arXiv subjects

Andreas Ekström

Publications and source records attributed to Andreas Ekström.

At least 19 recordsLinked to original sources

arXiv2026

Linking Electromagnetic Moments to Nuclear Interactions with a Global Physics-Driven Machine-Learning Emulator

Understanding how specific components of the nuclear interaction shape observable properties of atomic nuclei remains a central challenge in nuclear structure research. While previous studies have focused on bulk observables such as nuclear energies and charge radii, it is unclear how distinct operator components of nuclear interactions impact complementary observables such as nuclear electromagnetic moments. Here, we develop a global, physics-constrained emulator to establish a quantitative link between electromagnetic moments and components of chiral nuclear forces. Unlike traditional sensitivity analyses that vary low-energy constants independently, we quantify parameter contributions while accounting for correlations within the physically supported parameter manifold. We show that, unlike bulk observables, electromagnetic moments probe complementary spin and isospin sectors of the interaction and exhibit a pronounced isotope-dependent sensitivity. These developments enable a quantitative assessment of the importance of prospective measurements, providing predictions with quantified uncertainties for observables that may be beyond the current experimental reach.

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arXiv2026

Perturbative calculations of light nuclei up to N$^3$LO in chiral effective field theory

We predict ground-state energies of $^3$H, $^4$He, and $^6$Li in chiral effective field theory up to next-to-next-to-next-to-leading-order (N$^3$LO) using a power counting guided by renormalization-group invariance. Subleading two-nucleon interactions are treated perturbatively, and for $^4$He and $^6$Li, we calculate the perturbative corrections from numerical derivatives of ground-state energies obtained with Lanczos diagonalization. We find that including the $^3$H binding energy in the calibration is essential for robust predictions of $^4$He and $^6$Li. This work demonstrates that the employed power counting can be applied to construct nuclear interactions with predictive power for light nuclei, bringing nuclear structure predictions closer to a foundation in quantum chromodynamics.

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arXiv2025

Inferring the breakdown scales of the chiral expansions for $g_A$ and $m_N$

We apply Bayesian inference to the order-by-order chiral perturbation theory ($χ$PT) expansions for the axial-vector coupling constant $g_A$ and the nucleon mass m_N, and thereby infer the scales at which $χ$PT breaks down for these two observables. Using a pointwise Bayesian analysis, we find that the inferred breakdown scales are notably different for the two observables. For the chiral expansion of $g_A$, we obtain $251^{+20}_{-50}$ MeV and $211^{+20}_{-30}$ MeV using two distinct sets of low-energy constants, while for the chiral expansion of $m_N$ we infer a significantly larger breakdown scale of $491^{+60}_{-90}$ MeV.

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arXiv2025

Perturbative $χ$EFT calculations of the deuteron and triton up to N$^2$LO

We extend previous studies of the deuteron and triton ground-state energies to next-to-next-to-leading order (N$^2$LO) in chiral effective field theory, employing a power counting in which subleading interactions are treated perturbatively. Triton calculations are performed using the no-core shell model, and we demonstrate converged perturbative results for regulator cutoffs up to $Λ\approx 1200$ MeV. We analyze exceptional cutoffs in the $^3P_0$ and $^3S_1 \mathrm{-}^3D_1$ nucleon-nucleon channels and find a resulting cutoff dependence in the triton ground-state energy at N$^2$LO. The effect associated with the exceptional cutoff in the $^3P_0$ channel can be mitigated by redefining the leading-order wave function within the freedom allowed by the effective field theory. The same approach applied in the $^3S_1 \mathrm{-}^3D_1$ channel remedies the effect of this exceptional cutoff on the ground state energy of the deuteron, but not for the triton.

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arXiv2025

The breakdown scale of pionless effective field theory in the three-nucleon sector

We make order-by-order predictions of neutron-deuteron total cross sections up to next-to-next-to-leading order in pionless effective field theory. Using Bayesian methods, we infer a posterior distribution for the breakdown scale. The result shows a mode near 100 MeV, and a combined analysis with neutron-proton scattering further sharpens the inference, placing the mode close to the pion mass scale, consistent with the expected range of pionless effective field theory.

