Recent Developments in Nuclear Physics

Abstract

Nuclei exhibit features that are described in superficially contradictory terms according to the different degrees of freedom that are excited by probes of different scale in space and in time. After giving some examples I concentrate on the hadron degrees of freedom such as the nucleon, the pion and the .6. isobar. These are the effective degrees of freedom on the level of intermediate resolution: about 0·5-1 fm in distance and correspondingly in time. A prime example is the deuteron :which has a nearly model-independent description in terms of pion physics to very high precision. In nuclear matter the pion propagates in close analogy to the propagation of light in a dielectric. This permits the explanation of a number of features in nuclei related to the chiral symmetry limit in which the pion mass vanishes. A consequence of this description is the analogy of the equations for the pion and its effective field with the Maxwell equations for a dielectric. A pionic collective mode should appear strongly and with characteristic properties for a well chosen probe. It is difficult to explore its properties directly and in particular physical pions are not useful for this purpose. I will discuss different alternatives involving 'virtual pion beams'. There is recent evidence for such a collective state in forward charge exchange reactions throughout the periodic system.