Single Spin Asymmetries from the Mainz A4 Experiment

The A4 collaboration at the MAMI accelerator in Mainz measures single spin asymmetries in the cross section of elastic scattering of polarized electrons off unpolarized protons. The electrons can be polarized longitudinally as well as transversely. Longitudinal polarization leads to a parity violating (PV) asymmetry from which the contribution of strange sea quarks to the vector form factors of the proton can be derived. Transverse polarization leads to an azimuthal ϕ -dependent asymmetry which gives acces to the imaginary part of the 2 γ -exchange amplitude. Measurements at forward angles θ = ( 35 ± 5 ) ° and two different momentum transfers Q 2 of 0.23 ( GeV / c ) 2 and 0.11 ( GeV / c ) 2 have been performed so far. Measurements at backward angles θ = ( 145 ± 5 ) ° are in preparation.