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Neutron-rich nuclei: Nuclear structure at the extremes

• Can nuclei be described in terms of our understanding of the underlying fundamental interactions?
• What is the role of 3-body forces?
• How does the ordering of quantum states change in exotic nuclei?

To address these questions, we lead experimental studies of neutron-rich nuclei. We are performing large-scale experiments at leading facilities world-wide, such as the R3B setup at GSI (cartoon of the setup below), at the S800 spectrometer of the NSCL facility, at the ATLAS facility at ANL, at JYFL and at the SAMURAI setup in RIKEN etc.

Connecting nuclear structure and dynamics with the theory of the strong interaction, quantum chromodynamics (QCD), is one of the overarching goals of the nuclear science community. The understanding of nuclei based on QCD will have strong predictive power with direct implications to other science frontiers, such as astrophysics and its quest to understand the origin of the elements. A new theoretical concept developed recently brings nuclear structure physics closer to reaching this goal for the first time. This new and promising approach exploits chiral Effective Field Theories (χEFTs) of the strong interaction, as first proposed by Steven Weinberg [We90], to derive nuclear interactions that can be used in ab initio calculations, such as Lattice EFT, No Core Shell Model, Green’s Function Monte Carlo and Coupled Cluster [Ep09]; χEFTs allow for connections with fundamental non-perturbative studies of few-nucleon systems (lattice QCD). Indeed, modern ab initio calculations are attempting to describe, self-consistently, neutron-rich nuclei and neutron stars [Ly16] and to calculate the size of nuclei [Ha16] starting from chiral forces. Moreover, three-nucleon forces play a prominent role in neutron-rich nuclear systems, as has been confirmed by recent theoretical studies [He15 and references therein]. We are leading an ambitious experimental programme that strongly feeds directly into the theoretical efforts at this pioneering frontier, providing experimental benchmarks for ab initio theories and realistic interactions in light neutron-rich nuclei.

The concept of nuclear shells and single-particle states is one of the most useful elements of understanding properties of composite self-bound systems such as atomic nuclei. Although a link between many-body observables and single-particle properties can be rigorously formulated [Ba70], experimental data are most often analysed in terms of Density Functional Theory (DFT) or shell-model approaches, where single-particle energies appear explicitly. In this language, the robustness of the nuclear magic numbers has already been challenged by exploring nuclei far from stability. The effort nowadays is to understand the mechanism responsible for this shell evolution, with many efforts focusing on: (i) the variations of nuclear mean field with neutron excess due to tensor interactions; (ii) the importance of many-body correlations; and (iii) the influence of open channels on properties of weakly bound and unbound nuclear states, as presented in [Do07].

To this respect, we lead a programme of experiments to study neutron-rich nuclei in key regions of the nuclear chart to advance our understanding of the underlying mechanism responsible for the shell evolution.

The experimental findings obtained are analysed by performing advanced DFT calculations. Thanks to the remarkable progress in computer science we can now perform systematic DFT calculations over the entire nuclear chart from drip-line to drip-line [Go09]. DFT is thus the tool of choice to perform studies of neutron-rich nuclei far away from the valley of stability. These systems are of particular interest for improving current models due to their strong isospin asymmetry and the vicinity of the continuum which enhances correlations effects (such as pairing effects). On the theory side, we want to tackle a long standing question in nuclear structure: the role of the effective tensor forces [Ot05, Le07, Su16]. 

[Ba70] M. Baranger, Nucl. Phys. A 149, 225 (1970)
[Do07] J. Dobaczewski, N. Michel, W. Nazarewicz, M. Ploszajczak and  J. Rotureau, Prog. Part.
Nucl. Phys 59, 432 (2007)
[Ep09] E. Epelbaum et al., Rev. Mod. Phys. 81, 1773 (2009)
[Go09] S. Goriely, N. Chamel and M. Pearson; Phys. Rev. Lett. 102, 152503 (2009)
[Ha16] G. Hagen et al, Nature Physics 12, 186-190 (2016)
[He15] K. Hebeler et al., Annu. Rev. Nucl. Part. Sci. 65, 457 (2015)
[Le07] T. Lesinski, M. Bender, K. Bennaceur, T. Duguet, J. Meyer Phys. Rev. C 76, 014312
(2007)
[Ly16] J. Lynn et al., Phys. Rev. Lett. 116, 062501 (2016)
[Ot10] T. Otsuka et al., Phys. Rev. Lett. 95, 232502 (2005)
[Su16] Y. Suzuki, H. Nakada and S. Miyahara, Phys. Prev. C 94, 024343 (2016)
[We90] S. Weinberg, Phys. Lett. B 251, 288 (1990)

Highlights

• Experiment at Argonne National Laboratory to study excited states of 16 C utilizing the Gammasphere array coupled to the Microball particle detector successfully performed in July 2016.

• Experiment 015085 at NSCL to study excited states of 21 O utilizing the GRETINA detector coupled to the S800 spectrometer and the plunger device for lifetime measurements of excited nuclear states was successfully performed in March 2016.

• Experiment NP1406-SAMURAI19R1 at the SAMURAI setup at RIKEN to study the four-neutron resonant system (to be performed).