| 1. |
- Djärv, Tor, 1991, et al.
(författare)
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Bayesian predictions for A=6 nuclei using eigenvector continuation emulators
- 2022
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Ingår i: Physical Review C. - 2469-9985 .- 2469-9993. ; 105:1
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Tidskriftsartikel (refereegranskat)abstract
- We make ab initio predictions for the A=6 nuclear level scheme based on two- and three-nucleon interactions up to next-to-next-to-leading order in chiral effective field theory (χEFT). We utilize eigenvector continuation and Bayesian methods to quantify uncertainties stemming from the many-body method, the χEFT truncation, and the low-energy constants of the nuclear interaction. The construction and validation of emulators is made possible via the development of jupiterncsm - a new M-scheme no-core shell model code that uses on-the-fly Hamiltonian matrix construction for efficient, single-node computations up to Nmax=10 for Li6. We find a slight underbinding of He6 and Li6, although consistent with experimental data given our theoretical error bars. As a result of incorporating correlated χEFT-truncation errors we find more precise predictions (smaller error bars) for separation energies: Sd(Li6)=0.89±0.44MeV, S2n(He6)=0.20±0.60MeV, and for the beta decay Q value: Qβ-(He6)=3.71±0.65MeV. We conclude that our error bars can potentially be reduced further by extending the model space used by jupiterncsm.
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| 2. |
- Djärv, Tor, 1991
(författare)
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JupiterNCSM: A Pantheon of Nuclear Physics —an implementation of three-nucleon forces in the no-core shell model—
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- It is well established that three-nucleon forces (3NFs) are necessary for achieving realistic and accurate descriptions of atomic nuclei. In particular, such forces arisenaturally when using chiral effective field theories (χEFT). However, due to the huge computational complexity associated with the inclusion of 3NFs in many-body methods they are often approximated or neglected completely. In this thesis, three different methods to include the physics of 3NFs in the ab initio no-core shell-model(NCSM) have been implemented and tested. In the first method, we approximate the 3NFs as effective two-body operators by exploiting Wick’s theorem to normal order the 3NF relative a harmonic-oscillator Slater determinant reference state and discarding the remaining three-body term. We explored the performance of this single-reference normal-ordered two-body approximation on the ground-state energies of the two smallest closed-core nuclei, 4He and 16O, in particular focusing on consequences of the breaking of translational symmetry. The second approach is a full implementation of 3NFs in a new NCSM code, named JupiterNCSM, that we provide as an open-source research software. We have validated and benchmarked JupiterNCSM against other codes and we have specifically used it to investigate theeffects of different 3NFs on light p-shell nuclei 6He and 6Li. Finally, we implement the eigenvector continuation (EVC) method to emulate the response of ground-state energies of the aforementioned A = 6 nuclei to variations in the low-energy constants of χEFT that parametrize the 3NFs. In this approach, the full Hamiltonian is projected onto a small subspace that is constructed from a few selected eigenvectors. These training vectors are computed with JupiterNCSM in a large model space for a small set of parameter values. This thesis provides the first EVC-based emulation of nuclei computed with a Slater-determinant basis. After the training phase, we find that EVC predictions offer a very high accuracy and more than seven orders of magnitude computational speedup. As a result we are able to perform rigorous statistical inferences to explore the effects of 3NFs in nuclear many-body systems.
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| 3. |
- 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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| 4. |
- Djärv, Tor, 1991
(författare)
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Three-nucleon forces in nuclear physics simulations
- 2019
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The aim of this licentiate thesis is to evaluate the normal-ordered two-body (NO2B)-approximation as a computationally promising way to incorporate realistic three-nucleon forces (3NFs) in nuclear many-body simulations using the no-core-shell-model. The existence and importance of 3NFs is predicted in chiral effective field theories of the strong-nuclear force. However, the inclusion of 3NFs renders simulations computationally demanding and this severely limits the size of nuclei that can be studied. Clearly, approximation schemes are needed. In the specific version of the NO2B-approximation that is studied here, the 3NF is normal-ordered with respect to a single Slater-determinant reference state constructed from harmonic-oscillator states, yielding an expansion with zero-, one-, two- and three-nucleon terms. The irreducible three-nucleon part of the original 3NF is assumed to be small and therefore discarded, thus leaving an effective two-nucleon potential. It is found that the predicted ground-state energy of 4-He in the NO2B-approximation depends strongly on the choice of many-body basis. The NO2B-approximation breaks the translational symmetry of the Hamiltonian and therefore introduces strong center-of-mass (CM)-mixing in the 4-He ground-state. This CM-mixing is shown to be an important reason for the observed basis dependence. Thus, the NO2B approximation is most likely more useful for studies of heavier nuclei. Indeed, the approximation error for the ground-state energy of 16-O is observed to be smaller and the results exhibit a weaker dependence on the choice of many-body basis. Finally, it is recommended that CM mixing should be used as a diagnostic to assess the reliability of the NO2B approximation.
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| 5. |
- Jiang, Weiguang, 1989, et al.
(författare)
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Emulating ab initio computations of infinite nucleonic matter
- 2024
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Ingår i: Physical Review C. - 2469-9985 .- 2469-9993. ; 109:6
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Tidskriftsartikel (refereegranskat)abstract
- We construct efficient emulators for the ab initio computation of the infinite nuclear matter equation of state. These emulators are based on the subspace-projected coupled-cluster method for which we here develop a new algorithm called small-batch voting to eliminate spurious states that might appear when emulating quantum many-body methods based on a non-Hermitian Hamiltonian. The efficiency and accuracy of these emulators facilitate a rigorous statistical analysis within which we explore nuclear matter predictions for >106 different parametrizations of a chiral interaction model with explicit Δ-isobars at next-to-next-to leading order. Constrained by nucleon-nucleon scattering phase shifts and bound-state observables of light nuclei up to He4, we use history matching to identify nonimplausible domains for the low-energy coupling constants of the chiral interaction. Within these domains we perform a Bayesian analysis using sampling and importance resampling with different likelihood calibrations and study correlations between interaction parameters, calibration observables in light nuclei, and nuclear matter saturation properties.
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| 6. |
- Jiang, Weiguang, 1989, et al.
(författare)
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Nuclear-matter saturation and symmetry energy within Δ -full chiral effective field theory
- 2024
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Ingår i: Physical Review C. - 2469-9985 .- 2469-9993. ; 109:6
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Tidskriftsartikel (refereegranskat)abstract
- Nuclear saturation and the symmetry energy are key properties of low-energy nuclear physics that depend on fine details of the nuclear interaction. The equation of state around saturation is also an important anchor for extrapolations to higher densities and studies of neutron stars. Here we develop a unified statistical framework that uses realistic nuclear forces to link the theoretical modeling of finite nuclei and infinite nuclear matter. We construct fast and accurate emulators for nuclear-matter observables and employ an iterative history-matching approach to explore and reduce the enormous parameter domain of Δ-full chiral interactions. We perform rigorous uncertainty quantification and find that model calibration including O16 observables gives saturation predictions that are more precise than those that only use few-body data.
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