Probing the Strangeness Content of the Proton and the Neutron Radius of 208Pb using Parity-Violating Electron Scattering

Precision measurement of the parity-violating asymmetry in electron-hadron scattering has been an extremely useful tool to study the structure of the hadrons. This thesis reports work on two such measurements: the measurement of the strange formfactors (FFs), G s E and Gs M , by the Hall A Proton Parity Experiment (HAPPEX)-III collaboration, and the first direct measurement of the nucleon skin thickness in a heavy nucleus by the Lead ( 208 Pb) Radius Experiment (PREX) collaboration. In HAPPEX-III, the parity-violating cross-section asymmetry in the elastic scattering of polarized electrons from unpolarized protons was measured at an average four-momentum transfer squared, 〈Q 2 〉, of 0.624 GeV 2 . A parity-violating asymmetry, A PV , of −23.80±0.78(stat)±0.36(syst) parts-per-million (ppm) was measured, which corresponds to a linear combination of the strange FFs, G s E + 0.517Gs M = 0.003±0.010(stat)±0.004(syst)±0.009(ff). The errors stat and syst are experimental statistical and systematic errors respectively. The error ff arises due to limits on precision of the electromagnetic FFs and radiative corrections. This result is consistent with zero contribution from the strange quarks to the proton FFs. Combined with the existing data on strange FFs, this result constrains the contribution of the strange FFs to the nucleon FFs to a few percent of the nucleon FFs. In PREX, the parity-violating asymmetry in the elastic scattering of polarized electrons from unpolarized 208 Pb was measured at an average 〈Q 2 〉 of 0.0088 GeV 2 . A parity-violating asymmetry, A PV , of 656±60(stat)±14(syst) parts-per-billion (ppb) was measured, which corresponds to a difference between the neutron and proton distribution radii in the 208 Pb nucleus of R n −R p = 0.33 +0.16 −0.18 fm. This result is the first electroweak evidence supporting the existence of a neutron skin in a neutron-rich nucleus. One class of systematic uncertainty that both of these experiments were sensitive to is the non-parity violating asymmetries that resulted from the helicity-correlated (pulse-to-pulse) differences in the electron beam parameters. Therefore, considerable effort was invested to understand, and suppress the asymmetries arising from these effects. Although both HAPPEX-III and PREX benefited from this effort, the result of this work was much more important for PREX due to its much higher precision goal compared to that of HAPPEX-III.

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