Collective Modes in Light Nuclei from First Principles

Abstract

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]).

Symmetry-guided large-scale shell-model theory

“…Remarkably, the outcome of these studies has revealed that typically only one or two symplectic irreducible representations (irreps), also referred to as "vertical cones" of many-particle basis states, suffice to represent a large fraction of each of the ab initio wave functions of 12 C and 16 O, typically in excess of about 80% of the physics. Such a symplectic pattern has been also observed in ab initio SA-NCSM results for light and intermediate-mass nuclei using symmetry-adapted SU(3)-scheme basis states [20]. Implications of these studies to understanding the nature of nuclear dynamics and symmetry-guided applications are reviewed in Sec.…”

“…With an expanding body of experimental evidence, it is becoming evident that non-zero deformation is far more widespread than zero deformation and that even nuclei that are spherical in their ground state have low-lying deformed excited states [36]. In fact, first-principle calculations in the SA-NCSM [20] (see also Sec. 5.2) have unveiled that even the lightest of nuclei, exemplified by 6 Li, in their ground state exhibit considerable collectivity, as seen by the dominance of prolate deformed configurations in the 6 Li wave function.…”

“…We focus on the symplectic Sp(3, R) symmetry and its deformation-related SU(3) subgroup, which underpins the Elliott model [39,40], the symplectic model [41,42] and the ab initio SA-NCSM [20], and review the broad scope of nuclei, where these symmetries have been found to play a key role -from the lightest p-shell systems of 6 Li and 8 Be, through sd-shell intermediate-mass nuclei, up to strongly deformed nuclei of the rare-earth and actinide regions (see also the review Ref. [43]).…”

“…Several ab initio nuclear-theory approaches have been recently advanced, including Green's function Monte Carlo (GFMC) [6,7], no-core shell model (NCSM) [8][9][10][11][12] together with NCSM with a core [13] and importance truncation NCSM [14], coupledcluster method (CC) [15,16], lattice effective field theory [17], in-medium SRG [18,19], symmetryadapted no-core shell model (SA-NCSM) [20], Monte Carlo NCSM [21], and self-consistent Green's function [22]. Ab initio approaches build upon a 'first principles' foundation.…”

Alpha particle clusters and their condensation in nuclear systems

“…And the intensity of this type of studies does not seem to slow down. For example very recently ab initio studies for cluster states appeared both using the nuclear effective-fieldtheory (EFT) approaches [41,42,43,44] and the symplectic no-core shell model (NCSM) [45,46,47] as we will discuss later in the main text.…”