Results for ab initio no-core shell model calculations in a symmetry-adapted SU(3)-based coupling scheme demonstrate that collective modes in light nuclei emerge from first principles. The low-lying states of 6 Li, 8 Be, and 6 He are shown to exhibit orderly patterns that favor spatial configurations with strong quadrupole deformation and complementary low intrinsic spin values, a picture that is consistent with the nuclear symplectic model. The results also suggest a pragmatic path forward to accommodate deformation-driven collective features in ab initio analyses when they dominate the nuclear landscape.Introduction. -Major progress in the development of realistic inter-nucleon interactions along with the utilization of massively parallel computing resources [1][2][3] have placed ab initio approaches [4][5][6][7][8][9][10][11][12][13][14] at the frontier of nuclear structure explorations. The ultimate goal of ab initio studies is to establish a link between underlying principles of quantum chromodynamics (quark/gluon considerations) and observed properties of atomic nuclei, including their structure and related reactions. The predictive potential that ab initio models hold [15,16] makes them suitable for targeting short-lived nuclei that are inaccessible by experiment but essential to modeling, for example, of the dynamics of X-ray bursts and the path of nucleosynthesis (see, e.g., [17,18]).

Exact symmetry and symmetry-breaking phenomena play a key role in providing a better understanding of the physics of many-particle systems, from quarks and atomic nuclei, to molecules and galaxies. In atomic nuclei, exact and dominant symmetries such as rotational invariance, parity, and charge independence have been clearly established. However, even when these symmetries are taken into account, the structure of nuclei remains illusive and only partially understood, with no additional symmetries immediately evident from the underlying nucleon-nucleon interaction. Here, we show through ab initio large-scale nuclear structure calculations that the special nature of the strong nuclear force determines additional highly regular patterns in nuclei that can be tied to an emergent approximate symmetry. We find that this symmetry is remarkably ubiquitous, regardless of its particular strong interaction heritage, and mathematically tracks with a symplectic group. Specifically, we show for light to intermediate-mass nuclei that the structure of a nucleus, together with its low-energy excitations, respects symplectic symmetry at about 70-80% level, unveiling the predominance of only a few equilibrium shapes, deformed or not, with associated vibrations and rotations. This establishes the symplectic symmetry as a remarkably good symmetry of the strong nuclear force, in the low-energy regime. This may have important implications to studies, e.g., in astrophysics and neutrino physics that rely on nuclear structure information, especially where experimental measurements are incomplete or not available. A very important practical advantage is that this new symmetry can be utilized to dramatically reduce computational resources required in ab initio large-scale nuclear structure modeling. This, in turn, can be used to pioneer predictions, e.g., for short-lived isotopes along various nucleosynthesis pathways. arXiv:1810.05757v1 [nucl-th]

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We present an ab initio symmetry-adapted no-core shell-model description for 6 Li. We study the structure of the ground state of 6 Li and the impact of the symmetry-guided space selection on the charge density components for this state in momentum space, including the effect of higher shells. We accomplish this by investigating the electron scattering charge form factor for momentum transfers up to q ∼ 4 fm −1 . We demonstrate that this symmetry-adapted framework can achieve significantly reduced dimensions for equivalent large shell-model spaces while retaining the accuracy of the form factor for any momentum transfer. These new results confirm the previous outcomes for selected spectroscopy observables in light nuclei, such as binding energies, excitation energies, electromagnetic moments, E2 and M 1 reduced transition probabilities, as well as point-nucleon matter rms radii.

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We report on the computational characteristics of ab initio nuclear structure calculations in a symmetry-adapted no-core shell model (SA-NCSM) framework. We examine the computational complexity of the current implementation of the SA-NCSM approach, dubbed LSU3shell, by analyzing ab initio results for 6 Li and 12 C in large harmonic oscillator model spaces and SU(3)-selected subspaces. We demonstrate LSU3shell's strong-scaling properties achieved with highly-parallel methods for computing the many-body matrix elements. Results compare favorably with complete model space calculations and significant memory savings are achieved in physically important applications. In particular, a well-chosen symmetry-adapted basis affords memory savings in calculations of states with a fixed total angular momentum in large model spaces while exactly preserving translational invariance.