| 1. |
- Acharya, B., et al.
(författare)
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Uncertainty quantification for proton-proton fusion in chiral effective field theory
- 2016
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Ingår i: Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 760, s. 584-589
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Tidskriftsartikel (refereegranskat)abstract
- We compute the S-factor of the proton-proton (pp) fusion reaction using chiral effective field theory (chi EFT) up to next-to-next-to-leading order (NNLO) and perform a rigorous uncertainty analysis of the results. We quantify the uncertainties due to (i) the computational method used to compute the pp cross section in momentum space, (ii) the statistical uncertainties in the low-energy coupling constants of chi EFT, (iii) the systematic uncertainty due to the chi EFT cutoff, and (iv) systematic variations in the database used to calibrate the nucleon-nucleon interaction. We also examine the robustness of the polynomial extrapolation procedure, which is commonly used to extract the threshold S-factor and its energy-derivatives. By performing a statistical analysis of the polynomial fit of the energy-dependent S-factor at several different energy intervals, we eliminate a systematic uncertainty that can arise from the choice of the fit interval in our calculations. In addition, we explore the statistical correlations between the S-factor and few-nucleon observables such as the binding energies and point-proton radii of H-2,H-3 and He-3 as well as the D-state probability and quadrupole moment of H-2, and the beta-decay of 3H. We find that, with the state-of-the-art optimization of the nuclear Hamiltonian, the statistical uncertainty in the threshold S-factor cannot be reduced beyond 0.7%.
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| 2. |
- Ekstrom, A., et al.
(författare)
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Accurate nuclear radii and binding energies from a chiral interaction
- 2015
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Ingår i: Physical Review C - Nuclear Physics. - 2469-9985 .- 2469-9993. ; 91:5
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Tidskriftsartikel (refereegranskat)abstract
- With the goal of developing predictive ab initio capability for light and medium-mass nuclei, two-nucleon and three-nucleon forces from chiral effective field theory are optimized simultaneously to low-energy nucleon-nucleon scattering data, as well as binding energies and radii of few-nucleon systems and selected isotopes of carbon and oxygen. Coupled-cluster calculations based on this interaction, named NNLOsat, yield accurate binding energies and radii of nuclei up to Ca-40, and are consistent with the empirical saturation point of symmetric nuclear matter. In addition, the low-lying collective J(pi) = 3(-) states in O-16 and 40Ca are described accurately, while spectra for selected p- and sd-shell nuclei are in reasonable agreement with experiment.
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| 3. |
- Ekstrom, A., et al.
(författare)
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Statistical uncertainties of a chiral interaction at next-to-next-to leading order
- 2015
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Ingår i: Journal of Physics G: Nuclear and Particle Physics. - : IOP Publishing. - 0954-3899 .- 1361-6471. ; 42:3
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Tidskriftsartikel (refereegranskat)abstract
- We have quantified the statistical uncertainties of the low-energy coupling-constants (LECs) of an optimized nucleon-nucleon interaction from chiral effective field theory at next-to-next-to-leading order. In addition, we have propagated the impact of the uncertainties of the LECs to two-nucleon scattering phase shifts, effective range parameters, and deuteron observables.
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| 4. |
- Ekström, Andreas, et al.
(författare)
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Effects of Three-Nucleon Forces and Two-Body Currents on Gamow-Teller Strengths
- 2014
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Ingår i: Physical Review Letters. - 1079-7114 .- 0031-9007. ; 113:26, s. 262504-
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Tidskriftsartikel (refereegranskat)abstract
- We optimize chiral interactions at next-to-next-to leading order to observables in two- and three-nucleon systems and compute Gamow-Teller transitions in C14 and O22,24 using consistent two-body currents. We compute spectra of the daughter nuclei N14 and F22,24 via an isospin-breaking coupled-cluster technique, with several predictions. The two-body currents reduce the Ikeda sum rule, corresponding to a quenching factor q2≈0.84–0.92 of the axial-vector coupling. The half-life of C14 depends on the energy of the first excited 1+ state, the three-nucleon force, and the two-body current.
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| 5. |
- Forssen, Christian, 1974, et al.
(författare)
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Large-scale exact diagonalizations reveal low-momentum scales of nuclei
- 2018
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Ingår i: Physical Review C. - 2469-9985 .- 2469-9993. ; 97:3
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Tidskriftsartikel (refereegranskat)abstract
- Ab initio methods aim to solve the nuclear many-body problem with controlled approximations. Virtually exact numerical solutions for realistic interactions can only be obtained for certain special cases such as few-nucleon systems. Here we extend the reach of exact diagonalization methods to handle model spaces with dimension exceeding 10(10) on a single compute node. This allows us to perform no-core shell model (NCSM) calculations for Li-6 in model spaces up to N-max = 22 and to reveal the He-4+d halo structure of this nucleus. Still, the use of a finite harmonic-oscillator basis implies truncations in both infrared (IR) and ultraviolet (UV) length scales. These truncations impose finite-size corrections on observables computed in this basis. We perform IR extrapolations of energies and radii computed in the NCSM and with the coupled-cluster method at several fixed UV cutoffs. It is shown that this strategy enables information gain also from data that is not fully UV converged. IR extrapolations improve the accuracy of relevant bound-state observables for a range of UV cutoffs, thus making them profitable tools. We relate the momentum scale that governs the exponential IR convergence to the threshold energy for the first open decay channel. Using large-scale NCSM calculations we numerically verify this small-momentum scale of finite nuclei.
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| 6. |
- Hagen, G., et al.
