Nucleon electromagnetic structure: past, present, and future

Abstract

We present the experimental status of electromagnetic hadron form factors. New and surprising results, based on polarization measurements, have been recently obtained for the electric proton and neutron form factors. In particular, the electric and magnetic distributions inside the proton appear not to be the same, in disagreement with results extracted from the unpolarized cross section, using the Rosenbluth separation. The new findings have given rise to a large number of papers and different speculations, as they question directly the models of nucleon structure and the reaction mechanism itself (based on $1\gamma$-exchange), with a possible revision of the calculation of radiative corrections, two-photon contribution etc. New data in time-like region are also available, through annihilation reactions. A large interest in this field arises, due also to the possibility of new measurements in polarized electron nucleon elastic scattering at JLab, and also in the time-like region, at Frascati and at the future FAIR international facility.Comment: 10 pages, 3 figures, to be included in a special issue of Nuovo Cimento