Spin Vibrations in Nuclei

ample for the application of quantum statistics to finite non-equilibrium sys­ tems. The study of "cold" (non-thermalized) fusion and fission processes, in­ teresting in itself, is also relevant to the production of new transuranium ele­ ments and possibly even superheavy nu­ clei. And in supercritical nuclear sys­ tems (Z1+Z2> 137), position creation has been observed which is a signal for new phenomena such as the formation of superheavy nuclear molecules. Eventually our colleague on his round trip would get involved in the discussion of the consequences of the discoveries in particle physics. How do quark de­ grees of freedom manifest themselves in a bound system of nucleons? How can the mesonic description of nuclear forces be embedded in the evolving the­ ory of strong interaction? Are there many body forces? How can the shellmodel description be reconciled with a picture of quark-bags in the nucleus? Clearly, the perspectives are changing considerably. In the past, the nucleus could be considered as a system of bound nucleons in their ground-state in a similar way that molecules and crys­ tals are systems of atoms in their ground state. Through higher energies and pro­ bes it now becomes possible, however, to excite quark structures inside a bound nucleon. Thus the nucleus has to be des­ cribed in more general terms as a multibaryon system. It is like going from the phonon spectrum of solids to the study of electronic excitations, which, of course, are different from those of isola­ ted atoms. These effects may be studied by electromagnetic probes like electrons and muons or by hadronic probes like Kmesons which allow a single nucleon in­ side a nucleus to be marked with a strange quark in a way quite similar to atoms being marked in a molecule by a radioactive tracer-atom. In the last analysis, a nucleus would be described as a system, not of nucleons, but of quarks and gluons, although this makes little sense except for very high energy densities. The road to study such extreme conditions of matter is opened up by the develop­ ments in heavy ion physics. Heavy ions are not only a tool to excite spec­ troscopically interesting states and to produce new nuclides far off stability but their collisions allow us also to study the global properties of nuclear matter. First steps in this direction have been taken, but much higher heavy ion energies than presently available will be needed to force the quark-bags in a nucleus to overlap and to form a quarkgluon-plasma. Such states of matter may have existed in the first micro seconds after the beginning of the Universe. Unavoidably our visitor would be con­ fronted with many ideas and proposals for new facilities which are needed to take the next steps into the strange world of nuclei and their substructure. The talk would probably be aboutamultiGeV high intensity electron accelerator, high precision beams for intermediate energies, relativistic heavy ion beams, the secondary beams of high-intensity proton-accelerators and K-meson fac­ tories. He realizes that bringing to life such projects in Europe would certainly depend on a substantial degree of inter­ national cooperation. For the moment, however, our returning colleague deci­ des to read in more detail this present issue of Europhysics News to get more insight into some of the problems he has just heard about.