Title Previous: Preamble
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:
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:
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:
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.
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.
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.
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/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
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.
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/
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.
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 -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
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.
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
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.
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
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 51Cr
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.
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.
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
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
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
Title Previous: Preamble