Reaching heavier masses

Low energy nuclear physics is currently going through an unprecedented revival due to several combining factors. Explicit links with the underlying Quantum Chromo Dynamics have allowed to developed models of the nuclear interaction soft enough to converge in quantum many-body calculations, say up to A=100 nucleons. Chiral perturbation theory not only gives a clear way to devise coherent models of two-, three- and many-nucleon forces but very recent applications have also proven unprecedented predictive power even for isotopes in the mid-mass range. Key factors to the development of ab-initio theory toward heavier masses are the availability quantum-many body techniques that scales gently with the number of nucleon, advances of algorithms and HPC, and the introduction of novel techniques that can address efficiently pairing effect and open shell systems.

The program on "Computational and Theoretical Advances for Exotic Isotopes in the Medium Mass Region" brought together practitioners in ab-initio many-body theory in order to address key questions for future progress, including: How can we extend current ab-initio methods to describe open-shell and deformed nuclei? How can we include the effects of three-nucleon forces (3NF) in a computationally efficient manner? Can we develop reliable theoretical error estimates for benchmarking and accuracy? How can we describe the onset of pairing in nuclei within various ab-initio frameworks? How can we generate reliable predictions for the drip-lines? How can we use ab-initio theory to constrain EDF methods and derive microscopically-informed parameterizations of the associated energy density functional kernels?

Over 40 participants attended the program which was subdivided in a week of discussions on development of realistic nuclear interactions, one week of confrontation on many-body approaches for finite systems between nuclear physics and quantum chemistry, and two weeks focused on advances in ab-initio nuclear structure. Strong momentum came from a three-day workshop in which nuclear physicists and quantum chemists were present. The introduction of ideas based on symmetry breaking and restoration to ab-initio theory has been identified as valid alternative to multi reference in both disciplines. Theory and implementation of Gorkov Green's function method was discussed along with similar recent developments along (e.g. in-medium similarity renormalization group and Bogoliubov Coupled cluster). Concerning nuclear interactions, highlights were the discussion of alternatives to Weinberg's power counting and new high-quality fits (POUNDerS). Chiral three-nucleon forces at leading order have been shown to determine the dynamics at the neutron driplines for several isotopes, from nitrogen and fluorine up to calcium. Quantum Monte Carlo calculations for infinite neutron matter have now been achieved with chiral interactions in momentum space. Several other interesting and novel ideas were aired during the program, which will stimulate further advances of this field.

Breakthroughs of ab-initio theory in heavier regions of the nuclear chart are very welcome, as they would eventually provide reliable nuclear input to several problems. These include astrophysics, e.g. the understanding of the explosive nucleosynthesis of heavy elements in the universe or the physics of neutron stars, and tests of the Standard Model, e.g. the unitarity of the Cabibbo-Kobayashi-Maskawa matrix.