INT Program INT192a
Nuclear Structure at the Crossroads
July 1  August 2, 2019
Overview
Effective Field Theories (EFTs) applied to nuclei and nuclear matter have had many phenomenological successes. In particular, nuclear interactions derived from chiral EFT and advances in nuclear manybody techniques have enabled rapid progress in ab initio nuclear structure calculations up to mass numbers A~100. However, the limits in mass and density of this approach are currently not known and there are open issues regarding the convergence and power counting of chiral EFT. Alternative lowerresolution EFTs have shown advantages for particular aspects of nuclear structure and reactions while EFT for energydensity functionals has many open questions. In the meantime, quantitative matching of QCD to nuclear EFT is becoming increasingly feasible. The goal of this program is to review recent progress in nuclear structure research using EFTs and to set future priorities in the light of upcoming experiments.
Background on recent developments

The combination of chiral EFT using Weinberg power counting with powerful manybody methods has dramatically extended ab initio calculations to mediumheavy nuclei. This has led to major successes in nuclear structure theory, but also highlights open problems in the present status of nuclear forces and currents.
The Weinberg approach has been criticized for the small range of cutoff variations possible, and some quantities are sensitive to the regulators used. Whether this is still a problem at high enough orders or a fundamental consequence of nonperturbative renormalization and the small separation of scales in nuclei is a matter of current debate. There are alternative approaches where the cutoff can be varied over a larger range, but their implementation in manybody methods needs to be explored.
Statistical methods for robust uncertainty quantification of nuclear EFTs has become an active area of research. These can also be used to assess alternative power counting schemes and to provide model selection and validation.
EFTs with a lower resolution scale make emergent degrees of freedom and universal relations between observables more transparent and can simplify certain calculations. Such EFTs include pionless EFT for fewnucleon systems and halo EFT for halo and cluster nuclei. More recently, an EFT for collective degrees of freedom has also been constructed, with very encouraging results.
The access to the heaviest nuclei and to global predictions of the nuclear chart is facilitated by energydensity functionals. Thus they provide a key piece towards the goal of describing the whole nuclear chart from first principles. While progress has been made in deriving energydensity functionals from EFT, phenomenological energy functionals are still more accurate to date.
Lattice QCD calculations of fewnucleon observables have become available in the last ten years and are now approaching pion masses close to the physical value. This makes matching to the fundamental theory of QCD at physical pion masses possible by the time of the program and will provide an anchor point for an EFTbased description of the nuclear chart firmly rooted in QCD.
Program format
The program will focus on key questions addressing open issues and priorities for future research on EFTs for lowenergy nuclear physics. We have divided the program into two overlapping topical blocks:
Weeks 13: Foundations, advances and limits of EFTs for nuclei and nuclear matter
Are the small cutoff variations in chiral EFT a real deficit or a fundamental consequence of nonperturbative renormalization and/or the small separation of scales in nuclei? What are viable alternative schemes and what are the key observables to test them? What are the limits of chiral EFT in heavy nuclei? Does the Fermi momentum emerge naturally in chiral EFT or should it be included explicitly as an additional scale? How can one quantify the limits in mass and density? How do Bayesian statistical methods help in addressing these questions?
Weeks 35: More effective EFTs, energydensity functionals, and lattice QCD
How do new degrees of freedom emerge from ab initio calculations? What are viable strategies for the development of more effective EFTs and their matching to ab initio calculations as well as experimental data? How can this program help ongoing efforts to integrate structure and reaction calculations? What strategies should be pursued for developing new expansions for energydensity functionals as well as improving the accuracy of EFTbased energy functionals? What are the key observables and strategies for matching EFTs of nuclei to lattice QCD?
We encourage all scientists interested in these questions to apply, using the link in the sidebar on the left of this page. The participation of early career researchers as well as underrepresented groups in the physics community are particularly encouraged.
Supported in part by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation)  Projektnummer 279384907  SFB 1245.