Measurement of the depolarizing vectorpe cross section using co-moving electrons

In order to perform experiments with polarised antiprotons (PAX-Collaboration) one needs an effective way to produce a stored polarized antiproton beam. FILTEX showed that an unpolarised proton beam is polarised when travelling through a polarised hydrogen target. For this result there exist two competing theoretical explanations. One with a strong contribution from electrons while the other sees an almost exact cancellation of all electron effects. New calculations by Walcher and Arenhoevel suggest a very large cross section for the spin-exchange between protons and electrons of left angle σPzz right angle ∼2.1013 barn at small relative velocities T∼ 1.7 keV. First results of a depolarising measurement at these low energies are shown in this talk