Measurement of the neutron magnetic form factor from inclusive quasielastic scattering of polarized electrons from polarized 3HE

[摘要] The results of a measurement of the spin-dependent asymmetry in quasielastic scattering of longitudinally polarized electron from polarized 3He target are eported. The neutron magnetic form factor at Q2=0.19 (GeV/c)2 has been extracted from the measured asymmetry and is reported in this thesis work. The experiment was performed at the MIT-Bates Linear Accelerator Center with an incident electron beam energy of 370 MeV. Two magnetic spectrometers, MEPS and OHIPS, were used to detect the scattered electrons independently, with MEPS at the kinematics maximally sensitive to the spin-dependent transverse response function, RT', at Q2=0.19 (GeV/c)2 and OHIPS at the kinematics maximally sensitive to the spin-dependent transverse-longitudinal response function, RTL', at Q2=0.14 (GeV/c)2. The elastic asymmetry and the quasielastic asymmetry ATL' in the threshold region for three-body breakup have also been measured in OHIPS and reported. A cryogenic polarized 3He gas target using a metastability-exchange optical pumping technique was employed in this experiment. The target was operated at 13K during the experiment with an average target thickness of 3.3x10(18)cm(-2) MEPS and 1.6x10(19)cm(-2) (OHIPS). The average target polarization with 25 µA average beam current was over 38%. A GaAs source produced the polarized electrons and a Wien spin rotator was employed to rotate the spin to give longitudinally polarized electrons at the 3He target. The average beam polarization for the experiment was 36.5%.The measured 3He elastic asymmetry agrees very well with the calculated values using the 3He form factors measured by Rosenbluth separation. The measured spin-dependent transverse asymmetry, AT', which is expected to be sensitive to the neutron magnetic form factor is in good agreement with recent PWIA calculations using a spin-dependent spectral function. The neutron magnetic form factor at Q2=0.19(GeV/c)2 has been extracted from the measured asymmetry based on the recent PWIA calculations. The extracted GnM value agrees very well with the standard dipole parametrization at this Q2. This experiment represents the first measurement of the neutron magnetic form factor using spin-dependent electron scattering. The uncertainty of the extracted GnM2 is dominated by the statistical uncertainty and the uncertainty due to model dependence is found to be comparatively small. The measured ATL' in the threshold region has shown very large positive asymmetry close to the three-body breakup threshold. To interpret the AT' data, a complete calculation of the 3He asymmetry ATL' in the elastic-threshold region using a spin-dependent spectral function with final state interactions (FSI) and meson exchange currents (MEC) taken into account is probably necessary.

[发布机构] University:California Institute of Technology;Department:Physics, Mathematics and Astronomy

[关键词] Physics