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Introduction

Nuclear physics is the science of atomic nuclei, their properties, their interactions and their constituents. Nuclei are complex quantal systems with a finite number of particles, synthesised by nature from two kinds of nucleon, protons and neutrons. Their binding energies are determined by the interplay between the strong, the electromagnetic and the weak interaction. At low energies, the nuclear properties are described in terms of nucleons and mesons, with empirically deduced effective interactions between them. Information on these properties is derived from nuclear structure studies using advanced spectroscopic tools. At high energies, the substructure of nucleons and mesons in terms of quarks and gluons becomes visible. The strong interaction between these fundamental building blocks of matter is described by quantum chromodynamics (QCD). One of the most exciting challenges for modern nuclear physics is to study how the elementary fields of QCD -- quarks and gluons -- build up particles such as nucleons and mesons, and to investigate the effects of quark and gluon degrees of freedom on nuclear structure and hadron dynamics at short distances. The structure and the dynamics of hadrons as composites of quarks and gluons are intimately connected. Information on hadron structure is derived from spectroscopy, from improved measurements of form factors and from deep-inelastic lepton scattering. Hadron dynamics enters crucially into the production and propagation of hadrons in complex nuclear systems. A key question in this context, both from the point of view of QCD and for nuclear physics, is about the way in which hadron properties change when they are embedded in nuclear matter. Collisions between relativistic heavy ions allow heating and compressing of nuclear matter and thus create the extreme conditions under which such questions can be explored. Great efforts are devoted to detecting the deconfinement of the quarks and gluons into a plasma phase within such a hot and compressed volume of nuclear matter. Nuclear physics has not only many links to particle physics, but also to astrophysics. Modern research in these fields is closely related to the description of the various phases in the evolution of the universe: The experiments to detect the quark-gluon plasma and to study the abundance of the lightest elements, the attempts to model the production of the heavier nuclei and to determine the equation of state of nuclear matter, and the search for neutrino masses and dark matter are motivated by open problems shared by nuclear physics, particle physics and astrophysics. The five chapters in this report describe the main activities and trends in nuclear physics and their relation to research in these neighbouring fields: Chapter one discusses the properties of nuclei at low energies and describes the extreme states of such finite quantal systems with respect to their neutron-to-proton ratio, angular momentum and temperature. Chapter two deals with experiments intended to heat and compress nuclear matter during the collision between relativistic heavy ions and to detect possible phase transitions, in particular the transition to the quark-gluon plasma. Chapter three discusses the substructure of strongly interacting particles and the dynamics of quarks, in particular the attempts to connect high energy QCD to nuclear physics at low energies. Chapters four and five set out the various connections between nuclear physics, particle physics and astrophysics and summarise the recent developments in neutrino physics.
 

• Nuclear Structure under Extreme Conditions of Isospin, Mass, Spin and Temperature
• The Physics Case:
• Highlights and Present Activities:
• Nucleus-Nucleus Collisions and the Phase Transitions of Nuclear Matter
• The Physics Case:
• Highlights and Present Activities:
• Quark and Hadron Dynamics
• The Physics Case:
• Highlights and Present Activities:
• Nuclear and Particle Astrophysics
• The Physics Case:
• Highlights and Present Activities:
• Search for Particles or Radiation Incident from outer Space:
• Laboratory Assisted Simulations:
• Neutrino Physics
• The Physics Case:
• Highlights and Present Activities:
• Trends and Directions

Nuclear Structure under Extreme Conditions of Isospin, Mass, Spin and Temperature

The Physics Case:

