Organizers: Richard J. Furnstahl, Hans-Werner Hammer, Achim Schwenk

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 many-body 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 lower-resolution EFTs have shown advantages for particular aspects of nuclear structure and reactions while EFT for energy-density functionals has many open questions. In the meantime, quantitative matching of QCD to nuclear EFT is becoming increasingly feasible. We are thus at a point where it is important to identify the priorities in moving forward in low-energy nuclear theory, i.e., at the "crossroads," particularly with respect to the role of effective field theory (EFT).

This 5-week program provided a meeting place for different approaches to nuclear structure with the goal of addressing open issues and priorities for future research. The program was divided into two topical blocks:

• Foundations, advances and limits of EFTs for nuclei and nuclear matter
• More effective EFTs, energy-density functionals, and lattice QCD

The vigorous discussions centered around a number of key questions:

• Which degrees of freedom should be used for which observables?
• Which Hamiltonians and currents should be used in ab initio calculations?
• What are strengths and weaknesses of ab initio and other methods?
• What are the status and perspectives of lattice QCD in nuclear physics?
• How can we understand energy density functionals as an EFT?
• What is the role of Bayesian statistical methods and uncertainty quantification in general?

The program identified important future calculations and paths forward. In addition, there was large consensus that uncertainty quantification for theory is essential and should be included in any future EFT calculation. We anticipate that the program has helped to focus the efforts in nuclear structure calculations using EFT and reevaluate the priorities in the field.