A precision measurement of the transverse asymmetry AT̳' from quasi-elastic ³sHe [right arrow] (e [right arrow], e') process and the neutron magnetic form factor Gn̳m̳ at low Q²

Abstract

Electromagnetic form factors are fundamental quantities in describing the underlying electromagnetic structure of nucleons. While proton electromagnetic form factors have been determined with good precision, neutron form factors are known poorly, largely due to the lack of free neutron targets. Jefferson Lab Hall A experiment E95-001, "a precise measurement of the transverse asymmetry AT' from the quasi-elastic 3He [right arrow](e [right arrow], e') process", was therefore designed to determine precisely the neutron magnetic form factor, GnM at low momentum transfer values and was successfully completed in Spring 1999. High precision AT, data in the quasi-elastic region at Q2 values of 0.1 to 0.6 (GeV/c)2 were obtained using a high-pressure spin-exchange optically-pumped polarized 3He gas target with an average polarization of 30%, a longitudinally polarized e- beam, and two High Resolution Spectrometers: HRSe and HRSh. HRSe was employed to detect scattered electrons from the quasi-elastic kinematic region, and HRSh was employed as a elastic polarimetry to monitor the product of the beam and target polarizations. The extraction of form factors is usually model-dependent. Significant constraints on theoretical calculations are provided by additional high precision quasi-elastic asymmetry data at Q2 values of 0.1 and 0.2 (GeV/c)2 in 3He breakup region, where effects of final state interactions(FSI) and meson exchange currents (MEC) are expected to be large [71]. GnM is extracted from a non-relativistic Faddeev calculation which includes both FSI and MEC at Q2 values of 0.1 and 0.2 (GeV/c)2.

(cont.) The uncertainties of GnM at these Q2 values are comparable to those of recent experiments with deuterium targets [58]. At the higher Q2 values from this experiment, GnM is extracted from Plane-Wave Impulsive Approximation (PWIA) calculations with a relatively large theoretical uncertainty of 2-4%. Thus a reliable extraction of GnM from T, at higher Q2 values (especially at Q2 values of 0.3 and 0.4 (GeV/c)2) requires improved theoretical calculations including FSI, MEC, and relativistic effects. However, those GnM results extracted from PWIA at higher Q2 values from this experiment still show overall a good agreement with the most recent deuterium measurements. The analysis of asymmetries and the extraction of GnM from both the Faddeev calculations and the PWIA calculations are reported in this thesis.