Determination of the neutron electric form factor from the reaction 3He(e, e′n) at medium momentum transfer

The electric form factor of the neutron G(En) has been determined in double polarized exclusive He-3(e, e'n) scattering in quasi-elastic kinematics by measuring asymmetries A(perpendicular to), A(parallel to) of the cross section with respect to helicity reversal of the electron, with the nuclear spin being oriented perpendicular to the momentum transfer q in case of A(perpendicular to) and parallel in case of A(parallel to). The experiment was performed at the 855 MeV c. w. microtron MAMI at Mainz. The degree of polarization of the electron beam and of the gaseous He-3 target were each about 50 %. Scattered electrons and neutrons were detected in coincidence by detector arrays covering large solid ang les. Quasi-elastic scattering events were reconstructed from the measured electron scattering angles theta(e), phi(e) and the neutron momentum vector p'(n) in the plane wave impulse approximation. We obtain the result [G(En)] (0.27 less than or equal to Q(2)c(2)/GeV2 less than or equal to 0.5) = 0.0334 +/- 0.0033(stat) +/- 0.0028(syst) which is averaged over the indicated range of Q(2), the squared momentum transfer. This G(En) value is significantly smaller than measured from the D(e, e'n) reaction under similar kinematical conditions. To what extent final state interactions in He-3 quench the G(En) result is subject of calculations currently in progress elsewhere.