| 1. |
- Djärv, Tor, 1991, et al.
(författare)
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Normal-ordering approximations and translational (non)invariance
- 2021
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Ingår i: Physical Review C. - 2469-9985 .- 2469-9993. ; 104:2
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Tidskriftsartikel (refereegranskat)abstract
- Normal ordering provides an approach to approximate three-body forces as effective two-body operators and it is therefore an important tool in many-body calculations with realistic nuclear interactions. The corresponding neglect of certain three-body terms in the normal-ordered Hamiltonian is known to influence translational invariance, although the magnitude of this effect has not yet been systematically quantified. In this paper we study in particular the normal-ordering two-body approximation applied to a single harmonic-oscillator reference state. We explicate the breaking of translational invariance and demonstrate the magnitude of the approximation error as a function of model space parameters for He-4 and O-16 by performing full no-core shell-model calculations with and without three-nucleon forces. We combine two different diagnostics to better monitor the breaking of translational invariance. While the center-of-mass effect is shown to become potentially very large for He-4, it is also shown to be much smaller for O-16 although full convergence is not reached. These tools can be easily implemented in studies using other many-body frameworks and bases.
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| 2. |
- Koszorús, Agota, et al.
(författare)
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Charge radii of exotic potassium isotopes challenge nuclear theory and the magic character of N = 32
- 2021
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Ingår i: Nature Physics. - : Springer Science and Business Media LLC. - 1745-2481 .- 1745-2473. ; 17:4, s. 439-443
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Tidskriftsartikel (refereegranskat)abstract
- Nuclear charge radii are sensitive probes of different aspects of the nucleon–nucleon interaction and the bulk properties of nuclear matter, providing a stringent test and challenge for nuclear theory. Experimental evidence suggested a new magic neutron number at N = 32 (refs. 1–3) in the calcium region, whereas the unexpectedly large increases in the charge radii4,5 open new questions about the evolution of nuclear size in neutron-rich systems. By combining the collinear resonance ionization spectroscopy method with β-decay detection, we were able to extend charge radii measurements of potassium isotopes beyond N = 32. Here we provide a charge radius measurement of 52K. It does not show a signature of magic behaviour at N = 32 in potassium. The results are interpreted with two state-of-the-art nuclear theories. The coupled cluster theory reproduces the odd–even variations in charge radii but not the notable increase beyond N = 28. This rise is well captured by Fayans nuclear density functional theory, which, however, overestimates the odd–even staggering effect in charge radii. These findings highlight our limited understanding of the nuclear size of neutron-rich systems, and expose problems that are present in some of the best current models of nuclear theory.
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| 3. |
- Wesolowski, S., et al.
(författare)
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Rigorous constraints on three-nucleon forces in chiral effective field theory from fast and accurate calculations of few-body observables
- 2021
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Ingår i: Physical Review C. - 2469-9985 .- 2469-9993. ; 104:6
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Tidskriftsartikel (refereegranskat)abstract
- We explore the constraints on the three-nucleon force (3NF) of chiral effective field theory (χEFT) that are provided by bound-state observables in the A=3 and A=4 sectors. Our statistically rigorous analysis incorporates experimental error, computational method uncertainty, and the uncertainty due to truncation of the χEFT expansion at next-to-next-to-leading order. A consistent solution for the H3 binding energy, the He4 binding energy and radius, and the H3β-decay rate can only be obtained if χEFT truncation errors are included in the analysis. The β-decay rate is the only one of these that yields a nondegenerate constraint on the 3NF low-energy constants, which makes it crucial for the parameter estimation. We use eigenvector continuation for fast and accurate emulation of no-core shell model calculations of the few-nucleon observables. This facilitates sampling of the posterior probability distribution, allowing us to also determine the distributions of the parameters that quantify the truncation error. We find a χEFT expansion parameter of Q=0.33±0.06 for these observables.
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| 4. |
- Yang, Chieh-Jen, 1977, et al.
(författare)
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Power counting in chiral effective field theory and nuclear binding
- 2021
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Ingår i: Physical Review C. - 2469-9985 .- 2469-9993. ; 103:5
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Tidskriftsartikel (refereegranskat)abstract
- Chiral effective field theory (chi EFT), as originally proposed by Weinberg, promises a theoretical connection between low-energy nuclear interactions and quantum chromodynamics (QCD). However, the important property of renormalization-group (RG) invariance is not fulfilled in current implementations and its consequences for predicting atomic nuclei beyond two- and three-nucleon systems has remained unknown. In this work we present a systematic study of recent RG-invariant formulations of chi EFT and their predictions for the binding energies and other observables of selected nuclear systems with mass numbers up to A = 16. Specifically, we have carried out ab initio no-core shell-model and coupled cluster calculations of the ground-state energy of H-3, He-3,He-4, Li-6, and O-16 using several recent power-counting (PC) schemes at leading order (LO) and next-to-leading order, where the subleading interactions are treated in perturbation theory. Our calculations indicate that RG-invariant and realistic predictions can be obtained for nuclei with mass number A <= 4. We find, however, that O-16 is either unbound with respect to the four alpha-particle threshold, or deformed, or both. Similarly, we find that the Li-6 ground-state resides above the alpha-deuteron separation threshold. These results are in stark contrast with experimental data and point to either necessary fine-tuning of all relevant counterterms, or that current state-of-the-art RG-invariant PC schemes at LO in chi EFT lack necessary diagrams-such as three-nucleon forces-to realistically describe nuclei with mass number A > 4.
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