190 research outputs found
Emergent symmetries in atomic nuclei from first principles
An innovative symmetry-guided approach and its applications to light and intermediate-mass nuclei is discussed. This approach, with Sp(3, R) the underpinning group, is based on our recent remarkable finding, namely, we have identified the symplectic Sp(3,R) as an approximate symmetry for low-energy nuclear dynamics. This study presents the results of two complementary studies, one that utilizes realistic nucleon-nucleon interactions and unveils symmetries inherent to nuclear dynamics from first principles (or ab initio), and another study, which selects important components of the nuclear interaction to explain the primary physics responsible for emergent phenomena, such as enhanced collectivity and alpha clusters. In particular, within this symmetry-guided framework, ab initio applications of the theory to light nuclei reveal the emergence of a simple orderly pattern from first principles. This provides a strategy for determining the nature of bound states of nuclei in terms of a relatively small fraction of the complete shell-model space, which, in turn, can be used to explore ultra-large model spaces for a description of alpha-cluster and highly deformed structures together with associated rotations. We find that by using only a fraction of the model space extended far beyond current no-core shell-model limits and a long-range interaction that respects the symmetries in play, the outcome reproduces characteristic features of the low-lying 0+ states in 12C (including the elusive Hoyle state of importance to astrophysics) and agrees with ab initio results in smaller spaces. For these states, we offer a novel perspective emerging out of no-core shell-model considerations, including a discussion of associated nuclear deformation, matter radii, and density distribution. The framework we find is also extensible beyond 12C, namely, to the low-lying 0+ states of 8Be as well as the ground-state rotational band of Ne, Mg, and Si isotopes
SU(3)-guided Realistic Nucleon-nucleon Interactions for Large-scale Calculations
We examine nucleon-nucleon realistic interactions, based on their SU(3)
decomposition to SU(3)-symmetric components. We find that many of these
interaction components are negligible, which, in turn, allows us to identify a
subset of physically relevant components that are sufficient to describe the
structure of low-lying states in C and related observables, such as
excitation energies, electric quadrupole transitions and rms radii. We find
that paring the interaction down to half of the SU(3)-symmetric components or
more yields results that practically coincide with the corresponding ab initio
calculations with the full interaction. In addition, we show that while various
realistic interactions differ in their SU(3) decomposition, their renormalized
effective counterparts exhibit a striking similarity and composition that can
be linked to dominant nuclear features such as deformation, pairing,
clustering, and spin-orbit effect.Comment: 9 pages, 7 figure
Electromagnetic Excitations and Responses in Nuclei from First Principles
We discuss the role of clustering on monopole, dipole, and quadrupole
excitations in nuclei in the framework of the ab initio symmetry-adapted
no-core shell model (SA-NCSM). The SA-NCSM starts from nucleon-nucleon
potentials and, by exploring symmetries known to dominate the nuclear dynamics,
can reach nuclei up through the calcium region by accommodating ultra-large
model spaces critical to descriptions of clustering and collectivity. The
results are based on calculations of electromagnetic sum rules and discretized
responses using the Lanczos algorithm, that can be used to determine response
functions, and for 4He are benchmarked against exact solutions of the
hyperspherical harmonics method. In particular, we focus on He, Be, and O
isotopes, including giant resonances and monopole sum rules.Comment: 6 pages, 4 figures, Proceedings of the Fourth International Workshop
on State of the Art in Nuclear Cluster Physics, Galveston, TX, USA, May
13-18, 201
Electromagnetic Excitations and Responses in Nuclei from First Principles
We discuss the role of clustering on monopole, dipole, and quadrupole
excitations in nuclei in the framework of the ab initio symmetry-adapted
no-core shell model (SA-NCSM). The SA-NCSM starts from nucleon-nucleon
potentials and, by exploring symmetries known to dominate the nuclear dynamics,
can reach nuclei up through the calcium region by accommodating ultra-large
model spaces critical to descriptions of clustering and collectivity. The
results are based on calculations of electromagnetic sum rules and discretized
responses using the Lanczos algorithm, that can be used to determine response
functions, and for 4He are benchmarked against exact solutions of the
hyperspherical harmonics method. In particular, we focus on He, Be, and O
