This thesis project concerns the precise measurement of the electroweak mixing angle with the P2 experiment, at the MESA accelerator, in Mainz. The measurement will make it possible to test, for the first time, the prediction of the Standard Model for the evolution of this fundamental parameter as a function of the energy scale, and the effects of possible new particles or interactions.

The determination of the mixing angle is based on a precise measurement of the variation of the scattering cross section of an electron beam on a liquid hydrogen target, as a function of the polarization of the beam. This asymmetry, measured in scattering at forward angles, is affected by significant systematic uncertainties linked to the structure of the proton. A measurement of the scattering asymmetry in the backward direction, using a dedicated detector, makes it possible to reduce these uncertainties, and constitutes the subject of this thesis.

The thesis project arrives at a crucial moment in the development of the experiment, and will allow the student to participate directly in the construction of a very high performance detector, its installation in the P2 experiment, and its scientific exploitation.