First determination of generalized polarizabilities of the proton by a virtual compton scattering experiment

Absolute differential cross sections for the reaction ep --> ep gamma have been measured at a four-momentum transfer with virtuality Q(2) = 0.33 GeV2 and polarization epsilon = 0.62 in the range 33.6 to 111.5 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.