
Recent years have seen exciting new developments and progress in nuclear structure theory,
reaction theory, and experimental techniques, that allow us to move towards a description of exotic systems and environments, setting the stage for new discoveries. The purpose of the 5week
program was to bring together physicists from the lowenergy nuclear structure and reaction communities to identify avenues for achieving reliable and predictive descriptions of reactions involving
nuclei across the isotopic chart. The 4day embedded workshop focused on connecting theory developments to experimental advances and data needs for astrophysics and other applications.
Nuclear theory must address phenomena from laboratory experiments to stellar environments,
from stable nuclei to weaklybound and exotic isotopes. Expanding the reach of theory to these
regimes requires a comprehensive understanding of the reaction mechanisms involved as well
as detailed knowledge of nuclear structure. A recurring theme throughout the program was
the desire to produce reliable predictions rooted in either ab initio or microscopic approaches.
At the same time it was recognized that some applications involving heavy nuclei away from
stability, e.g. those involving fission fragments, may need to rely on simple parameterizations of
incomplete data for the foreseeable future. The goal here, however, is to subsequently improve
and refine the descriptions, moving to phenomenological, then microscopic approaches. There
was overarching consensus that future work should also focus on reliable estimates of errors in
theoretical descriptions.
Highlights from the program include:
Ab initio theory  Using the most advanced supercomputer facilities, solutions based on firstprinciple approaches to scattering and nuclear reactions for light nuclei have become feasible.
Successful applications to fusion reactions and astrophysics have been reported, e.g., the
^{11}C(p, γ)^{12}N Sfactor. We learned that we are close to seeing a numerical solution of the
FaddeevYakubovsky equations for the fivebody n + α system, and that ab initio theories
have aggressively pushed toward intermediate and mediummass nuclei, with selected heavy
nuclei beyond, reaching as far as ^{208}Pb. Benchmark calculations have demonstrated the
accuracy of different approaches for nuclear bulk properties. Ab initio methods that build
upon physically relevant modes in nuclei have been shown to offer computational advantages
and the ability to capture important characteristic, such as alphaclustering and deformation.
These developments make it possible to inform the NN (and 3N) interactions and motivate
the search for reliable interactions. The need to critically examine how to best approach
nuclear interactions was a theme discussed several times during our program.
(see talks by B. Barrett, A. Calci, T. Dytrych, G. Hagen, K. Launey, R. Lazauskas, P. Navratil, N. Nevo
Dinur, G. Rupak, and S. Quaglioni)
Effective field theory (EFT)  EFT approaches are seen to play an important role to provide a framework for improving nuclear descriptions via welldefined expansion schemes and
identifying the relevant degrees of freedom, as demonstrated by presentations on, e.g., pionless EFT, halo (cluster) EFT, and EFT for deformed nuclei. Most models rely on available
experimental data, but recent efforts focus on calculating model parameters within another
theoretical framework, e.g., ab initio structure calculations provide input to halo EFT. Similarly, computational advantages have been noted in methods that can provide a separation
of scales, such as a renormalization group evolution.
(see talks by P. Capel, E.A. Coello Prez, R. Furnstahl, C. Ji, T. Papenbrock, D. Phillips, G. Rupak, U. van
Kolck)
Effective intercluster interactions (EICI) or optical potentials  Reaction theory is needed to describe elastic and inelastic scattering processes, the fusion of nuclei, as well as transfers of nucleons or groups of nucleons between projectile and target. The complexity of the
problem requires the elimination of possible reaction channels from explicit consideration to
arrive at a (usually fewbody) problem which can be computed exactly. As a consequence
of this reduction to a set of manageable channels, the introduction of effective interactions
(often referred to as optical potentials) is mandatory. To date, these have been available
and widely used in the form of phenomenological optical potentials.
Remarkable progress has been reported on ab initio EICI, but these methods are still in the
early stages of development and have suggested that, to produce reliable EICI, the ab initio
structure models, from which the EICI are derived, need to reproduce matter radii and to
properly take into account collectivity/clustering degrees of freedom. For heavier nuclei,
methods based on densityfunctional and (Q)RPA approaches are being employed to obtain
microscopic potentials for the calculation of scattering observables, for both spherical and
deformed nuclei. The importance of obtaining ab initio and microscopic EICI was highlighted, as these can be used in fewbody reaction calculations and can provide guidance for
developing a new generation of phenomenological opticalmodel interactions, which should
be dispersive and nonlocal.
