Universidad de Salamanca
Introduction
That part of science which is known as Nuclear Physics is, like all live research areas, constantly evolving as progress is made and new frontiers open up. In the past Nuclear Physics could be defined simply as the science of the atomic nucleus. The features observed and the associated forces were found to differe so drastically from what could be studied in other systems that both the identity and the fundamental character of that science were established beyond any doubt. Furthermore, it was assumed, sometimes implicitly, that the nucleus provided us with a specific field where one could study the strong interaction, one of the fundamental interactions at work in nature. Yet, it is clear now that this interaction, although it plays an essential role in nuclei, does not appear in all its clarity but intervenes only through that part of the force which spills out of the quark bag.
Thus, in retrospect one can only wonder at the quality and predictive value of nuclear models which have been elaborated over the last 40 years. Yet, al least for the time being, a global description of nuclei cannot be formally and directly derived from a fundamental interaction. Frustated in its ambition to be the field where the fundamental strong interaction could be elucidated, Nuclear Physics has actually developed into a very original and fundamental branch of science, the science of complex systems of elementary particles.
Basic Constituents and Interactions
The structure of the atomic and sub-atomic world as we know it today is illustrated in Figure 1. The atom consists of a small, positively charged, very dense, central nucleus surrounded by negatively charged electrons held in orbit by the electromagnetic (Coulomb) force. The nucleus is made up of neutral neutrons and charged protons, which are in turn composed of quarks. The quarks are pointlike objects with a charge of -1/3 or +2/3. They are bound inside the nucleons by strong force mediated by the exchange of gluons. The proton and neutron are made up mainly of combinatios of only two types of quarks, the u(up) and d(down) quark. Together with the pointlike electrons they are the building blocks of all matter with which we are familiar. There is, in adition to the two forces mentioned above, the short range weak force which is responsible for the ß-decay processes. It is due to the exchange of the heavy W- and Z-bosons.
The elementary particles and interactions of the sub-nucleonic world are successfully described in the Standard Model. It states that there are two types of spin 1/2-particles (fermions), the quarks and the leptons, pointlike particles which are the basic constituents of all matter. Quarks are sensitive to the strong, electromagnetic and weak interaction, charged leptons only to the electromagnetic and weak one. Like the electromagnetic interaction whose strength is given by the coupling constant a = 1/137, the strong interaction is characterised by a "strong" colour charge and a strong coupling constant as. Strong interaction field theory, Quantum Chromo Dynamics (QCD), has the unique property of confinement; at small distance the force between quarks is small. At larges distances ( > 0.5 fm) the interactions become increasingly stronger. At that distance scale, the first manifestation of structured objects occurs in nature due to the phenomenon of confinement. Very little is known about the long-distance properties of QCD that gives rise to these structured objects (nucleons and mesons). If a quark inside a nucleon is hit by a probe transferring a large momentum to the struck quark, it will move out the confinement region, hadronise and form a jet of mesons.
The description of a nucleus in terms of nucleons as the fundamental constituents, interacting through the exchange of mesons, holds as long as one restricts oneself to distances larger than about 1 fm, or equivalently to momenta smaller than 400 MeV/c. At much higher momenta or shorter distances, smaller than 0.2 fm, a description in terms of quarks and gluons seems appropiate. One of the basic problems in strong interaction physics is how to describe the intermediate non-perturbative region where neither the short nor the long distance approach applies. The study of this problem is one major goal in both Particle and Nuclear Physics.
Nuclei as a System of Bound Nucleons
Traditionally Nuclear Physics studies the properties of a system of nucleons interacting through the exchange of virtual mesons. The interaction has a long range attractive part and a short range repulsive one. It is a many body, quantal system made of a finite number of particles. Due to the complexity of the system, a fully microscopic description is beyond our means except for A < 5, thus necessitating the introduction of models.
The liquid drop model is based on the observation that the average internucleon distance is about equal to the range of the internucleon force which looks very much like a van der Waals force. It has been successfully applied to collective phenomena such as fission and certain vibrations of nuclei.
