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 ≈ 0.001 the cross section for spin exchange is as large as 〈σPzz〉 ≈ 2 ·10 barn as shown by new QED-calculations. Two possibilities to provide 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 · 10/(mm s) appears to be feasible by means of a radioactive C dc-source. A more involved proposal is the production of polarised antiprotons 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 (100 MeV) as well as at high (1 GeV) energy storage rings providing a time of about one hour for polarisation build-up of about 10 antiprotons. A comparison with other proposals show a factor of about a hundred gain in the figure-ofmerit. PACS. 13.88.+e Polarisation in interactions and scattering – 29.20.Dh Storage rings – 29.25.Bx Electron sources – 29.27.Hj Polarised beams