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arXiv2024

Unexpected Rise in Nuclear Collectivity from Short-Range Physics

We discover a surprising relation between the collective motion of nucleons within atomic nuclei, traditionally understood to be driven by long-range correlations, and short-range nucleon-nucleon interactions. Specifically, we find that quadrupole collectivity in low-lying states of $^6$Li and $^{12}$C, calculated with state-of-the-art ab initio techniques, is significantly influenced by two opposing $S$-wave contact couplings that subtly alter the surface oscillations of one largely deformed nuclear shape, without changing that shape's overall contribution within the nucleus. The results offer new insights into the nature of emergent nuclear collectivity and its link to the underlying nucleon-nucleon interaction at short distances.

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arXiv2024

Quantifying the breakdown scale of pionless effective field theory

We use Bayesian statistics to infer the breakdown scale of pionless effective field theory in its standard power counting and with renormalization of observables carried out using the power-divergence subtraction scheme and cutoff regularization. We condition our inference on predictions of the total neutron-proton scattering cross section up next-to-next-to leading order. We quantify a median breakdown scale of approximately 1.4$m_π$. The 68% degree of belief interval is $[0.96,1.69]m_π$. This result confirms the canonical expectation that the pion mass is a relevant scale in low-energy nuclear physics.

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arXiv2024

Bayesian estimation of the low-energy constants up to fourth order in the nucleon-nucleon sector of chiral effective field theory

We use Bayesian methods and Hamiltonian Monte Carlo (HMC) sampling to infer the posterior probability density function (PDF) for the low-energy constants (LECs) up to next-to-next-to-next- to-leading order (N3LO) in a chiral effective field theory ($χ$EFT) description of the nucleon-nucleon interaction. In a first step, we condition the inference on neutron-proton and proton-proton scattering data and account for uncorrelated $χ$EFT truncation errors. We demonstrate how to successfully sample the 31-dimensional space of LECs at N3LO using a revised HMC inference protocol. In a second step we extend the analysis by means of importance sampling and an empirical determination of the neutron-neutron scattering length to infer the posterior PDF for the leading charge-dependent contact LEC in the $^{1}S_0$ neutron-neutron interaction channel. While doing so we account for the $χ$EFT truncation error via a conjugate prior. We use the resulting posterior PDF to sample the posterior predictive distributions for the effective range parameters in the $^{1}S_0$ wave as well as the strengths of charge-symmetry breaking and charge-independence breaking. We conclude that empirical point-estimate results of isospin breaking in the $^{1}S_0$ channel are consistent with the PDFs obtained in our Bayesian analysis and that, when accounting for $χ$EFT truncation errors, one must go to next-to-next-to-leading order to confidently detect isospin breaking effects.

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arXiv2024

Inference of the low-energy constants in $Δ$-full chiral effective field theory including a correlated truncation error

We sample the posterior probability distributions of the low-energy constants (LECs) in $Δ$-full chiral effective field theory ($χ$EFT) up to third order. We use eigenvector continuation for fast and accurate emulation of the likelihood and Hamiltonian Monte Carlo to draw effectively independent samples from the posteriors. Our Bayesian inference is conditioned on the Granada database of neutron-proton ($np$) cross sections and polarizations. We use priors grounded in $χ$EFT assumptions and a Roy-Steiner analysis of pion-nucleon scattering data. We model correlated EFT truncation errors using a two-feature Gaussian process, and find correlation lengths for $np$ scattering energies and angles in the ranges 45--83 MeV and 24--39 degrees, respectively. These correlations yield a non-diagonal covariance matrix and reduce the number of independent scattering data with a factor of 8 and 4 at the second and third chiral orders, respectively. The relatively small difference between the second and third order predictions in $Δ$-full $χ$EFT suppresses the marginal variance of the truncation error and the effects of its correlation structure. Our results are particularly important for analyzing the predictive capabilities in \textit{ab initio} nuclear theory.

