Measurement of the generalized polarizabilities of the proton by virtual Compton scattering at MAMI and Q 2 = 0.2 GeV 2

Abstract

This work presents the measurement of the generalized electric αE (Q2) and magnetic βM (Q2) polarisabilities (GPs) of the proton. The GPs are deffined in the Virtual Compton Scattering (VCS) context, i.e. the reaction γ p → γp, where Q is the four-momentum transfer of the virtual photon. The GPs are a generalization of polarizabilities measured in real Compton scattering (RCS) by taking into account the Q2-dependency. They are dynamical properties of the proton when it is deformed by an applied electromagnetic field. αE (Q2) (or βM (Q2)) gives access to the local polarization (or magnetization) density of the deformed proton. The studied VCS process is accessible by the photon-electroproduction reaction (ep→ epγ ). The GP effect is a 1 − 15% contribution to the the ep → epγ cross section, requiring a high-precision measurement. The present work is a part of an experiment conducted by the A1 collaboration at MAMI at three new Q2 values: 0.1,0.2 and 0.45 GeV . This thesis details the extraction at 0.2 GeV2 . The experiment uses the 1 GeV electron beam, the 5 cm liquid hydrogen target and spectrometers A (B) to detect the final electron (proton). The first-level analysis includes a detailed calibration of experimental data, and the use of a simulation of the experiment. The measurement of the unpolarized ep → epγ cross section is described with two of its important features: the correction of the radiative effects and the renormalization. Two different frameworks are used to extract the GPs: the Low Energy Theorem (LET) and the dispersion relation model (DR). The two extractions lead to results in good agreement. The world data still raises question about the Q -behavior of the GPs.