[en] Based on the fundamental symmetries of QCD, chiral effective field theory (EFT) provides two- (NN), three- (3N), four- (4N), and many-nucleon interactions in a consistent and systematic scheme. Recent developments to construct chiral NN+3N interactions at different chiral orders and regularizations enable exciting nuclear structure investigations as well as a quantification of the fundamental uncertainties resulting from the chiral expansion and regularization. We present the complete toolchain to employ the present and future chiral NN, 3N, and 4N interactions in nuclear structure calculations and emphasize technical developments in the three- and four-body space, such as the similarity renormalization group (SRG), the frequency conversion, and the transformation to the JT-coupled scheme. We study the predictions of the chiral NN+3N interactions in ab initio nuclear structure calculations with the importance-truncated no-core shell model and coupled-cluster approach. We demonstrate that the inclusion of chiral 3N forces improves the overall agreement with experiment for excitation energies of p-shell nuclei and it qualitatively reproduces the systematics of nuclear binding energies throughout the nuclear chart up to heavy tin isotopes. In this context it is necessary to introduce truncations in the three-body model space and we carefully analyze their impact and confirm the reliability of the reported results. The SRG evolution induces many-nucleon forces that generally cannot be included in the calculations and constitute a major limitation for the applicability of SRG-evolved chiral forces. We study the origin and effect of the induced many-nucleon forces and propose a modification of the interaction, which suppresses the induced beyond-3N forces. This enables applications of the chiral interactions far beyond the mid-p shell. Furthermore, we test alternative formulations of SRG generators aiming to prevent the induced many-body forces from the outset. The extension of the SRG evolution and matrix-element treatment to the four-body space allows for an explicit inclusion of induced and initial 4N forces. We discuss the truncations and limitations in the four-body space and present first ab initio nuclear structure calculations for p-shell nuclei with induced 4N forces. By changing the parameters of the local 3N force we perform a comprehensive sensitivity analysis for nuclear spectra in the p shell that provides constraints for the construction of chiral interactions. Moreover, we identify certain correlations that prevent an accurate description of the experimental results by an adjustments of the local 3N force at N²LO. We report first results obtained with a next-generation NN+3N interaction at N³LO and compare the spectra obtained with several chiral interactions at different chiral orders, varying the regulator function and cutoff. These studies present a first step towards a systematic uncertainty quantification for the chiral forces and we show that p-shell spectroscopy is a sensitive diagnostic for chiral 3N interactions.

[en] We present ab initio calculations of nuclear binding energies using chiral two- plus three-nucleon Hamiltonians evolved consistently with the similarity renormalization group. We use coupled-cluster theory at the singles and doubles excitations level in combination with a non-iterative treatment of triples excitations in order to obtain accurate solutions of the many-body problem. We explore different chiral two- and three-nucleon interactions and the systematics of ground-state energies up to ¹³²Sn. These calculations open the door to ab initio calculations in the regime of heavy nuclei and demonstrate the predictive power of chiral Hamiltonians.

[en] Nucleon-nucleon (NN) and three-nucleon (3N) interactions derived from chiral effective field theory, transformed using the similarity renormalization group (SRG), have been successful in the ab initio description of nuclear-structure. Previous investigations have shown strong indications for sizable effects originating by SRG-induced four-nucleon (4N) contributions. To obtain reliable results, the 4N contributions can either be suppressed or included in nuclear-structure calculations. We present an efficient scheme to take induced as well as initial 4N forces into account by performing the SRG transformation in four-body space and extending the importance-truncated no-core shell model (IT-NCSM) for their explicit inclusion. We investigate the effect of induced forces beyond the three-body level in ab initio nuclear-structure calculations and present results for p-shell nuclei up to ¹⁶O.

[en] Chiral effective field theory provides a systematic scheme to obtain two- (NN), three-body (3N) and higher-body interactions based on the fundamental symmetries of QCD. In the past years several chiral interactions were developed using different regulator functions, cutoffs and chiral expansion orders. Due to the recent extension of the Similarity Renormalization Group (SRG) beyond the two-body level, we are able to probe these interactions in ab initio nuclear structure calculations for nuclei in the p- and lower sd-shell. By applying the importance-truncated no-core shell model (IT-NSCM) to specific nuclei in the p-shell, we provide an uncertainty quantification for various NN+3N interaction e.g. by varying the cutoffs of the chiral order. In this context we demonstrate the importance of the three- and four-body contributions induced during the SRG transformation and give a perspective for the handling of SRG transformed chiral interactions including four-body contributions.