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arXiv2024

Eigenvector Continuation and Projection-Based Emulators

Eigenvector continuation is a computational method for parametric eigenvalue problems that uses subspace projection with a basis derived from eigenvector snapshots from different parameter sets. It is part of a broader class of subspace-projection techniques called reduced-basis methods. In this colloquium article, we present the development, theory, and applications of eigenvector continuation and projection-based emulators. We introduce the basic concepts, discuss the underlying theory and convergence properties, and present recent applications for quantum systems and future prospects.

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arXiv2024

Perturbative computations of neutron-proton scattering observables using renormalization-group invariant $χ$EFT up to N$^3$LO

We predict neutron-proton scattering cross-sections and polarization observables up to next-to-next-to-next-to leading order in a renormalization-group invariant description of the strong nucleon-nucleon interaction. Low-energy constants are calibrated to phase shifts, sub-leading corrections are computed in distorted-wave perturbation theory, and we employ momentum-cutoff values 500 and 2500 MeV. We find a steady order-by-order convergence and realistic descriptions of scattering observables up to a laboratory scattering energy of approximately 100 MeV. We also compare perturbative and non-perturbative calculations for phase shifts and cross sections and quantify how unitarity is gradually restored at higher orders. The perturbative approach offers an important diagnostic tool for any power counting and our results suggest that the breakdown scale in chiral effective field theory might be significantly lower than estimates obtained in non-perturbative calculations.

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arXiv2024

Uncertainty Quantification of Collective Nuclear Observables From the Chiral Potential Parametrization

We perform an uncertainty estimate of quadrupole moments and B(E2) transition rates that inform nuclear collectivity. In particular, we study the low-lying states of 6Li and 12C using the ab initio symmetry-adapted no-core shell model. For a narrow standard deviation of approximately 1% on the low-energy constants which parametrize high-precision chiral potentials, we find output standard deviations in the collective observables ranging from approximately 3-6%. The results mark the first step towards a rigorous uncertainty quantification of collectivity in nuclei that aims to account for all sources of uncertainty in ab initio descriptions of challenging collective and clustering observables.

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arXiv2023

Bayesian Analysis of $χ$EFT at Leading Order in a Modified Weinberg Power Counting Approach

We present a Bayesian analysis of renormalization-group invariant nucleon-nucleon interactions at leading order in chiral effective field theory ($χ$EFT) with momentum cutoffs in the range 400--4000 MeV. We use history matching to identify relevant regions in the parameter space of low-energy constants (LECs) and subsequently infer the posterior probability density of their values using Markov chain Monte Carlo. All posteriors are conditioned on experimental data for neutron-proton scattering observables and we estimate the $χ$EFT truncation error in an uncorrelated limit. We do not detect any significant cutoff dependence in the posterior predictive distributions for two-nucleon observables. For all cutoff values we find a multimodal LEC posterior with an insignificant mode harboring a bound $^1{S}_0$ state. The $^3P_0$ and $^3P_2$ phase shifts emerging from the Bayesian analysis are less constrained and typically more repulsive compared to the results of a phase shift optimization. We expect that our inference will impact predictions for nuclei. This work demonstrates how to perform inference in the presence of limit-cycle-like behavior and spurious bound states, and lays the foundation for a Bayesian analysis of renormalization-group invariant $χ$EFT interactions beyond leading order.

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arXiv2023

Understanding the Effect of Chiral NN Parametrization on Nuclear Shapes From an Ab Initio Perspective

The ab initio symmetry-adapted no-core shell model naturally describes nuclear deformation and collectivity, and is therefore well-suited to studying the dynamics and coexistence of shapes in atomic nuclei. For the first time, we analyze how these features in low-lying states of 6Li and 12C are impacted by the underlying realistic nucleon-nucleon interaction. We find that the interaction parametrization has a notable but limited effect on collective shapes in the lowest 6Li and 12C states, while collective structures in the excited 2+ state of 12C are significantly more sensitive to the interaction parameters and exhibits emergent shape coexistence.

