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4.3 Nuclear Structure and Dynamics
Ab-initio methods, large-scale Shell-Model (SM) calculations,
Energy Density Functional (EDF) methods,
and symmetry-dictated models, present complementary
approaches to the nuclear many-body problem. A successful
description of the rich nuclear phenomenology
must, of course, ultimately be related to the underlying
fundamental theory of strong interactions. Ab-initio
methods are being developed to establish this link on a
quantitative level and, in addition, it will be important to
extend the domain of applicability of these approaches
toward heavier nuclei. Large-scale SM methods and
EDF-based models currently provide the most accurate
and complete set of tools for a systematic description
and interpretation of data that will be produced by the
next generation of experimental facilities, focused on the
physics of nuclei far from stability. Symmetry-dictated
approaches provide insight on how simple patterns
emerge in the structure of complex nuclei. The goal
for the next decade will be to develop a microscopic
description of nuclear structure and reaction phenomena
that can be extrapolated very far from beta-stability, and
simultaneously provide reliable error estimates. The aim
will also be to firmly establish the microscopic foundation
of low-energy nuclear theory. This will only be
possible by methodically developing and exploring the
various interdependent theoretical and computational
approaches described in this overview.
4.3.3 Onset of Complexity
Linking nucleons with nuclei
The domain of light nuclei is the natural testing ground
for linking nucleons and sub-nucleon degrees of freedom
with complex nuclei. Thus the related experiments
should provide precise data to constrain the modern
ab-initio calculations. Not only energies and spin-parities
of states, but also electro-weak transition rates, spectroscopic
factors, ground state moments, radii and masses
are needed. Furthermore, it is crucial to investigate the
role of the continuum, both concerning reaction mechanism
and structure.
Weakly bound and unbound states
A large part of the motion of exotic systems at the very
limits of nucleonic binding is in classically forbidden
regions and therefore their properties are profoundly
influenced by both the continuum and many-body
correlations. The importance of the continuum for the
description of resonances is obvious. Weakly bound
states cannot be described within the closed quantum
system formalism. A consistent description of the interplay
between scattering states, resonances and bound
states requires an open quantum system formulation of
the nuclear many-body problem.
The main challenges in the coming years will be
(i) to address the structure/reaction aspects in long
chains of isotopes,
(ii) to handle very large model spaces and to introduce
a correct treatment of continuum states in ab-initio
many-body framework,
(iii) to develop microscopic reaction theories for the
description of complex reactions with multi-nucleon
continuum and/or more than two fragments in the
final state.
In recent years, the availability of RIBs and associated
new equipment has enabled studies of continuum
properties of unbound and weakly bound nuclei through
transfer or knock-out reactions. Open questions arising
from these experiments are the role of resonances
and non-resonant continuum as well as shell structure
changes beyond the drip lines.
The observed systems beyond the drip-lines are so far
restricted to the light masses, such as 5,7 H, 7,9,10 He and
10,12,13 Li (Figure 2) on the neutron-rich side, or 10,11 N and
12 O on the neutron-deficient side. The existing results
are hampered by low statistics, resolution and/or selectivity
very often leading to conflicting conclusions on
the properties of more loosely bound systems. Higher
RIB intensities and next generation instrumentation are
required to clarify the situation and to explore the neutron
drip-line towards heavier elements.
13
Li
Figure 2. The unbound 13 Li was observed after nucleon knockout
reactions at relativistic energies with 14 Be impinging on a
liquid hydrogen target. Data are from the ALADIN+LAND setup at
GSI. The 11 Li+2n data demonstrate components that cannot be
attributed to initial correlations in 14 Be (red line) and thus give the
first indication for the existence of a 13 Li resonance at 1.47 MeV
(blue line)
110 | Perspectives of Nuclear Physics in Europe – NuPECC Long Range Plan 2010