arXiv subjects

T. Duguet

Publications and source records attributed to T. Duguet.

At least 19 recordsLinked to original sources

arXiv2026

The iconic $^{238}$U: ab initio nuclear structure theory towards the limit of the periodic table

The ab initio description of heavy and superheavy nuclei constitutes one of the holy grails of nuclear theory, bearing on the synthesis of the heaviest elements and the limits of nuclear stability. Over the last fifteen years, many-body expansion methods, whose numerical cost scales polynomially with system size, have extended first-principles calculations to medium-mass nuclei and a few spherical closed-shell heavy systems. The largest portion of the nuclear chart is however composed of heavy deformed doubly open-shell nuclei and has remained completely out of reach. This is due to two major obstacles: (i) the huge computational cost of beyond mean-field calculations in very large single-particle bases, and (ii) a dubious collapse of the mean-field energy at large prolate deformation. While a highly efficient numerical implementation of the novel deformed self-consistent Green's function formalism removes the first difficulty, the second is cured by the inclusion of many-body correlations beyond the deformed mean field. Presenting the first ab initio calculation of the iconic $^{238}$U nucleus, this work brings the upper-end of the nuclear chart within reach of theoretical predictions based on first principles.

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arXiv2026

Deformed self-consistent Green's function method for atomic nuclei at second and third order in the algebraic diagrammatic construction

The description of atomic nuclei from first principles constitutes one of the central goals of nuclear theory. Polynomial-scaling expansion methods have extended \textit{ab initio} calculations at sub-percent accuracy to medium-mass nuclei and a few closed-shell heavy nuclei, but deformed doubly open-shell heavy and superheavy nuclei remain out of reach. The self-consistent Green's function (SCGF) formalism is here extended to doubly open-shell nuclei by allowing the one-body propagator to spontaneously break SU(2) rotational symmetry. The resulting deformed SCGF (dSCGF) scheme, based on the algebraic diagrammatic construction truncated at first, second, and third order, is implemented in a newly developed many-body suite, \texttt{FoxTrot}. Numerical strategies required to handle the associated, symmetry-unrestricted $M$-scheme working basis are discussed in detail. The method is illustrated through a study of $^{28}$Si based on the 1.8/2.0 (EM) Hamiltonian. The impact of the three-nucleon interaction and of its rank-reduction approximation on the deformed Hartree-Fock total energy curve is examined, and the correlated curve obtained from constrained dSCGF calculations is shown to differ appreciably from the mean-field one. Physical solutions appearing as minima of the correlated curve are shown to be reachable via unconstrained calculations starting from any point along the deformed Hartree-Fock curve, demonstrating the self-consistent character of the method. The $^{28}$Si ground-state binding energy at third order, extrapolated to the infinite basis-size limit, reproduces experiment within $2.1\%$, while the excited prolate solution is consistent with the observed $0^+_3$ shape isomer. The present developments open the way to an accurate ab initio description of all (very) heavy nuclei in the near future.

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arXiv2026

Balancing theory uncertainties in ab initio nuclear structure calculations: Many-body truncation versus finite basis size

First-principles calculations of atomic nuclei are necessarily incomplete as the Schrödinger equation is solved using approximate methods and due to the finite dimension of the employed Hilbert space. By balancing many-body truncation and basis-size uncertainties, we formalize a criterion for the optimal one-body basis dimension in a given ab initio nuclear structure computation. Next, it is demonstrated that higher-order many-body contributions can be computed using smaller basis sizes than used for the lower orders when a consistent accuracy in the calculation is targeted. Our findings are empirically validated using many-body perturbation theory and coupled-cluster calculations of nuclei spanning a large portion of the nuclear chart using two sets of chiral two- and three-nucleon interactions. The results suggest that considerable computational savings can be obtained using many-body-order-dependent one-body basis sizes.

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arXiv2026

Vibrational Modes in Strongly Deformed Nuclei

Low-energy vibrational excitations associated with the fluctuation of quadrupole deformed shapes are discussed within the frame of state-of-the-art Configuration Interaction calculations, actually performed via the Quasi-particle Vacua Shell Model version of the Monte Carlo Shell Model. Recently, low-lying $γ$ bands in heavy strongly deformed nuclei were shown to be rotational $K^P$ = 2$^+$ excitations of triaxially deformed states (see T. Otsuka \etal, Eur. Phys. J. A 61, 126 (2025)) rather than vibrational excitations as traditionally interpreted. In this context, it is important to identify possible low-lying vibrational excitations and to characterize the excitation energy at which they emerge. Focusing on two typical examples, $^{166}$Er and $^{162}$Dy, vibrational states are indeed identified above the $γ$ band using an extended version of the so-called T-plot. The phenomenon of shape coexistence is also shown to produce low-lying states below such vibrational band heads. These results suggest novel and rich structures in heavy deformed nuclei. While experimental counterparts are seen for some of such states, others are predictions opening doors to future dedicated experiments.