(see talks by G. Blanchon, W. Dickhoff, L. Hlophe, J. Holt, A. Idini, F. Nunes)
Microscopic manybody approaches and statistical reaction theory  An important objective of nuclear theory is to achieve reliable predictions for reactions involving heavy isotopes away from stability, e.g. those produced in rprocess nucleosynthesis. In the past,
phenomenological approaches (describing the bulk properties of nuclei and using parameterizations fitted to data for stable isotopes) were widely used as input for statistical (HauserFeshbach) calculations for heavy nuclei. The program featured recent developments in the
framework of microscopic approaches for calculating reaction observables or the required
nuclear structure input, such as level densities, gammaray strength functions, fission barriers, as well as optical models and preequilibrium descriptions. Approaches employing
(Q)RPA calculations with finiterange interactions, multiphonon descriptions, and shellmodel MonteCarlo methods, as well as the Gamow shell model, were discussed. It was
stressed that replacing older phenomenological models by such microscopic descriptions is
crucial for allowing for controlled extrapolations to very exotic isotopes. In addition, possible
observations of nonstatistical effects, the question of energy averaging, and a new method to
describe fission in an approach that goes beyond the traditional phenomenological approach
involving a simple fission barrier model without resorting to computationallychallenging
microscopic generatorcoordinate methods, were discussed. It became obvious that properly
including higherorder effects in the reaction theories, such as breakup effects and multistep
processes in inelastic scattering and transfer reactions, is crucial for interpreting experiments,
both for extracting structure information and for determining cross sections indirectly.
(see talks by Y. Alhassid, N. Auerbach, D. Baye, G. Bertsch, A. Bonaccorso, M. Dupuis, J. Escher, L. Jin,
M. Ploszajczak, G. Potel Aguilar, I. Stetcu, I. Thompson, A. Tonchev, N. Tsoneva)
Workshop "Nuclear Reactions: A Symbiosis between Experiment,
Theory and Applications"
A recurring theme throughout the workshop centered around how to best make connections between theory and experiment, as well as experimental measurements that will serve as crucial tests
of the predictive power of theory. Questions discussed included how to best disseminate information (logistics of databases and their maintenance), which quantities are the most appropriate for
comparisons, and reliable estimation (and reporting) of uncertainties.
Overviews of nuclear physics facilities and research programs  these included NSCL/
FRIB, TRIUMF, and ANL and other universities and national laboratories. With new
facilities coming online, the ability to study the properties of exotic nuclei will significantly
improve in the near future. These facilities promise not only significant improvements in
the breadth and intensity of radioactive ion beams, but also multiuser capabilities that
could substantially increase the volume and impact of the experimental data. Looking to
the future, several proposed upgrades to FRIB could further extend the scientific reach by
implementing higher beam energies, a harvesting capability for longlived isotopes, and an
ISOL capability.
Open questions  Major scientific themes included the structure of light exotic nuclei that
are accessible by ab initio theory, how to improve the treatment of the continuum, how to
better constrain neutron capture on radioactive nuclei, and what measurements will have the
most impact on theoretical developments. It was recognized that, in some cases, collecting
data under different conditions, such as determining spectroscopic factors and ANCs at
different beam energies, could provide additional information to limit the model dependence
of the interpretation. Other important open questions remain: For example, how good is
mirror symmetry and how large of an uncertainty do we assign to results which rely on this
symmetry? Broad resonances remain a challenge, both for measurements and for theoretical
descriptions. How well do we understand the evolution of statistical properties of nuclei, such
as the lowenergy behavior of the photon strength function or the pygmy dipole resonance,
as we move away from the valley of stability? How do we best account for the differences
between the conditions present in indirect measurements and those relevant to the quantity
we are interested in?
Astrophysics and data libraries  The theoretical and experimental developments required
to address the nuclear properties needed for astrophysics involve a large number of unstable
nuclei. Improved nuclear physics input is needed to interpret the everincreasing number
of astrophysics observations. In some cases, the increased intensity of reaccelerated beams
of shortlived nuclei will allow direct measurements of the reactions of interest, such as
(p, γ) reactions of importance for understanding the rpprocess. However, for most reactions
needed for nuclear astrophysics and applications, direct measurement will remain inaccessible
due to beam or target limitations. For these cases, indirect approaches which depend on
a close interplay of experimental observables and reaction theory are needed. For nuclei on the nucleosynthesis paths that will remain unmeasured, theoretical predictions will be indispensable. Another topic of discussion was how to better integrate experimental and
theoretical efforts to improve data libraries.
(see talks by T. Ahn, B. Back, D. Bardayan, J. Blackmon, C. Brune, J. Cizewski, A. Garnsworthy, B. Jurado, F.
Montes, G. Nobre, A. Couture, G. Perdikakis, G. Rogachev, N. Scielzo, R. Surman)
The program attracted a mix of scientists at different stages in their careers, from students and postdocs to senior members
and retirees. Our discussion sessions benefited from
this diverse mix, as lessons
learned from much earlier
work were considered along
with very recent results.
