, measurements of the polarization buildup of stored antiprotons are required to define the optimum parameters of a future, dedicated Antiproton Polarizer Ring (APR), intended to feed a double–polarized asymmetric pbar-p collider with polarized antiprotons. Such a machine has recently been proposed by the PAX collaboration for the new Facility for Antiproton and Ion Research (FAIR) at GSI in Darmstadt, Germany. The availability of an intense stored beam of polarized antiprotons will provide access to a wealth of single– and double–spin observables, thereby opening a new window on QCD spin physics. A recent experiment at COSY revealed that ep spin–flip cross sections are too small to cause a detectable depolarization of a stored proton beam. This measurement rules out a proposal to use polarized positrons to polarize an antiproton beam by e+pbar spin–flip interactions. Thus, our approach to provide a beam of polarized antiprotons is based on spin filtering, using an internal polarized hydrogen gas target – a method that has been tested with stored protons. We expect to produce a polarized antiproton beam with at least ten orders of magnitude higher intensity than a secondary polarized antiproton beam previously available. Provided that antiproton beams with a polarization of about 15% can be obtained with the APR, the antiproton machine at FAIR (the High Energy Storage Ring) could be converted into a double–polarized asymmetric pbar-p collider by installation of an additional COSY–like ring. In this setup, antiprotons of 3.5 GeV/c collide with protons of 15 GeV/c at c.m. energies of √s ≈ √200 GeV with a luminosity in excess of 10^31 cm−2s−1. The PAX physics program proposed for FAIR has been highly rated, and would include, most importantly, a first direct 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 G_E,M of the proton.