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arXiv2026

From closed shells to open shells: Coupled-cluster calculations of atomic nuclei

Coupled-cluster theory is a powerful tool for first-principles calculations of atomic nuclei, enabling accurate predictions of nuclear observables across the Segrè chart. While coupled-cluster computations are especially efficient at shell closures, extensions have been developed to tackle open-shell nuclei, by exploiting the equation-of-motion method or by expanding the coupled-cluster wave function on top of a symmetry-breaking (either deformed or superfluid) reference state. In this study, we provide a comprehensive comparison of these different formulations applied to the calcium and nickel isotopes using nuclear two- and three-body interactions from chiral effective field theory. Based on ground-state energies, two-neutron separation energies, and two-neutron shell gaps, different coupled-cluster computations - based on symmetry-broken reference states and equation-of-motion techniques - offer consistent descriptions of bulk properties across medium-mass isotopic chains.

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arXiv2025

Topical issue on the intersection of low-energy nuclear structure and high-energy nuclear collisions

High-energy heavy-ion physics and low-energy nuclear structure physics have historically been disconnected fields. The hydrodynamic description of the quark-gluon plasma (QGP) requires input from nuclear structure to model the initial states of the colliding nuclei. Advances in both theory and experiment now show that the hydrodynamic evolution of the QGP is sensitive to the detailed features of the colliding nuclei, with remarkable consequences for experimental observables. The topical collection represents a joint effort between the low- and high-energy nuclear communities, reflecting the growing recognition that precision modeling of nuclear structure is essential for interpreting high-energy collision data. This new experimental approach opens outstanding opportunities to deepen our understanding of strong-interaction matter. Indeed, by probing many-body correlations of nucleons directly in the nuclear ground state, high-energy collisions provide a unique way to "image" nuclei, fully complementary to the techniques of low-energy experiments, where nuclear collectivity is usually inferred from spectroscopic information on excited states. Do emergent many-body QCD phenomena in nuclei manifest consistently across experiments and energy scales? Addressing this question requires synergy between collider data and state-of-the-art nuclear structure calculations. In view of the rapid progress of ab initio methods based on low-energy effective field theories of QCD, the implications are far-reaching: heavy-ion collisions can probe nuclear forces, while nuclear structure insights refine our understanding of QGP dynamics.

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arXiv2025

Extremely large oblate deformation of the first excited state in $^{12}$C: a new challenge to modern nuclear theory

A Coulomb-excitation study of the high-lying first excited state at 4.439 MeV in the nucleus $^{12}$C has been carried out using the $^{208}$Pb($^{12}$C,$^{12}$C$^*$)$^{208}$Pb$^*$ reaction at 56 MeV and the {\sc Q3D} magnetic spectrograph at the Maier-Leibnitz Laboratorium in Munich. High-statistics achieved with an average beam intensity of approximately 10$^{11}$ ions/s together with state-of-the-art {\it ab initio} calculations of the nuclear dipole polarizability permitted the accurate determination of the spectroscopic quadrupole moment, $Q_{_S}(2_{_1}^+) = +0.076(30)$~eb, in agreement with previous measurements. Combined with previous work, a weighted average of $Q_{_S}(2_{_1}^+) = +0.090(14)$ eb is determined, which includes the re-analysis of a similar experiment by Vermeer and collaborators, $Q_{_S}(2_{_1}^+) = +0.103(20)$~eb. Such a large oblate deformation challenges modern nuclear theory and emphasizes the need of $α$ clustering and associated triaxiality effects for full convergence of $E2$ collective properties.

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arXiv2025

Refined topology of the N = 20 island of inversion with high precision mass measurements of $^{31-33}$Na and $^{31-35}$Mg

Mass measurements of $^{31-33}$Na and $^{31-35}$Mg using the TITAN MR-TOF-MS at TRIUMF's ISAC facility are presented, with the uncertainty of the $^{33}$Na mass reduced by over two orders of magnitude. The excellent performance of the MR-TOF-MS has also allowed the discovery of a millisecond isomer in $^{32}$Na. The precision obtained shows that the binding energy of the normally closed N = 20 neutron shell reaches a minimum for $^{32}$Mg but increases significantly for $^{31}$Na, hinting at the possibility of enhanced shell strength toward the unbound $^{28}$O. We compare the results with new ab initio predictions that raise intriguing questions of nuclear structure beyond the dripline.

