First measurement of the electric formfactor of the neutron in the exclusive quasielastic scattering of polarized electrons from polarized 3He

A first measurement of the asymmetry in quasielastic scattering of longitudinally polarized electrons from a polarized 3He gas target in coincidence with the knocked out neutron is reported. This measurement was made feasible by the cw beam of the 855 meV Mainz Microtron MAMI. It allows a determination of the electric formfactor of the neutron GnE independent of binding effects to first order. At Q2=0.31 (GeV/c)2 two asymmetries A∥(S↘He∥q↘) and A⊥(S↘He⊥q↘) have been measured giving A∥=(−7.40±0.73%) and A⊥=(0.89±0.30)%. The ratio A⊥/A∥ is independent of the absolute value of the electron and target polarization and yields GnE=0.035±0.012±0.005.