isotopes, including giant resonances and monopole sum rules.Comment: 6 pages, 4 figures, Proceedings of the Fourth International Workshop
on State of the Art in Nuclear Cluster Physics, Galveston, TX, USA, May
13-18, 201
No-core Symplectic Model: Exploiting Hidden Symmetry in Atomic Nuclei
We report on recent developments within the framework of the no-core symplectic shell model (NCSpM) that complements the no-core shell model (Navrátil, Vary, and Barrett) by exploiting the algebraic features of the symplectic shell model (Rowe and Rosensteel) while also allowing for high-performance computing applications, but in highly truncated, physically relevant subspaces of the complete space. The leading symplectic symmetry typically accounts for 70% to 90% of the structure of the low-lying states, a result that is only moderately dependent on the details of the selected inter-nucleon interaction. Examples for6Li,12C,16O, and20Ne are shown to illustrate the efficacy the NCSpM, and as well the strong overlap with cluster-like and pairing configurations that dominate the dynamics of low-lying states in these nuclei
Nuclear spin features relevant to ab initio nucleon-nucleus elastic scattering
Background: Effective interactions for elastic nucleon-nucleus scattering
from first principles require the use of the same nucleon-nucleon interaction
in the structure and reaction calculations, as well as a consistent treatment
of the relevant operators at each order.
Purpose: Previous work using these interactions has shown good agreement with
available data. Here, we study the physical relevance of one of these
operators, which involves the spin of the struck nucleon, and examine the
interpretation of this quantity in a nuclear structure context.
Methods: Using the framework of the spectator expansion and the underlying
framework of the no-core shell model, we calculate and examine spin-projected,
one-body momentum distributions required for effective nucleon-nucleus
interactions in nuclear states.
Results: The calculated spin-projected, one-body momentum distributions for
He, He, and He display characteristic behavior based on the
occupation of protons and neutrons in single particle levels, with more
nucleons of one type yielding momentum distributions with larger values.
Additionally, we find this quantity is strongly correlated to the magnetic
moment of the excited state in the ground state rotational band for each
nucleus considered.
Conclusions: We find that spin-projected, one-body momentum distributions can
probe the spin content of a wave function. This feature may allow future
\textit{ab initio} nucleon-nucleus scattering studies to inform spin properties
of the underlying nucleon-nucleon interactions. The observed correlation to the
magnetic moment of excited states illustrates a previously unknown connection
between reaction observables such as the analyzing power and structure
observables like the magnetic moment.Comment: 13 pages, 7 figures, 1 tabl
Ab initio Leading Order Effective Potentials for Elastic Nucleon-Nucleus Scattering
Background: Calculating microscopic effective interactions (optical
potentials) for elastic nucleon-nucleus scattering has already in the past led
to a large body of work. For first-order calculations a nucleon-nucleon
(\textit{NN}) interaction and a one-body density of the nucleus were taken as
input to rigorous calculations of microscopic full-folding calculations.
Purpose: Based on the spectator expansion of the multiple scattering series
we employ a chiral next-to-next-to-leading order (NNLO) nucleon-nucleon
interaction on the same footing in the structure as well as in the reaction
calculation to obtain an in leading-order consistent effective potential for
nucleon-nucleus elastic scattering, which includes the spin of the struck
target nucleon.
Methods: The first order effective folding potential is computed by first
deriving a nonlocal scalar density as well as a spin-projected momentum
distribution. Those are then integrated with the off-shell Wolfenstein
amplitudes , , and . The resulting nonlocal potential serves as input
to a momentum-space Lippmann-Schwinger equation, whose solutions are summed to
obtain the nucleon-nucleus scattering observables.
Results: We calculate elastic scattering observables for He, He,
He, C, and O in the energy regime between 100 and 200 MeV
projectile kinetic energy, and compare to available data. We also explore the
extension down to about 70 MeV, and study the effect of ignoring the spin of
the struck nucleon in the nucleus.
Conclusions: In our calculations we contrast elastic scattering off
closed-shell and open-shell nuclei. We find that for closed-shell nuclei the
approximation of ignoring the spin of the struck target nucleon is excellent.
We only see effects of the spin of the struck target nucleon when considering
He and He, which are nuclei with a ratio larger than 1.Comment: 13 pages, 13 figure
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