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arXiv2025

Mean-field approximation on steroids: exact description of the deuteron

The present article demonstrates that the deuteron, i.e. the lightest bound nuclear system made of a single proton and a single neutron, can be accurately described within a mean-field-based framework. Although paradoxical at first glance, the deuteron ground-state binding energy, magnetic dipole moment, electric quadrupole moment and root-mean-square proton radius are indeed reproduced with sub-percent accuracy via a low-dimensional linear combination of non-orthogonal Bogoliubov states, i.e. with a method whose numerical cost scales as $n_{\text{dim}}^4$, where $n_{\text{dim}}$ is the dimension of the basis of the one-body Hilbert space. By further putting the system into a harmonic trap, the neutron-proton scattering length and effective range in the ${}^{3}S_1$ channel are also accurately reproduced. To achieve this task, (i) the inclusion of proton-neutron pairing through the mixing of proton and neutron single-particle states in the Bogoliubov transformation and (ii) the restoration of proton and neutron numbers before variation are shown to be mandatory ingredients. This unexpected result has implications regarding the most efficient way to capture necessary correlations as a function of nuclear mass and regarding the possibility to ensure order-by-order renormalizability of many-body calculations based on chiral or pionless effective field theories beyond light nuclei. In this context, the present study will be extended to $^{3}$H and $^{3,4}$He in the near future as well as to the leading order of pionless effective field theory.

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arXiv2024

Towards heavy-mass ab initio nuclear structure: Open-shell Ca, Ni and Sn isotopes from Bogoliubov coupled-cluster theory

Recent developments in nuclear many-body theory enabled the description of open-shell medium-mass nuclei from first principles by exploiting the spontaneous breaking of symmetries within correlation expansion methods. Once combined with systematically improvable inter-nucleon interactions consistently derived from chiral effective field theory, modern ab initio nuclear structure calculations provide a powerful framework to deliver first-principle predictions accompanied with theoretical uncertainties. In this Letter, controlled ab initio Bogoliubov coupled cluster (BCC) calculations are performed for the first time, targeting the ground-state of all calcium, nickel and tin isotopes up to mass A ~ 180. While showing good agreement with available experimental data, the shell structure evolution in neutron-rich isotopes and the location of the neutron drip-lines are predicted. The BCC approach constitutes a key development towards reliable first-principles simulations of heavy-mass open-shell nuclei.

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arXiv2024

Tensor factorization in ab initio many-body calculations: Triaxially-deformed (B) MBPT calculations in large bases

Whether for fundamental studies or nuclear data evaluations, first-principle calculations of atomic nuclei constitute the path forward. Today, performing \textit{ab initio} calculations (a) of heavy nuclei, (b) of doubly open-shell nuclei or (c) with a sub-percent accuracy is at the forefront of nuclear structure theory. While combining any two of these features constitutes a major challenge, addressing the three at the same time is currently impossible. From a numerical standpoint, these challenges relate to the necessity to handle (i) very large single bases and (ii) mode-6, \textit{i.e.} three-body, tensors (iii) that must be stored repeatedly. Performing second-order many-body perturbation theory(ies) calculations based on triaxially deformed and superfluid reference states of doubly open-shell nuclei up to mass $A=72$, the present work achieves a significant step forward by addressing challenge (i). To do so, the memory and computational cost associated with the handling of large tensors is scaled down via the use of tensor factorization techniques. The presently used factorization format is based on a randomized singular value decomposition that does not require the computation and storage of the very large initial tensor. The procedure delivers an inexpensive and controllable approximation to the original problem, as presently illustrated for calculations that could not be performed without tensor factorization. With the presently developed technology at hand, one can envision to perform calculations of yet heavier doubly open-shell nuclei at sub-percent accuracy in a foreseeable future.

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arXiv2024

Symmetry-restored Skyrme-Random-Phase-Approximation calculations of the monopole strength in deformed nuclei

Within the Energy Density Functional (EDF) approach, the use of mean-field wave-functions deliberately breaking (some) symmetries of the underlying Hamiltonian is an efficient and largely utilized way to incorporate static correlations. However, the restoration of broken symmetries is eventually mandatory to recover the corresponding quantum numbers and to achieve a more precise description of nuclear properties. While symmetry-restored calculations are routinely performed to study ground-state properties and low-lying excitations, similar applications to the nuclear response are essentially limited to either formal studies or to schematic models. In the present paper, the effect of angular momentum restoration on the monopole and quadrupole responses of doubly open-shell nuclei is investigated. Based on deformed Skyrme-Random Phase Approximation (RPA) calculations, the exact Angular Momentum Projection (AMP) is implemented in the calculation of the multipole strength functions, thus defining a projection after variation (PAV-RPA) scheme. The method is employed for the first time in a realistic study to investigate the effect of AMP on the coupling of monopole and quadrupole modes in $^{24}$Mg resulting from its intrinsic deformation.