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arXiv2023

How chiral forces shape neutron-rich Ne and Mg nuclei

We compute the structure of the exotic even nuclei $^{20-34}$Ne and $^{34-40}$Mg using interactions from chiral effective field theory (EFT). Our results for the ground-state rotational bands in $^{20-32}$Ne and $^{36-40}$Mg agree with data. We predict a well-deformed $^{34}$Ne and find that $^{40}$Mg exhibits an oblate deformed band close to the prolate ground-state, indicating the emergence of shape co-existence at the neutron dripline. A global sensitivity analysis shows that the subleading singlet $S$-wave contact and a pion-nucleon coupling strongly impact deformation in chiral EFT.

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arXiv2023

Ab Initio Symmetry-Adapted Emulator for Studying Emergent Collectivity and Clustering in Nuclei

We discuss emulators from the ab initio symmetry-adapted no-core shell-model framework for studying the formation of alpha clustering and collective properties without effective charges. We present a new type of an emulator, one that utilizes the eigenvector continuation technique but is based on the use of symplectic symmetry considerations. This is achieved by using physically relevant degrees of freedom, namely, the symmetry-adapted basis, which exploits the almost perfect symplectic symmetry in nuclei. Specifically, we study excitation energies, point-proton root-mean-square radii, along with electric quadrupole moments and transitions for 6Li and 12C. We show that the set of parameterizations of the chiral potential used to train the emulators has no significant effect on predictions of dominant nuclear features, such as shape and the associated symplectic symmetry, along with cluster formation, but slightly varies details that affect collective quadrupole moments, asymptotic normalization coefficients, and alpha partial widths up to a factor of two. This makes these types of emulators important for further constraining the nuclear force for high-precision nuclear structure and reaction observables.

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arXiv2022

Posterior predictive distributions of neutron-deuteron cross sections

We quantify the posterior predictive distributions (PPDs) of elastic neutron-deuteron ($nd$) scattering cross sections using nucleon-nucleon ($NN$) interactions from chiral effective field theory ($χ$EFT) up to and including next-to-next-to-next-to-leading order (N$^3$LO). These PPDs quantify the spread in $nd$ predictions due to the variability of the low-energy constants (LECs) inferred from $NN$ scattering data. We use the wave-packet continuum discretization method to solve the Alt-Grassberger-Sandhas form of the Faddeev equations for elastic scattering. We draw 100 samples from the PPDs of $nd$ cross sections up to 67 MeV in scattering energy, i.e., in the energy region where the effects of three-nucleon forces are expected to be small. We find that the uncertainty about $NN$ LECs inferred from $NN$ scattering data, when assuming uncorrelated errors, does not translate to significant uncertainty in the low-energy $nd$ continuum. Based on our estimates, the uncertainty of $nd$ predictions are dominated by the $χ$EFT truncation error, at least below N$^3$LO. At this order, the 90% credible interval of the PPD and the truncation error are comparable, although both are very small on an absolute scale.

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arXiv2022

Ab initio predictions link the neutron skin of ${}^{208}$Pb to nuclear forces

Heavy atomic nuclei have an excess of neutrons over protons, which leads to the formation of a neutron skin whose thickness is sensitive to details of the nuclear force. This links atomic nuclei to properties of neutron stars, thereby relating objects that differ in size by orders of magnitude. The nucleus ${}^{208}$Pb is of particular interest because it exhibits a simple structure and is experimentally accessible. However, computing such a heavy nucleus has been out of reach for ab initio theory. By combining advances in quantum many-body methods, statistical tools, and emulator technology, we make quantitative predictions for the properties of ${}^{208}$Pb starting from nuclear forces that are consistent with symmetries of low-energy quantum chromodynamics. We explore $10^9$ different nuclear-force parameterisations via history matching, confront them with data in select light nuclei, and arrive at an importance-weighted ensemble of interactions. We accurately reproduce bulk properties of ${}^{208}$Pb and determine the neutron skin thickness, which is smaller and more precise than a recent extraction from parity-violating electron scattering but in agreement with other experimental probes. This work demonstrates how realistic two- and three-nucleon forces act in a heavy nucleus and allows us to make quantitative predictions across the nuclear landscape.

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