Date Thesis Awarded
Honors Thesis -- Open Access
Bachelors of Science (BS)
The goal of the PREx II and CREx experiments are to determine the size of the neutron distribution in 208Pb and 48Ca, respectively. Because these isotopes have a large discrepancy in between the number of neutrons and protons, theoretical models predict that some of the neutrons will arrange themselves in a “skin” on the nucleus’s surface. Prior results from the PREx I experiment confirmed the existence of the neutron skin in 208Pb. All of the mentioned experiments employ the technique of parity-violating electron scattering. Electron-nucleus weak interactions are dominated by electron-neutron scattering, which is an order of magnitude more probable than weak electron-proton scattering. As the weak force is parity violating, right-handed and left-handed electrons will scatter at different rates; these rates can be measured and used to calculate the parity-violating asymmetry. Such a measurement is complicated by the fact that changes in the electron beam’s properties (such as energy, position, or angle) may be systematically related to the beam’s parity and thus can yield false asymmetries.
A beam modulation system has been developed to measure and account for the effects of these changes. As the beam passes through the accelerator, the beam modulation system makes small changes, known as ‘kicks’, to its position and energy. The beam’s position and energy are then correlated with the size of the kick and detector response are then correlated to determine how changes in the beam parameters affect the final measurements. In order for this analysis to be possible, a sufficient number of the applied kicks must produce linearly independent responses in the beam position. The Electron Generation and Tracking software, known as elegant, can be used to simulate beam modulation with a given set of parameters, but its accuracy with respect to experimental data was unknown prior to the start of PREx II. This research has established that a qualitative analysis of elegant simulations can be used to predict whether a certain set of parameters will result in a feasible beam modulation analysis. However, a quantitative analysis of the simulations yields results that differ significantly from those observed in real data. The beam modulation system is a permanent fixture in the accelerator hardware and likely will be used in future experiments. Therefore, knowledge of elegant’s capabilities and limitations can be used to assist in the process of data collection for future experiments.
Metts, Carrington, "Studying and Correcting for Helicity-Correlated False Asymmetries in the PREx II and CREx Experiments" (2020). Undergraduate Honors Theses. William & Mary. Paper 1438.