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arXiv2023

Cross-shell states in $^{15}$C: a test for p-sd interactions

The low-lying structure of $^{15}$C has been investigated via the neutron-removal $^{16}$C$(d,t)$ reaction. Along with bound neutron sd-shell hole states, unbound p-shell hole states have been firmly confirmed. The excitation energies and the deduced spectroscopic factors of the cross-shell states are an important measure of the $[(p)^{-1}(sd)^{2}]$ neutron configurations in $^{15}$C. Our results show a very good agreement with shell-model calculations using the SFO-tls interaction for $^{15}$C. However, a modification of the $p$-$sd$ and $sd$-$sd$ monopole terms was applied in order to reproduce the $N=9$ isotone $^{17}$O. In addition, the excitation energies and spectroscopic factors have been compared to the first calculations of $^{15}$C with the $ab~ initio$ self-consistent Green's function method employing the NNLO$_{sat}$ interaction. The results show the sensitivity to the size of the $N=8$ shell gap and highlight the need of going beyond the current truncation scheme in the theory.

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arXiv2022

Rooting the EDF method into the ab initio framework. PGCM-PT formalism based on MR-IMSRG pre-processed Hamiltonians

Recently, ab initio techniques have been successfully connected to the traditional valence-space shell model. In doing so, they can either explicitly provide ab initio shell-model effective Hamiltonians or constrain the construction of empirical ones. In the present work, the possibility to follow a similar path for the nuclear energy density functional (EDF) method is analyzed. For this connection to be actualized, two theoretical techniques are instrumental: the recently proposed ab initio PGCM-PT many-body formalism and the MR-IMSRG pre-processing of the nuclear Hamiltonian. Based on both formal arguments and numerical results, possible new lines of research are briefly discussed, namely to compute ab initio EDF effective Hamiltonians at low computational cost, to constrain empirical ones or to produce them directly via an effective field theory that remains to be invented.

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arXiv2022

Zero- and finite-temperature electromagnetic strength distributions in closed- and open-shell nuclei from first principles

Ab initio approaches to the nuclear many-body problem have seen their reach considerably extended over the past decade. However, collective excitations have been scarcely addressed so far due to the prohibitive cost of solving the corresponding equations of motion. Here, a numerically efficient method to compute electromagnetic response functions at zero- and finite-temperature in superfluid and deformed nuclei from an ab initio standpoint is presented and applied to $^{16}$O, $^{28}$Si, $^{46}$Ti and $^{56}$Fe. This work opens the path to systematic ab initio calculations of nuclear responses to electroweak probes across a significant portion of the nuclear chart.

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arXiv2022

A First Glimpse at the Shell Structure beyond $^{54}$Ca: Spectroscopy of $^{55}$K, $^{55}$Ca, and $^{57}$Ca

States in the $N=35$ and 37 isotopes $^{55,57}$Ca have been populated by direct proton-induced nucleon removal reactions from $^{56,58}$Sc and $^{56}$Ca beams at the RIBF. In addition, the $(p,2p)$ quasi-free single-proton removal reaction from $^{56}$Ca was studied. Excited states in $^{55}$K, $^{55}$Ca, and $^{57}$Ca were established for the first time via in-beam $γ$-ray spectroscopy. Results for the proton and neutron removal reactions from $^{56}$Ca to states in $^{55}$K and $^{55}$Ca for the level energies, excited state lifetimes, and exclusive cross sections agree well with state-of-the-art theoretical calculations using different approaches. The observation of a short-lived state in $^{57}$Ca suggests a transition in the calcium isotopic chain from single-particle dominated states at $N=35$ to collective excitations at $N=37$.

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arXiv2022

Angular-momentum projection in coupled-cluster theory: structure of $^{34}$Mg

Single-reference coupled-cluster theory is an accurate and affordable computational method for the nuclear many-body problem. For open-shell nuclei, the reference state typically breaks rotational invariance and angular momentum must be restored as a good quantum number. We perform angular-momentum projection after variation and employ the disentangled coupled-cluster formalism and a Hermitian approach. We compare our results with benchmarks for $^8$Be and $^{20}$Ne using a two-nucleon interaction from chiral effective field theory and for $pf$-shell nuclei within the traditional shell model. We compute the rotational band in the exotic nucleus $^{34}$Mg and find agreement with data.

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arXiv2021

Nuclear Charge Radii of the Nickel Isotopes $^{58-68,70}$Ni

Collinear laser spectroscopy is performed on the nickel isotopes $^{58-68,70}$Ni, using a time-resolved photon counting system. From the measured isotope shifts, nuclear charge radii $R_c$ are extracted and compared to theoretical results. Three ab initio approaches all employ, among others, the chiral interaction NNLO$_{\rm sat}$, which allows an assessment of their accuracy. We find agreement with experiment in differential radii $δ\left\langle r_\mathrm{c}^2 \right\rangle$ for all employed ab initio methods and interactions, while the absolute radii are consistent with data only for NNLO$_{\rm sat}$. Within nuclear density functional theory, the Skyrme functional SV-min matches experiment more closely than the Fayans functional Fy($Δr$,HFB).

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