Uncertainties in G

An accurate knowledge of the magnetic form factor G is of particular importance for a determination of G because the relevant term of the measured asymmetry is proportional to G G. The present accuracy of G is not better than 10% due to the systematic uncertainties resulting from the deuteron model required to extract G from quasielastic d(e,e') and d(e,e'p) data.

Part of the collaboration is involved in experiments using a method that is not subject to these limitations. These experiments determine G via a measurement of the cross section ratio R = d(e,e'n)/d(e,e'p) in quasifree kinematics which is independent of the deuteron model used because the initial momentum distribution for proton and neutron are identical. The price one pays is that in such a measurement the absolute neutron detection efficiency of the detector must be known with great precision.

Such an approach was explored for the first time in an experiment at the NIKHEF electron accelerator at Q -values of 0.09 and 0.13 GeV/c . The neutron detector was calibrated at the neutron beam facility at PSI in Switzerland. The experiment will produce data with an accuracy of <3% in G. These measurements will be extended to cover a Q - range of 0.1 - 0.8 GeV/c using the electron and neutron beams at Mainz and PSI. Thus very precise data for G will be available at the time they are needed for the present experiment.