Virtual Compton Scattering

Virtual Compton Scattering off the proton: γ∗p → γp is a new field of investigation of nucleon structure. Several dedicated experiments have been performed at low c.m. energy and various momentum transfers, yielding specific information on the proton. This talk reviews the concept of nucleon Generalized Polarizabilities and the present experimental status. Virtual Compton Scattering (VCS) off the proton: γ∗p → γp has emerged within the last ten years as a powerful tool to study the internal structure of the nucleon, bringing forward exciting new concepts and observables. The field can be subdivided according to different c.m. energy domains. At high energy (s M N ) and large momentum transfer (Q M N), QCD factorization allows the use of the VCS process to access: i) the nucleon Generalized Parton Distributions (GPDs) via Deep VCS at small t [1] ; ii) the nucleon Distribution Amplitudes via hard Compton scattering at large t [2]. At low energy, the VCS process gives access to the nucleon Generalized Polarizabilities or GPs, which are the main focus of this talk. The basic concepts are introduced and an experimental review is given of the experiments performed in the threshold regime ( √ s ≤ (MN + Mπ)) and resonance region. 1 Concept of Generalized Polarizabilities These are the generalization to a non-zero Q of the polarizabilities introduced in Real Compton Scattering (RCS). Let us recall that polarizabilities measure how much the internal structure of a composite particle is deformed when an external EM field is applied. The description of