No abstract available

[en] The inclusion of three-nucleon (3N) interactions, that are important to understand the structure of nuclei, is still a challenging task in ab-initio nuclear-structure calculations, because of the tremendous computational cost. We have shown that normal ordering with respect to a single-reference state provides a helpful tool to derive an approximate lower-particle-rank form of any 3N interaction. This approximation, however, is limited to closed-shell nuclei. We now extend the normal-ordering approximation to open-shell systems using a multi-determinantal reference state, i.e. a linear combination of single Slater determinants. For the first time, we present spectra for p-shell nuclei, e.g. ⁶ Li and ¹⁰B, calculated in importance-truncated no-core shell model using chiral 3N interactions in normal-order approximation with respect to a multi-determinantal reference state. We compare them to calculations using full 3N interactions, and go beyond model-space sizes that can be handled with full 3N interactions.

[en] Chiral effective field theory provides two- (NN) and three-body (3N) interactions from QCD in a consistent manner. We employ these interactions in ab initio nuclear structure calculations within the importance-truncated no-core shell model (IT-NSCM) using an additional Similarity Renormalization Group (SRG) transformation to improve convergence. Formally, the SRG induces irreducible many-body contributions, which nowadays can be included up to the three-body level. Using a flow-parameter analysis we show that the conventional SRG evolution induces sizeable many-body contributions for nuclei in the mid-p-shell and beyond. We demonstrate that the induced many-body contributions originate from the two-pion exchange terms of the initial 3N interaction. Finally, the suppression of the induced many-body contributions either by an alternative formulation of the SRG or by a modification of the initial chiral interactions is discussed. This opens an opportunity to universal applications of chiral 3N interactions in nuclear structure.

[en] Chiral effective field theory provides the most systematic QCD-based interaction for nuclear structure calculations. We present ab-initio calculations of spectra and spectroscopic observables for p- and sd-shell nuclei obtained within the importance-truncated no-core shell model (IT-NCSM) using chiral two-body (NN) plus three-body (3N) interactions. To improve the convergence of the IT-NCSM calculations, we soften the nuclear NN+3N interactions via a consistent Similarity Renormalization Group transformation at the three-body level. The comparison of our predictions for spectra and spectroscopy to experimental data provides new benchmarks of the quality of chiral interactions. Furthermore, we study the sensitivity of the spectroscopy for A≥10 including the carbon isotopic chain to the parameters of the chiral forces, e.g. the low-energy constants (LECs) or the cut-off. With these results we investigate the propagation of the uncertainties of the LECs into nuclear structure.

[en] Neutron drops provide a simple test system of neutron-rich matter. Hence, ab initio calculations of these pure neutron systems may be used to constrain the large isospin properties of energy-density functionals and equations of state that are used for the description of very neutron-rich systems as they occur in astrophysical environments, e.g., neutron stars. For the first time, we calculate properties of neutron drops interacting via realistic two- and three-nucleon forces derived from chiral effective field theory using the ab initio importance-truncated no-core shell model (IT-NCSM). In contrast to nuclei, systems consisting of only neutrons are not self-bound and, therefore, need to be confined in an external potential well. Variations of the external potential, typically a harmonic oscillator potential, can be used to explore different density regimes. With this input, we compute a range of different properties of neutron drops, such as ground-state energies, radial densities and excitation energies, and compare our IT-NCSM results to previous calculations performed by using phenomenological interactions.

[en] Tremendous progress is being made on the experimental study of hypernuclei, especially on the spectroscopy of p-shell hypernuclei. Their theoretical description, however, is limited to phenomenological models or to very light (i.e. s-shell) systems. We present ab initio calculations of p-shell hypernuclei using chiral Hamiltonians including leading-order (LO) hyperon-nucleon as well as two- and three-nucleon interactions at N³LO and N²LO, respectively. To improve convergence with respect to model space size, the Hamiltonians are evolved using a Similarity Renormalization Group (SRG) transformation. The many-body calculations are carried out in the framework of the importance-truncated no-core shell model. We present the first ab initio results for the spectroscopy of _Λ⁷Li, _Λ⁹Be and _Λ¹³C obtained using chiral and phenomenological hyperon-nucleon interactions. We also discuss the role of SRG-induced hyperon-nucleon-nucleon (YNN) terms which hint at the impact of chiral YNN interactions.