Measurement of the Neutron Electric Form FactorGE,nin the QuasifreeH2(e→,e′n→)pReaction

The electric form factor of the neutron $G_{E,n}$ has been measured in the quasifree ${}^2\mathrm{H}\left(\overrightarrow{e}{,e}^{\prime }\overrightarrow{n}\right)p$ reaction using the 855 MeV polarized cw electron beam of the Mainz Microtron MAMI. The polarization of the scattered neutrons was analyzed in a polarimeter consisting of two walls of plastic scintillators. The precession of the neutron spin in a magnetic field was used for the first time to circumvent the measurement of the effective analyzing power of the polarimeter and the beam polarization. In this way $G_{E,n}$ could be determined with little model dependence and experimental uncertainties. The result $G_{E,n}\left(0.34\mathrm{GeV}{}^2/c^2\right)=0.0611\mathrm{}\pm {0.0069}_{\mathrm{stat}}(\{+0.0069\mathrm{}\}\{-0.0055\mathrm{}\}{)}_{\mathrm{syst}}$ is larger than previously assumed.