Measurement of the Neutron Electric to Magnetic Form Factor Ratio at Q2=1.58 GeV2 Using the Reaction He−→3(e⃗ ,e′n)pp

International audience; A measurement of beam helicity asymmetries in the reaction He−→3(e⃗ ,e′n)pp is performed at the Mainz Microtron in quasielastic kinematics to determine the electric to magnetic form factor ratio of the neutron GnE/GnM at a four-momentum transfer Q2=1.58 GeV2. Longitudinally polarized electrons are scattered on a highly polarized He3 gas target. The scattered electrons are detected with a high-resolution magnetic spectrometer, and the ejected neutrons are detected with a dedicated neutron detector composed of scintillator bars. To reduce systematic errors, data are taken for four different target polarization orientations allowing the determination of GnE/GnM from a double ratio. We find μnGnE/GnM=0.250±0.058(stat)±0.017(syst).