Abstract
The A4 experiment at the MAMI facility in Mainz measures a parity violating single spin asymmetry in the scattering of longitudinally polarized electrons off unpolarized protons. The longitudinal single spin asymmetry is parity violating and arises from the interference of the weak and electromagnetic one-boson exchange amplitude. It is sensitive to the strangeness contribution to the electromagnetic form factors of the nucleon. Recently the transverse single spin asymmetry has gained much interest. It is purely electromagnetic in origin and arises from the interference of the one-photon with the two-photon exchange amplitude. It is a time-odd quantity and sensitive to the imaginary part of the two-photon amplitude only. The A4 collaboration has measured both asymmetries at two different momentum transfer values. The physical background, the experimental apparatus, the experimental results and the implications are discussed.
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