( A New Proposal to Jefferson Lab PAC 34 ) Measurement of the Neutron Electromagnetic Form Factor Ratio

We propose a measurement of the electromagnetic form factor ratio of the neutron, GE /G n M, at high four–momentum transfer values of Q 2 = 5.0, 6.8, and 10.2 GeV2 in double polarized semi-exclusive 3 −→ He(~e,e′n)pp scattering in quasi–elastic kinematics by measuring the transverse asymmetry, A⊥, of the cross section. This quantity can then be used to quickly extract the electric form factor, GE , as more precise high Q 2 GM data becomes available. Results from the recent JLab experiments E93-027, E04-108 for elastic electronproton scattering, using a recoil polarization technique, show remarkable features for the proton electric form factor at these momentum transfers, whereas no data on GE are available. Our previous measurement of GE in experiment E02-013 provided data up to Q2 of 3.5 GeV2, which more than doubles the previously covered Q2 range. The recently developed approach for calculations of exclusive reactions in the Q2range between 1 and 10 GeV2 using generalized parton distributions (GPD) relates these elastic form factors and the results from deep inelastic scattering and deeply virtual Compton scattering. Data for GE at high Q 2 are necessary, in particular, to constrain spin-flip GPDs at high momentum transfer. The experiment utilizes the polarized 3He target and the polarized JLab beam at beam energies of 4.4, 6.6, and 8.8 GeV. The electrons will be detected in the BigBite spectrometer with a new GEM based tracker and the neutrons in an array of scintillators. Because of the high kinetic energy of the neutrons, a high neutron detection efficiency with an excellent background suppression can be achieved. Separation of recoiling protons and neutrons will be performed magnetically. Within 55 days of beamtime the ratio GE /G n M can be measured to an accuracy better than ∆(GE /G n M)= 0.20 for these three values of Q 2. With accurate measurements of GM, this would correspond to ∆(GE /GD) = 0.07, or ∆G n E= 3×10 −4 for our highest Q2 point. Such a measurement would significantly increase our knowledge about a fundamental property of the neutron in a region where no data are available.