First Measurement of the Q^{2} Dependence of the Beam-Normal Single Spin Asymmetry for Elastic Scattering off Carbon.

We report on the first Q(2)-dependent measurement of the beam-normal single spin asymmetry A(n) in the elastic scattering of 570 MeV vertically polarized electrons off C-12. We cover the Q(2) range between 0.02 and 0.05 GeV2/c(2) and determine A(n) at four different Q(2) values. The experimental results are compared to a theoretical calculation that relates A(n) to the imaginary part of the two-photon exchange amplitude. The result emphasizes that the Q(2) behavior of A(n) given by the ratio of the Compton to charge form factors cannot be treated independently of the target nucleus.