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The Neutron Electric Form Factor at Q2 up to 7 (GeV/c)2 from the Reaction 2H(e,e′n)1H via Recoil Polarimetry

We propose to extend our previous measurements of GnE from deuterium to Q2 = 6.88 (GeV/c)2 . Additional measurements at 5.22 and 3.95 (GeV/c)2 will provide continuity with our prior measurements up to Q2 = 1.45 (GeV/c)2 , and overlap with recent measurements from a polarized 3He target.

The JLab E93-038 collaboration measured GnE from the d(e, e′ n)p reaction on a liquid deuterium target at Q2 values of 0.45, 1.13, and 1.45 (GeV/c)2 . The experiment used a high-luminosity neutron polarimeter and the dipole neutron-spin-precession magnet [Charybdis] to measure the ratio of two scattering asymmetries associated with positive and negative precessions of the neutron polarization vector. In this ratio technique, systematic uncertainties are extremely small because the analyzing power of the polarimeter cancels in the ratio, and sensitivity to the beam polarization is reduced because it depends only on the small drift in polarization between sequential measurements. Use of a deuteron target yields a better separation between quasielastic and inelastic events, as well as a smaller proton background which must be cleanly separated from the neutron scattering events. Finally, the reaction mechanism and nuclear physics corrections [for FSI, MEC, and IC] are best understood and can be most reliably corrected for in the deuteron. The combination of these advantages is what yields 2–3% systematic uncertainties for the recoil polarization measurement, while the polarized 3He measurements typically have ≈10% systematic uncertainties.

The primary motivation for this proposed experiment is the ability to measure a fundamental quantity of the neutron -- one of the basic building blocks of matter. A successful model of confinement must be able to predict both neutron and proton electromagnetic form factors simultaneously. The neutron electric form factor is especially sensitive to the nucleon wave function, and differences between model predictions for GnE tend to increase rapidly with Q2 . Calculations and fits to the data up to 1.45 (GeV/c)2 show significant quantitative differences in the few (GeV/c)2 range, and make qualitatively different predictions for the behavior of GnE at higher Q2 values, with some showing GnE falling off more slowly than GnE, and others showing GnE falling rapidly to zero and becoming negative. The proposed measurements of GnE will be able to challenge theoretical calculations, including both models and new rigorous lattice QCD calculations, with a focus on the high Q2 range where the models of the nucleon are generally meant to be more complete. Finally, these measurements of GnE are also needed to understand electron scattering experiments that probe electric structure functions at high Q2, and will be important for the analysis of precision few-body data from measurements at Jefferson Lab.