Cold nuclei in or near to the valley of stability have been studied by decay spectroscopy, inelastic excitation and nuclear reactions. Their single particle structure and their collective excitation modes are rather well investigated and understood in terms of effective interactions. In particular, the interplay between the independent particle motion and the two-body character of the nucleon-nucleon force is modelled by a mean field, with collective and single particle degrees of freedom. In spite of this significant progress during the last decades, the extrapolation of our knowledge to hitherto unknown areas of the nuclear parameter space as to higher temperatures or to high angular momenta, to superheavy nuclei or to extreme proton-to-neutron ratios is hampered by the effective nature of our models. It is presently not even possible to predict the limits of the valley of stability. The investigation of nuclei under these extreme conditions is therefore the main object of present-day nuclear structure research. Fusion reactions are available to study nuclei up to the highest values of angular momentum that nuclear matter can hold. They allow the investigation of the entire range of bound states between the yrast-line, where the nucleus is internally cold and the angular momentum is carried by collective motion, and the line defined by the binding energy of the least bound nucleons of the spinning nucleus. Nuclear reactions with stable beams allow the production of nuclei away from the valley of stability. Fusion reactions, the fragmentation of relativistic projectiles and fission are used to synthesise nuclei with extreme neutron-to-proton ratios and the heaviest elements beyond uranium. However, a detailed spectroscopy of more exotic nuclei is not yet possible. Only the upgrade of existing fragmentation facilities to increase the production rates and the development of powerful radioactive beam facilities will make these investigations possible. 

Highlights and Present Activities:

The new generation of high resolution, highly segmented, full solid angle gamma-ray detector arrays have opened up a window for detailed studies of high spin states of cold and warm nuclei:

• The most conspicuous recent findings have shown the existence of several areas in the nuclear landscape containing superdeformed nuclei. Small systematic shifts in the energy levels of some of the rotational bands of superdeformed nuclei and the observation of such bands with almost identical level spacings in neighbouring nuclei pose exciting problems to nuclear structure theory. The dynamics of the decay between states at large deformation and those closer to sphericity, and the possibility of hyperdeformed shapes are among the intriguing open questions of this research.

Heavy-ion fusion, projectile fragmentation, projectile fission of uranium as well as light-particle induced fission are exploited in order to produce exotic nuclei far from the valley of stability:

• One of the surprising results concerns the discovery of nuclei such as ¹¹Li exhibiting an extended neutron halo; first detailed studies of these halo nuclei as well as the expectation to find heavier neutron-rich nuclei with a kind of neutron skin have prompted widespread theoretical efforts to understand this new kind of dilute neutron matter. Our knowledge of the N=Z nuclei with their unusual properties caused by the proton-neutron pairing interaction was pushed to heavier nuclei, and much of the close-by proton drip-line could be mapped out. The doubly magic ⁷⁸Ni and ¹⁰⁰Sn nuclei have been observed and their proper study brought within reach once we have higher intensity beams of unstable nuclei. Experiments and theory are indicating that shell closures may change far from stability, with consequences for the r-process of the nucleosynthesis yet to be explored. Another outstanding achievement is the synthesis of the heaviest elements, up to Z=112, with real prospects of extending the periodic table to even heavier elements.

Experimental progress in this field will strongly depend on the realisation of present and future upgrading and development programmes of radioactive beam facilities. Direct nuclear reaction studies can then be performed with the exotic nuclei as projectiles and in inverse kinematics, allowing single particle orbitals to be identified. Proton radioactivity of nuclei at the proton drip line begins to become a spectroscopic tool, because it makes it possible to identify the shell structure of the initial and final states and the angular momentum transfer. Coulomb excitation using relativistic projectiles will be used to study high-lying excitations of collective nature. Mass measurements with an accuracy of 10^-8 or better will be possible. New theoretical approaches such as relativistic Hartree-Fock and Monte Carlo shell model calculations have been developed to interpret these new data, in particular high spin states and the properties of nuclei close to the drip lines. Calculations for the shell model single particle structure of the heaviest nuclei have been extended to a high degree of sophistication and will help to guide the experimental efforts to produce the very heavy nuclei at and near those with Z=114. 

