Perspectives of Nuclear Physics in Europe - European Science ...

4.3 Nuclear Structure and Dynamics
of closed-shell nuclei within coupled-cluster theory,
using bare or transformed EFT interactions. The range
of NCSM calculations regarding model-space size and
particle number has been extended, not only through
computational advances but also through conceptual
developments, such as adaptive model space truncations.
The use of transformed interactions in a variety of
approximate many-body schemes, such as variational
or perturbative methods, provides valuable information
on the properties of QCD-based interactions for
nuclei beyond the reach of exact ab initio calculations.
Specialized many-body methods for the description of
cluster and halo structures, e.g., Fermionic Molecular
Dynamics (FMD), provide insight into phenomena that
cannot be described adequately in other methods.
The gap between nuclear structure and ab initio
nuclear reaction theory is being bridged for systems of
increasing complexity. Methods connecting the powerful
bound-state techniques discussed above with the
description of unbound and scattering states, such as
the Lorentz integral transform or the resonating group
method, have extended the domain of ab initio reaction
studies to systems significantly beyond the reach
of traditional few-body approaches (see “Reactions”,
page 109).
Together, exact and approximate ab initio methods
give access to a wealth of nuclear structure observables
based on the same Hamiltonian. The comparison with
experiment, in particular for the systematic evolution of
different observables from stable to exotic isotopes, will
provide decisive information on the predictive power and
the limitations of QCD-based nuclear interactions.
Perspectives
During the past few years exciting new avenues have
emerged in ab initio nuclear structure theory. Successful
first steps have been made and methodological refinements
and extensions as well as applications will be on
the agenda for the coming years.
Regarding the QCD-based interactions derived within
chiral EFT, there are a number of conceptual questions
such as proper power counting and regularization
schemes and the role of explicit Δ degrees of freedom
that will be addressed in the near future. Furthermore, the
derivation and implementation of the chiral three-body
interaction at next-to-next-to-next-to-leading order needs
to be completed. There are also questions as how to
combine the EFT description of the interaction with a consistent
many-body framework as well as how to connect
EFT-based energy density functionals and Hamiltonians.
In the sector of unitary and renormalization group transformations
of the Hamiltonian, further improvements
regarding the choice of the transformation and the
inclusion and treatment of three-body contributions are
needed. Detailed benchmarks of transformed interactions
in finite nuclei and nuclear matter using ab initio
methods are required. The comparison to experimental
data for light nuclei will provide a rigorous assessment
of the quality of QCD-based Hamiltonians.
A major task for many-body methods is the systematic
inclusion of two- plus three-nucleon interactions, be it
bare or transformed, for a wide array of nuclear structure
applications. Given the tremendous computational
effort associated with a full implementation of three-body
forces, approximate schemes have to be considered.
Further exact ab initio many-body methods have to be
extended for using QCD-based non-local interactions,
most notably Green’s Function or Auxiliary Field Monte
Carlo methods. Approximate many-body schemes using
the same QCD-based interactions will be refined and
applied for ab initio nuclear structure calculations beyond
the domain of the exact approaches.
In the coming decade QCD-based ab initio methods
will make decisive steps towards their major goals:
(i) To provide precise ab initio predictions for structure
and reactions of exotic nuclei that help to guide and
to interpret experiments.
(ii) To yield information on the properties and degrees of
freedom of QCD that are relevant for the understanding
of the plethora of nuclear structure phenomena.
(iii) To establish rigorous benchmarks for other nuclear
structure approaches, e.g. energy density functional
methods that eventually give access to the whole
nuclear chart.
Shell model
The Shell Model (SM) is a highly successful configuration
interaction approach for the microscopic description of
the structure of the nucleus. It fills the gap between ab
initio methods applicable to light nuclei and energy density
functional approaches which are the best adapted
for heavy nuclei. The SM is based on an effective interaction
acting within a limited model space of valence
nucleons. The computational requirements of the SM are
heavy and the applicability of the method relies on the
availability of large-scale computational resources.
The progress made in the last years has permitted the
configuration spaces tractable by the SM to be extended
considerably and as a consequence the nuclei that can
be studied. The SM plays also an important role in the
study of weakly bound and open systems when combined
with a proper treatment of resonant states and of
106 | Perspectives of Nuclear Physics in Europe – NuPECC Long Range Plan 2010