Have we seen meson-exchanges in nuclei

Over the past few years nuclear currents have been very accurately measured using both weak and electromagnetic interactions. These studies have revealed several processes dominated by meson-exchange currents. Specific meson-exchange processes have been isolated and their spatial distribution has been determined for relative distances between nucleons larger than 0.8 fm. On has clearly seen meson-exchange currents in nuclei. A fully consistent description of nuclear charge and current distributions in terms of mesons, nucleons and Δ-excitations has been achieved up to momentum transfers Q2 ≅ 1 (GeV/c)2. Long range interactions are determined by an one-pion exchange while medium range interactions are described via dispersion relations in terms of correlated 2π-exchanges. We have shown that the spatial distributions of meson-exchange currents have been determined by electron scattering for distances between two nucleons larger than 0.8 fm. At this scale, two-body operators are constrained by general theorems and, in the long wavelength limit, meson-exchange currents are given by chiral symmetry. This explains the importance of the pion-exchange current in transitions induced by both axial and vector probes. In order to understand higher momentum transfer data, a description of the nucleon-nucleon interaction consistent with the nucleon substructure is needed. This will require an understanding of the structure of the nucleon and the development of a theoretical framework based on QCD valid for both non-perturbative, long-wavelength interactions and perturbative, short-wavelength interactions. This is now one of the most interesting challenges to nuclear physics