Violation de parité et contenu étrange du nucléon : expériences G0 et SAMPLE

Abstract : The nucleon is described by QCD as being composed of three valence quarks (of flavor up and down) and a sea of gluons and quark – antiquark pairs of any flavor. The strange quark is the best candidate for the study of the sea quarks whose contribution to the nucleon structure is poorly known. Indeed, this quark is the lightest one having no valence contribution. Experimentally, this study can be done in three different sectors related to the mass, to the charge and current distributions, and to the spin of the nucleon. This thesis presents the G0 experiment at the Thomas Jefferson laboratory (USA) and the SAMPLE experiment at the MIT-Bates which measure the asymmetry of parity violation in elastic scattering of polarized electrons on a nucleon target. These experiments are performed at low energy, thus the asymmetry measured is of the order of 10-6. The sources of systematic effects should be found in order to decrease the influence of the related non physical asymmetries. From these measurements, we are able to extract the strange quark contribution to the charge and current distributions inside the nucleon as well as the axial form factor. This thesis describes the formalism related to the electroweak probe and give an overview of the existing theoretical predictions. The origins of the systematic changes in the electron beam properties have been studied and we developed feedback systems in order to reduce their effect for parity violation experiment. We detail the G0 setup, the background studies and their implications for the experimental strategy. We finally present the SAMPLE experiment and the data analysis which led to the determination of the axial form factor of the nucleon. This quantity is poorly described by theory due to higher order contributions which are difficult to calculate theoretically. This form factor has a contribution to the asymmetry therefore, it is important to measure it experimentally.
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Theses
Nuclear Theory [nucl-th]. Université Joseph-Fourier - Grenoble I, 2002. French


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Contributor : Raphael Tieulent <>
Submitted on : Friday, June 28, 2002 - 11:26:48 AM
Last modification on : Friday, June 28, 2002 - 11:26:48 AM

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Raphaël Tieulent. Violation de parité et contenu étrange du nucléon : expériences G0 et SAMPLE. Nuclear Theory [nucl-th]. Université Joseph-Fourier - Grenoble I, 2002. French. <tel-00001466>

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