Systematic studies of beam-normal single spin asymmetries at MAMI

Reaching new precision frontiers in nuclear physics raises new experimental challenges as well as the demand for more sophisticated theoretical calculations. Especially in parity-violation electron scattering experiments the contribution from higher order processes, such as two-photon exchange, is comparable in size with the observed asymmetry A₏ₕ. Hence, a precise knowledge of this contribution is mandatory to determine the systematic uncertainties. Beam-normal single spin asymmetries A₍ (or the so-called transverse asymmetries) are a direct probe of the imaginary part of the two-photon exchange amplitude in the elastic scattering of transversely polarized electrons from unpolarized nuclei. Up to now, there is significant disagreement between experiment and theory for ^208Pb, which motivates more measurements to study the Q^2 and Z dependence. During a successful campaign at the MAinz MIcrotron (MAMI), using the spectrometer set-up of the A1 collaboration, the Q^2 dependence of A₍ for ^12C was determined. Follow-up experiments on ^28Si and ^90Zr investigated the charge dependence of the transverse asymmetry and have paved the way for the future experiment on ^208Pb, thus benchmarking the theoretical calculations in the heavier mass regime.