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Doctor of Philosophy (PhD)




Paul A. Souder


beam modulation system, dithering analysis methodology, elastic electron scattering, helicity-correlated beam asymmetry, neutron radii in heavy nuclei, parity-violating asymmetry

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Over the past decade, in Hall-A of the Thomas Jefferson National Accelerator Facility (TJNAF), both the HAPPEX and PREx collaborations have carried out various high-precision polarized elastic electron scattering experiments to explore the nuclear structure, the nucleon form factor and the weak charge of proton and electron. They have done so through the technique of the parity-violating asymmetry measurement with limited theoretical uncertainties. My dissertation focuses on the study of nuclear structure, namely the thickness of the neutron skin, using elastic electron scattering experiments.

The direct measurement of the thickness of the neutron skin in heavy nuclei, where neutron are two-fold more than protons, constrains the slope of changes in binding energies of every single heavy nucleus with respect to the full nucleus density, including proton and neutron densities. In addition, a more precise description of the neutron density profile for each heavy nucleus can help us gain better understanding of nuclear binding energies and has astrophysical implications for neutron stars. As far as we know, the proton and charge RMS (root-mean-square) radii in heavy nuclei such as Lead (208/82 Pb) have been measured with an accuracy of 0.02 fm and 0.002 fm, respectively. However, there is no clear picture of the neutron density profile through a high precision neutron RMS radii measurement free from the strong interaction until now.

Through a series simulations, both theorists and experimentalists have studied the sensitivity of the parity-violating asymmetry to the extraction of the neutron radii in heavy nuclei. Under some specific conditions, for instance, a fixed scattering angle of 5 degrees and a fixed Q2 of 0.0088 GeV2, a 3% statistic uncertainty of parity-violating asymmetries corresponds to a merely 1% error of the neutron radii in Lead (208/82 Pb). That is, the uncertainties of neutron radii in Lead (208/82 Pb) is three-fold smaller than the error of the parity-violating asymmetry. Since Mar. 2010, we performed the first electroweak experiment to probe the neutron radii in Lead (208/82 Pb). The normalized parity-violating asymmetries, after addressing false asymmetries, background asymmetries, to the 90% partially polarized electron beam and the momentum-transfer (Q2) is 0.656 ± 0.06(stat) ± 0.014(sys) ppm (part-per-million), which corresponds to the thickness of the neutron skin of 0.33+0.16/-0.18 fm. One of the most significant systematic uncertainties results from the discrepancies in beam parameters such as position, angle and energy on the target, leading to the difference in the differential cross-section between two helicity states. The helicity-correlated (window-to-window or pulse-to-pulse) beam asymmetries thus arise. My primary contribution to this experiment is to establish an analysis strategy used to control the size of the helicity-correlated beam asymmetries during the data-taking period. This analysis is especially addressed in Chapter 6.

In sum, the neutron radii of 0.33+0.16/-0.18 fm in Lead (208/82 Pb) supports the existence of the neutron skin in the neutron-rich matter. A second future run will yield a much higher precision neutron radii measurement. Moreover, the strong correlation between the neutron skin in Lead (208/82 Pb) and the neutron start radius indicates an approach from nuclear physics to understand the astrophysical equation of state (EOS) for a neutron star.


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