Nucleus-Nucleus Collisions and the Phase Transitions of Nuclear Matter

The Physics Case:

Nucleus-nucleus collisions are a tool to heat and compress atomic nuclei. The choice of the incident energy and the size of the system serves to control the degree to which this happens. Collisions at incident energies above the Fermi energy create regions of hot and compressed nuclear matter. Densities of more than three times the normal nuclear matter density have been estimated for relativistic and ultra-relativistic energies. This compression phase is followed by a phase in which the system cools, expands and reaches densities much lower than normal nuclear density. The present activity in the field aims at the understanding of both the dynamical process leading to the formation of hot nuclei and the nature of the subsequent emission processes. Two phase transitions are predicted and are being investigated: a liquid-to-gas transition and the formation of a quark-gluon plasma. Evidence for the liquid-to-gas transition is discussed in connection with data on the production of intermediate mass fragments. A caloric curve may be derived resulting from an independent measurement of both temperature and excitation energy of a nuclear system. However, the results appear to depend on the initial conditions of the heavy ion collision, and are not conclusive at this point. Computer simulations of QCD on a lattice predict that the transition from nucleons and nucleon resonances to a deconfined quark-gluon plasma phase should occur in ultra-relativistic heavy ion collisions. Experiments involving Pb + Pb collisions at 158 GeV/u have been performed in order to produce and identify the quark-gluon plasma and study its properties. The suppression of J/$\chi$production in hadronic matter at high energy density is discussed as one of the possible signatures for this phase transition. 

Highlights and Present Activities:

Substantial progress has been made in the phenomenological description of the dynamical evolution of relativistic heavy ion collisions, and of the hot and dense interaction zone generated in such reactions. This refined understanding is based on exclusive measurements of nucleons, pions, kaons and fragments that became available with a good characterisation of the impact parameter:

• The measured Bremsstrahlung-spectrum allows the identification of the primary nucleon-nucleon collisions. Various flow modes caused by the collective motion of many nucleons have been observed. These processes are strongly impact parameter dependent and are caused by contributions from nucleon-nucleon scattering, the momentum dependence of the interaction and the mean field.
• The measured hadron yields are consistent with values expected for an equilibrated system. The particle production cross sections measured in heavy ion collisions are explained by a two-step mechanism in a high-density medium of hadrons, consisting in large part of nucleon resonances.
• The unusual suppression of J/$\chi$  production in Pb+Pb collisions, as compared to the proton + proton, proton + nucleus and sulphur + nucleus systems, may reflect Debye screening at the critical density of a deconfined quark-gluon plasma phase. The di-lepton spectrum obtained in Pb+Pb collisions is taken as an indication that the properties of hadrons are modified in a hadronic environment of high density.

The efforts in this field have resulted in very successful experiments, both in Europe and the US. The detector developments, the data analysis of the high-multiplicity events and a thorough discussion of the signatures for a quark-gluon plasma represent huge progress. However, despite some encouraging signals the phase transition cannot be identified unambiguously. What is needed is the correlated signal from several signatures characteristic of the phase transition. A next generation of experiments is being prepared. 

Quark and Hadron Dynamics

The Physics Case:

The properties of nuclei and nuclear reactions at low energies are described in terms of nucleons and mesons, interacting on a scale of 1 fm or larger. The nucleon-nucleon interaction at these distances is described by potentials derived from meson exchange models and adjusted to experimental data. However, nucleons and mesons, by themselves, have a substructure. Quarks and gluons are the building blocks of nuclear matter at small distances, large densities and/or high temperatures, and quantum chromodynamics (QCD) is the theory for the strong force. One of the most exciting challenges of present day nuclear physics is to study the influence of the quark and gluon substructure on the properties and interactions of the hadrons and nuclei, i.e. to connect the QCD and low energy model descriptions. The problem is that this requires an understanding of the region of non-perturbative QCD, and it is not a priori clear how to proceed. The internal structure of the free nucleons has been studied in great detail. Polarised lepton-polarised nucleon deep inelastic scattering allows to disentangle the contributions of quarks, anti-quarks and gluons to the spin of the nucleon. It is now generally accepted that the understanding of the nucleon spin requires consideration of the gluon contribution, and should take into account the strange $q\overline{q}$-pairs. 

