Nuclear physics beyond neutrons and protons

In the contemporary picture the consituents of the nucleus, neutrons and protons, are composed of quarks and gluons. Their dynamics is described by the QCD, in which the chiral symmetry is a basic ingredient. Around the centre of the nucleons this symmetry is very much obeyed, which near the surface is violated, giving rise to the non-conservation of the axial vector current. It is believed that on the surface of the nucleons this symmetry is restored by the birth of pions. Thus, looking from the inside of the nucleons the pions provide an additional axial vector current to restore the chiral symmetry and for the nuclear physics they provide the basic starting point for its dynamics. The nucleons themselves, in this picture, are surrounded by a cloud of pions and their resonances. Physically this picture leads to the differences in the measured radii of the nucleons using different probes and a simple description of the charge-exchange spin-isospin flip reactions. It also puts the propagation of pions and rho-mesons (a resonant state of two pions probably) in nuclear medium in a category of basic importance. The first excited state of the nucleon, the Δ-isobar, to which the pions couple very strongly, acquires, in a natural way, a very significant place in the nuclear dynamics. (author). 10 refs., 4 figs