Polarization observables in 3He(e,e'n) and the neutron electric form factor

Abstract

In the internal target facility of the MEA/AmPS electron accelerator and storage ring we measured the longitudinal A’z and sideways Ax spin-spin correlation functions (or “asymmetries”) for quasi-elastic neutron knock-out from 3Helium at Q2 = 0.2 GeV2/c2. The 720 MeV electron beam had an average longitudinal polarization of 60% and the average target polarizaton was 50%. Scattered electrons were detected in a large acceptance magnetic spectrometer and neutrons were detected by using the time-of- flight (TOF) technique. The large A’z asymmetry is theoretically well-understood and was mainly used to investigate systematic uncertainties. It turned out that the value of the asymmetry depends on the beam current for currents larger than 100 mA. This can partly be explained by current-dependent depolarization of the electron beam. The remaining variation is accounted for in the systematic error. A’x asymmetry depends (in theoretical calculations) on the electric form factor of the neutron. Using the continuous wave Faddeev calculations performed by the Bochum nuclear theory group we would obtain a value for GnE of 0.031 + 0.034 (purely statistical error is 0.023). However, the induced asymmetry A0y was also measured, using the same setup. The agreement of the Faddeev calculation of A0y with the experimental data is not satisfactory. This implies a theoretical uncertainty for the extracted value for GnE, which is hard to estimate quantitatively