The main topics of the workshop are:

Accurate interactions and uncertainty quantification

Current chiral EFT interactions are still not able to reproduce simultaneously few- and many-body data across the nuclear chart. In addition, the dominat theoretical uncertainties of ab initio calculations of nuclei and nuclear matter can be attributed to uncertainties of the underlying Hamiltonian. As a consequence, there are ongoing efforts to develop new and more systematic statistical methods that allow to determine the unknown low-energy coupling constants of the interactions at different orders in the EFT expansion and to include the EFT uncertainties rigorously in ab initio calculations. We will discuss various strategies for assessing correlated truncation errors and identify new strategies and open questions regarding the incorporation of different power counting and regularization schemes in the uncertainty analysis.

Universality in nuclei, multi-neutron systems (tetraneutron and beyond)

The structure of multi-neutron systems is challenging in experiment and theory because it involves understanding multi-body continuum states. The most convincing evidence for a resonance-like structure in the tetraneutron system to date was presented in an experiment using the hard knock-out reaction ⁸He(p,pα)4n. Whether the origin of this structure is a genuine resonance or some other mechanism, such as the final state interaction among di-neutrons and/or the valence neutron structure of the ⁸He projectile is an open question that will be discussed at the workshop. We will also elucidate to which extent quantum simulations with ultracold fermionic atoms can contribute to the resolution of this question. Finally, we plan to address multi-neutron systems beyond four neutrons, which will be investigated in future experiments.

Heavy nuclei and deformation

Nuclei away from shell closures are driven by complex many-body correlations, and deformation in particular. The importance of deformation becomes even moreprominent in heavy nuclei (A > 100) where shell closures are scarce and the vast majority of nuclei feature complex intrinsic shapes. So far, the ab initio description of deformed systems is only in its infancy and an accurate description of observables sensitive to deformation, e.g. electromagnetic transition strengths, defines an important future challenge. Due to its mild computational scaling, the use of phenomenological energy-density functionals is the method of choice when targeting heavy deformed systems. Ab initio approaches are strongly inspired by the spontaneous breaking (and restoration) of spatial and gauge symmetries, which is a key ingredient in the nuclear DFT approach. Finally, DFT calculations may guide the role of different symmetries such as the emergence of exotic deformation modes, e.g., for searches of parity-violating moments.

Electroweak interactions in nuclei

Nuclear electroweak processes are versatile tools to probe the structure of nuclei as well as beyond-the-standard-model (BSM) physics. One can either search for processes that would violate the standard model (such as neutrinoless double-beta decay) or look for BSM contributions to standard-model processes in precision experiments. In either case, to extract BSM physics from experiments, one needs reliable nuclear-theory predictions for the processes, including both accurate description of the structure of nuclei and the weak interaction mediating the process. To this end, recent progress in ab initio nuclear theory has allowed for consistent EFT description of the strong nuclear interaction and electroweak operators based on the EFT expansion allowing for robust uncertainty quantification. In particular, two-body currents have been shown to be crucial in accurate prescription of electroweak properties of nuclei, such as beta-decay rates and magnetic moments.

Collective phenomena, matching ab initio and DFT

While the ab initio description of bulk properties of light and medium-mass nuclei is well developed, there are clear limitations of ab initio theory when it comes to collective phenomena in nuclei. One aspect is the role of collectivity in the spectroscopy of low-lying states, typically connected to intrinsic deformations as already addressed in 3). The effect of collective correlations in ab initio calculations of electromagnetic moments and transition strength is a topic of intense investigations. This extends to the description of higher-lying collective excitations and giant resonances. Only few ab initio methods have been extended into this domain, which connects to a wealth of experimental data and important constraints for the equation of state. Both domains, collectivity in low-lying excitations and giant resonances, offer exciting perspectives for connecting ab initio studies with energy-density-functional based methods. Both sectors share similar many-body tools, e.g. the generator coordinate method or the random-phase approximation, and offer multiple synergies towards a comprehensive understanding of collective excitations.