In the mean field description one replaces in first approximation the interaction of a specific nucleon with all other nucleons by an average central potential where the nucleon moves independently of the other particles. The properties of this potential can be calculated in a self consistent way by solving the Schrödinger equation in the Hartree-Fock approximation and using some suitable parametrisation of the effective nucleon-nucleon force. The well known shell model is obtained by introducing, in addition, a spin-orbit interaction. Further refinements are obtained by introducing short and long range nucleon-nucleon interactions, an important example being pairing correlations. It is not only essential to an undestanding of the nuclear spectrum at low excitation energies but also gives rise to phenomena similar to superconductivity. Although most of these calculations have been performed in a nonrelativistic description, recent approaches are able to take relativity into account.
These refined models have been quite successful in describing low energy nuclear phenomena. They will be further tested by studying nuclei under extreme circumstances close to the limit of stability: fast spinning nuclei, nuclei with an extreme ratio of neutrons to protons and nuclei at high excitation energy.
Subnucleonic Degrees of Freedom.
The description of phenomena in terms of nucleons interacting by meson exchange is a long range approximation of the basic underlying structure of quarks and gluons. One of the key questions emerging in Nuclear Physics reserach is whether this model can be understood starting from QCD, the fundamental theory describing strong interactions at the quark gluon level. It is of particular importance to study how and to what extent the properties of a free nucleon or other hadrons are modified by the presence of nearby nucleons, as in a nucleus. For instance one might wonder to what extent the properties of nuclear matter are affected by the (virtual) excitation of nucleonic excited states and their propagation in nuclear matter, or whether the hadronisation process, which occurs when a quark is knocked out of a nucleon, would be affected by the presence of other quarks inside the surrounding nucleons. A systematic study of these phenomena may lead to new insights into QCD both in the perturbative and non-perturbative regions.
The modification of free particle properties in nuclear matter becomes increasingly significant, if the average distance between nucleons decreases, that is with increasing density of the nuclear matter. One might then speculate that a sufficiently high densities and temperatures of nuclear matter new phenomena occur where nucleons cease to exist and a transition to the real basic constituents, quarks and gluons, takes place. The search for and study of deconfinement, leading to a quark-gluon plasma, is one of the major new frontiers in Nuclear Physics research.
Connections to Other Disciplines
Nuclear Physics and other disciplines continuosly exchange a flow of concepts, methods and techniques. In addition to the increasing overlap with elementary particle physics, already referred to above, there is a considerable overlap with atomic physics and astrophysics. In this report it is not intended to discuss the many existing interconnections between these fields and Nuclear Physics. For completeness' sake only a brief summary will be given. Atomic spectra are affected by nuclear properties. From an accurate measuremente of atomic level shifts one obtains precise values of the spin, deformation and the difference in charge radii of nuclei. Nuclear properties can in principle be affected by the structure of the surrounding atomic cloud. An extreme example is the beta-decay of completely stripped nuclei to a final state in which the emitted electron is bound in an atomic orbit: the energy balance can be changed to the extent that normally stable nuclei become unstable. This kind of experiment is made possible by the availability of new facilities in which stripped heavy ions can be stored. These facilities also create new opportunities for advanced atomic experiments.
Nuclear Physics is of vital importance for astrophysics. Nearly all important events in astronomy, cosmogeny and cosmology have left nuclear clues. Specifically nuclear structure provides the basis for an understanding of the structure and evolution of stars, the generation of energy and synthesis of elements in stars, and the nuclear debris from the Big Bang. It is in the nature of astrophysics, however, that many of the processes and objects one tries to understand are physically inaccessible. Thus it is important that those aspects which can be studied in the laboratory should be well understood.
Extending our knowledge of nuclear poperties to nuclei far from the region of stability is relevant to an undestanding of the origin of elements supposed to be formed in the p-, s-, and r-processes as well as providing the basis for cosmochronology. Determining the very small reaction rates relevant in the static burning stages of stars, such as hydrogen burning requires the measurement of very small cross sections at very low bombarding energies: in most cases unstable nuclei such as 13N are involved. Knowledge of the nuclear equation of state at high densities is essential for a quantitative description of astrophysical phenomena such as supernova explosions and the formation and properties of neutron stars.
Without such improved nuclear data much of the work done in the field of nuclear astrophysics will be at least incomplete and at worst possibly incorrect. With the availability of improved observations of the universe and the objects therein, new nuclear clues will be observed requiring additional efforts in the laboratory to obtain the nuclear data necessary for their interpretation.
