First Determination of Generalized Polarizabilities of the Proton by a Virtual Compton Scattering Experiment

Absolute differential cross sections for the reaction $\mathrm{ep}\to \mathrm{ep}\gamma$ have been measured at a four-momentum transfer with virtuality $Q^2=0.33\mathrm{}{\mathrm{GeV}}^2$ and polarization $\epsilon =0.62\mathrm{}$ in the range 33.6 to $111.5\mathrm{MeV}/c$ for the momentum of the outgoing photon in the photon-proton center of mass frame. The experiment has been performed with the high-resolution spectrometers at the Mainz Microtron MAMI. From the photon angular distributions, two structure functions which are a linear combination of the generalized polarizabilities have been determined for the first time.