(författare)
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Neutron and weak-charge distributions of the Ca-48 nucleus
- 2016
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Ingår i: Nature Physics. - : Springer Science and Business Media LLC. - 1745-2481 .- 1745-2473. ; 12:2, s. 186-190
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Tidskriftsartikel (refereegranskat)abstract
- What is the size of the atomic nucleus? This deceivably simple question is difficult to answer. Although the electric charge distributions in atomic nuclei were measured accurately already half a century ago, our knowledge of the distribution of neutrons is still deficient. In addition to constraining the size of atomic nuclei, the neutron distribution also impacts the number of nuclei that can exist and the size of neutron stars. We present an ab initio calculation of the neutron distribution of the neutron-rich nucleus Ca-48. We show that the neutron skin (difference between the radii of the neutron and proton distributions) is significantly smaller than previously thought. We also make predictions for the electric dipole polarizability and the weak form factor; both quantities that are at present targeted by precision measurements. Based on ab initio results for Ca-48, we provide a constraint on the size of a neutron star.
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| 7. |
- Karlsson, Boris, 1987
(författare)
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Making predictions using χEFT
- 2015
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis we explore the merits of chiral effective field theory (χEFT) as a model for low-energy nuclear physics. χEFT is an effective field theory based on quantum chromo dynamics (QCD) describing low-energy interactions of nucleons and pions. We estimate the inherent uncertainties of χEFT and the accompanying methods used to compute observables in order to test the predictive power of the model. We use experimental pion-nucleon, nucleon- nucleon and few-nucleon data to perform a simultaneous fit of the low-energy constants in χEFT. This results in small statistical uncertainties in the model. The results show a clear order-by-order improvement of χEFT with the systematical model error dominating the total error budget.
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| 8. |
- Karlsson, Boris, 1987
(författare)
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Quantifying statistical uncertainties in ab initio nuclear physics using Lagrange multipliers
- 2017
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Ingår i: Physical Review C - Nuclear Physics. - 2469-9985 .- 2469-9993. ; 95:3, s. 034002-
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Tidskriftsartikel (refereegranskat)abstract
- Theoretical predictions need quantified uncertainties for a meaningful comparison to experimental results. This is an idea which presently permeates the field of theoretical nuclear physics. In light of the recent progress in estimating theoretical uncertainties in ab initio nuclear physics, I here present and compare methods for evaluating the statistical part of the uncertainties. A special focus is put on the (for the field) novel method of Lagrange multipliers (LM). Uncertainties from the fit of the nuclear interaction to experimental data are propagated to a few observables in light-mass nuclei to highlight any differences between the presented methods. The main conclusion is that the LM method is more robust, while covariance-based methods are less demanding in their evaluation.
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| 9. |
- Karlsson, Boris, 1987
(författare)
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The data perspective on chiral effective field theory
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The scientific method implies a dynamical relationship between experiment and theory. Indeed, experimental results are understood through theories, which themselves are of less value until confronted with experiment. In this thesis I study this relationship by quantifying two key properties of theories: theoretical uncertainties and predictive power.Specifically I investigate chiral effective field theory and the precision and accuracy by which it reproduces and predicts low-energy nuclear observables. I estimate both statistical and systematic uncertainties. The conclusion is that the latter, which in my approximation originates from omitted higher-order terms in the chiral expansion, are much larger than the former. In relation to this, I investigate the order-by-order convergence up to fourth order in the chiral expansion. I find that predictions generally improve with increasing order, while the additional low-energy constants (LECs) of the interaction makes it more difficult to fully constrain the theory. Furthermore, in order to accurately reproduce properties of heavier nuclei I see indications that it is necessary to include selected experimental data from such systems directly in the fitting of the interaction.In order to perform these studies I have developed accurate and efficient methods as well as computer codes for the calculation of observables. In particular, the application of automatic differentiation for derivative calculations is shown to be crucial for the minimization procedure. These developments open up new avenues for future studies. For example, it is now possible to do extensive sensitivity analyses of the experimental data and the model; to investigate the power counting from a data perspective; and incorporate more experimental data in the fitting procedure.
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| 10. |
- Karlsson, Boris, 1987, et al.
(författare)
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Uncertainty Analysis and Order-by-Order Optimization of Chiral Nuclear Interactions
- 2016
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Ingår i: Physical Review X. - 2160-3308. ; 6:1
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Tidskriftsartikel (refereegranskat)abstract
- Chiral effective field theory (chi EFT) provides a systematic approach to describe low-energy nuclear forces. Moreover, chi EFT is able to provide well-founded estimates of statistical and systematic uncertainties-although this unique advantage has not yet been fully exploited. We fill this gap by performing an optimization and statistical analysis of all the low-energy constants (LECs) up to next-to-next-to-leading order. Our optimization protocol corresponds to a simultaneous fit to scattering and bound-state observables in the pion-nucleon, nucleon-nucleon, and few-nucleon sectors, thereby utilizing the full model capabilities of chi EFT. Finally, we study the effect on other observables by demonstrating forward-error-propagation methods that can easily be adopted by future works. We employ mathematical optimization and implement automatic differentiation to attain efficient and machine-precise first-and second-order derivatives of the objective function with respect to the LECs. This is also vital for the regression analysis. We use power-counting arguments to estimate the systematic uncertainty that is inherent to chi EFT, and we construct chiral interactions at different orders with quantified uncertainties. Statistical error propagation is compared with Monte Carlo sampling, showing that statistical errors are, in general, small compared to systematic ones. In conclusion, we find that a simultaneous fit to different sets of data is critical to (i) identify the optimal set of LECs, (ii) capture all relevant correlations, (iii) reduce the statistical uncertainty, and (iv) attain order-by-order convergence in chi EFT. Furthermore, certain systematic uncertainties in the few-nucleon sector are shown to get substantially magnified in the many-body sector, in particular when varying the cutoff in the chiral potentials. The methodology and results presented in this paper open a new frontier for uncertainty quantification in ab initio nuclear theory.
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