Highlights and Present Activities:

New accelerators or storage rings for electrons and protons, with near to 100% duty cycle, polarised beams and polarised targets have recently become available. They have been used to obtain new experimental results:

• Short range correlations between bound nucleons are studied with reactions such as (e,e'p). Recent experiments have provided detailed and accurate information on the occupancy of the mean-field orbitals. The analysis shows that only 70% of the strength is contained in states below the Fermi surface. This result is one of the cleanest signals of nucleon-nucleon short-range correlations. The more direct study of these correlations by the eA $\rightarrow$ e'pp (A-2) reaction has provided first results.
• The understanding of the constituent quark structure in nucleons and nuclei requires high precision measurements of the electric and magnetic elastic form factor as well as of transition form factors to baryon resonances. The observation of parity violation in electron scattering experiments by the interference between weak and electromagnetic interaction, currently planned or being carried out in various laboratories, will provide additional information on the strangeness content of the nucleon. New approaches will be made to study the spin structure of the nucleon. The various theories put forward differ considerably in their prediction of the amount of spin carried by the gluons. Therefore several experiments have been proposed to measure the amount of gluon polarisation.
• Hadron dynamics is being investigated by particle production at threshold using photons or proton-nucleon collisions, and by studies of the modification of the properties of hadrons in a hadronic medium. Such processes can be analysed by an effective theory of hadron interactions based on chiral symmetry. First results on the production of neutral pions are available. Medium modifications are already expected at normal nuclear matter densities, and should be detectable in pion- or photon-induced reactions on nuclei. They should be enhanced in the higher density and temperature regimes encountered in relativistic heavy ion reactions. Experimental evidence for such a behaviour is found in the di-lepton signals from ultrarelativistic heavy ion collisions.

Nuclear and Particle Astrophysics

The Physics Case:

Major experimental developments in nuclear physics and in astrophysics have caused an explosion of activities in this new field. The goal is to study the evolution of the expanding and cooling universe, to understand the creation of the elementary particles, the nucleons and the lighter elements, the various burning phases of stars, and the production of the heavy elements.

Highlights and Present Activities:

Search for Particles or Radiation Incident from outer Space:

Highlights are the data obtained with the telescopes COBE, HUBBLE, ROSAT and GRO. They cover a large fraction of the electromagnetic spectrum. The X-ray and gamma astronomy are completely new fields which came into existence with the availability of detectors outside the earth's atmosphere. The origin of the gamma-ray bursts is being explored. Several large terrestrial detector arrays have been built which are optimised to look for various components of the cosmic radiation incident on the outer atmosphere. First data on the high energy part of the spectrum are just being obtained.

Laboratory Assisted Simulations:

The abundance of stable hydrogen, helium and lithium isotopes, together with the life-time of the neutron and the knowledge that three generations of fundamental particles exist, are the input to an understanding of the ``Big Bang Nucleosynthesis'', and thus of the dynamical behaviour of the very early phases of our universe. The production of the heavier elements is dominated by various ``cycles'', either hot or cold. The exit from such a cycle depends on the cross section for specific nuclear reactions and on the life-times of the nuclei involved. Radioactive beams allow data to be obtained which are relevant for the hot burning processes in stars. These are cross sections for reactions below or near the barrier, subject to the influence of resonances, the level structure of the nuclei involved, and their respective half-lives. Recent work has established the importance of the rapid proton-capture process in explosive phases, of the ``screening'' by the electrons on fusion reaction cross sections far below the barrier, and the role of bound-state beta-decay. Large computer codes have been devised in order to simulate the various scenarios considered in connection with the evolution of stars, up to the explosive phases, and with the synthesis of the elements. The ``physics of neutron stars'' depends on the equation of state for nuclear matter, and has been a long-debated topic, stimulated in particular by data on the supernovae 1987A. 

