A surprising method for polarising antiprotons

We propose a method for polarising antiprotons in a storage ring by means of a polarised positron beam moving parallel to the antiprotons. If the relative velocity is adjusted to v/c ≈ 0.002 the cross-section for spin-flip is as large as about 2 ^. 10¹³ barn as shown by new QED calculations of the triple spin cross-sections. Two possibilities for providing a positron source with sufficient flux density are presented. A polarised positron beam with a polarisation of 0.70 and a flux density of approximately 1.5 ^. 10¹⁰ /(mm² s) appears to be feasible by means of a radioactive ¹¹C dc-source. A more involved proposal is the production of polarised positrons by pair production with circularly polarised photons. It yields a polarisation of 0.76 and requires the injection into a small storage ring. Such polariser sources can be used at low (100MeV) as well as at high (1GeV) energy storage rings providing a time of about one hour for polarisation build-up of about 10¹⁰ antiprotons to a polarisation of about 0.18. A comparison with other proposals show a gain in the figure of merit by a factor of about ten.