Dispersion Analysis for Generalized Spin Polarizabilities

Over the past years, the virtual Compton scattering (VCS) process on the proton, accessed through the ep → epγ reaction, has become a powerful tool to provide new information on the internal structure of the nucleon . In the low energy regime below pion threshold, the outgoing photon in the VCS process plays the role of a quasi-constant applied electromagnetic dipole field and, through electron scattering, one measures the spatial distribution of the nucleon response to this applied field. The response is parametrized in terms of 6 generalized polarizabilities (GP’s) , which are functions of the square of the virtual photon four-momentum Q. The GP’s provide valuable non-perturbative nucleon structure information, and have been calculated in different approaches. In particular, the GP’s teach us about the interplay between nucleon-core excitations and pion-cloud effects. The first dedicated VCS experiment has been performed at MAMI 5 and two combinations of GP’s have been determined at Q = 0.33 GeV. Further VCS experiments are underway at lower Q at MIT-Bates 6 and at higher Q at JLab . At present, VCS experiments at low outgoing photon energies are analyzed in terms of a low-energy expansion (LEX) as proposed by Guichon et