Neutrino Physics

The Physics Case:

Within the ``Standard Model'' neutrinos are assumed to be mass-less, with no electromagnetic interactions, and with only two components for each of the three families, i.e. only left-handed neutrinos and only right-handed anti-neutrinos. The observation of finite neutrino masses, neutrino-less double-beta decay, neutrino oscillations, electromagnetic interactions of neutrinos or the missing right-handed/left-handed component of the neutrinos/antineutrinos would constitute evidence for physics beyond the ``Standard Model''. Limits for these quantities have been derived from studies of nuclear beta-decay spectra, from astrophysical arguments, from neutrino experiments at accelerators and reactors, and from the search for solar neutrinos There is no unambiguous empirical evidence in conflict with the simple picture provided by the ``Standard Model''. However there are also no fundamental symmetries known that would explain this amazing simplicity.

Highlights and Present Activities:

Large efforts are under way to improve the limits for neutrino masses, neutrino oscillations, and for the electromagnetic properties of neutrinos. Such limits result in ``exclusion plots'' for the oscillation parameters $\delta(m^2)$ and $sin^2(2\theta)$. Data on neutrino-electron scattering allow limits to be set for the electromagnetic interactions of neutrinos. Improved laboratory experiments, in which the shape of the beta-decay electrons was measured carefully, have determined the electron-neutrino rest mass to be less than 7 eV. The solar neutrino experiments GALLEX and SAGE are continuing to take data. One of the highlights is the precision with which GALLEX was calibrated with a reactor-made <sup>51</sup>Cr source. However, it becomes more and more obvious that the ``Solar Neutrino Puzzle'' is not solved. Therefore the GALLEX experiment has been extended to take data for at least another decade, and a new detector system (BOREXINO) is being constructed. The double-beta decay Germanium detector experiment is being continued, and the limits for a neutrino-less double-beta decay will continuously be reduced. Large efforts are being made to develop low temperature detector devices in order to detect dark matter, in particular weakly interacting massive particles (WIMPs). 

Trends and Directions

What are the perspectives of nuclear physics? We describe some observations which characterise the research activities presently performed in Europe. They can serve as a guide for future developments.

• A New Challenge: Nuclear Structure under Extreme Conditions. The development of radioactive beams and of a new generation of gamma ray detectors have stimulated various experimental studies of nuclear structure at extreme values of spin and isospin, and efforts to synthesise the spherical super-heavy nuclei around Z=114. It appears necessary, however, to encourage and support theoretical work which is necessary to identify the kind of data which might be most sensitive to an understanding of nuclei at their extremes.
• The Co-ordinated Effort to Identify the Quark Gluon Plasma. The search for the quark gluon plasma is being performed as a huge common effort by various large collaborations in Europe and in the US, guided by an ideally matched interplay of theory and experiment. It has become a demonstration of the way in which nuclear and particle physicists attack the problem of understanding a system of maximum complexity.
• The Ambitious Goal to Understand the Hadronic Interaction. Scientists from particle and nuclear physics work together in order to study and understand the hadronic interaction at short distances in terms of non-perturbative QCD. This problem requires a co-ordinated effort from the experimental and the theoretical side.
• New Possibilities for Research in Particle and Nuclear Astrophysics. This field is profiting from completely new data obtained by the various instruments operating in space, the possibilities to analyse the dynamical evolution of complex systems in large computer simulations, and the availability of radioactive beams which allow the scenario of such simulations to be defined.
• The Necessity to Work in Large International Collaborations. The complexity of modern detection devices, the need to optimise the available time for research at a given facility, and the funding policies of European agencies make it necessary to form large, and in general, international collaborations.

Modern research in nuclear physics has led to impressive technical achievements: Completely new detector concepts for gamma rays and charged particles have been invented, new electronic data handling systems devised, and fast pattern recognition algorithms developed. These achievements will be of use in various other fields of science.