Parity Violation In Electron Scattering and Measurement of Parity Violating Asymmetry in 208Pb
Date of Award
Doctor of Philosophy (PhD)
Parity-Violating (PV) experiments are designed to study nuclear structure and the Standard Model (SM) of nuclear and particle physics by precisely measuring the PV asymmetries in scattering cross sections. The PV asymmetries measured with longitudinally polarized electrons scattering off unpolarized neutron-rich targets provide a model independent probe of neutron densities which is free from most strong interaction uncertainties. This dissertation mainly focusses on two such parity violating experiments, namely Lead (208Pb) Radius Experiment (PREX-II), and Calcium Radius Experiment (CREX).
The goal of PREX-II and CREX is the precise measurement of the radius of neutron distribution 𝑅𝑛 and thus the neutron skin thickness (Δ𝑅 = 𝑅𝑛 − 𝑅𝑝), given that the radius of the proton distribution (𝑅𝑝) is well known. This skin thickness provides very important information about nuclear structure. PREX-II is a follow-up experiment to PREX which was conducted in 2010 at Jefferson Lab (JLab) and has already established the existence of the neutron skin, but was not of sufficient precision to constraint any theoretical model due to the limited statistics. PREX delivered ~9% statistical uncertainty on the parity violating asymmetry (𝐴𝑃𝑉). 3% or better uncertainty was needed for 1% or better accuracy on the neutron skin thickness (Δ𝑅). The precise measurement of the neutron radius of 208Pb has important implications for nuclear structure models and their application in atomic and astrophysics. PREX-II has delivered the desired 3% uncertainty on 𝐴𝑃𝑉 which is sufficient for constraining the density dependence of the symmetry energy of neutron rich matter and help study neutron star structure, heavy ion collisions, and atomic parity violation experiments. PREX-II was successfully conducted at JLab in Hall A from June 17 through Sept. 9, 2019 and is able to reduce the statistical error of PREX with precision in blinded 𝐴𝑃𝑉 of 2.8%. The blinded physics asymmetry in PREX-II is measured to be 𝐴𝑃𝑉 = 537 𝑝𝑝𝑏 ± 2.8% (𝑠𝑡𝑎𝑡. ) ± 2.4% (𝑠𝑦𝑠𝑡. ). I have used regression analysis to get this result. A different approach like dithering analysis could result in slightly different result.
This dissertation also focuses on design and development of the radiation shielding of an upcoming MOLLER experiment to be conducted at JLab, Virginia using Monte-Carlo simulations. The MOLLER experiment will perform an ultra-precise measurement of the Weak Mixing Angle using parity-violating asymmetry in the elastic scattering of longitudinally polarized electrons off unpolarized electrons. A precise estimation of the radiation dose which can occur during any nuclear physics experiment is very important. This helps us to design and develop the shielding for nuclear physics experiments accordingly and prevent any catastrophic damage to equipment. The simulation for the radiation dose measurement was done for the MOLLER experiment and the radiation shielding was optimized accordingly which helps reducing the overall structural cost while maintaining a safe radiation dose. I was able to reduce the amount of lead used in the originally proposed design by ~93% maintaining the safe radiation dose, which corresponds to a ~53% reduction in total mass of the shielding.
Bhatta Pathak, Devaki, "" (2020). Dissertation. 889.