arXiv subjects

Guillaume Hupin

Publications and source records attributed to Guillaume Hupin.

At least 19 recordsLinked to original sources

arXiv2026

Ab initio evidence for surface-dominated antiproton annihilation in ${}^4 \mathrm{He}$

Low-energy antiproton beams at CERN/AD open the possibility of probing exotic nuclear structure through annihilation at the nuclear surface. The use of antiprotons as the probe for the nuclear surface is based on the assumption that the annihilation takes place at the periphery of the target. We test this idea for the lightest tightly bound nucleus, i.e., ${}^4 \mathrm{He}$, using the ab initio no-core shell model combined with the resonating group method (NCSM/RGM), adapted to antiproton-nucleus dynamics. After validating our microscopic calculations against available atomic and scattering data, we use the microscopic annihilation density to examine where annihilation occurs inside the antiprotonic atom. We find that the annihilation peaks in the tail of the ${}^{4}\mathrm{He}$ density, around $r\approx 2$ fm, and is strongly suppressed in the nuclear interior. Although this density is representation dependent, a similarity renormalization group (SRG) analysis of the NCSM/RGM Hamiltonian shows that the low-energy annihilation strength remains localized at large intercluster distances. These results support the phenomenological picture underlying antiprotonic-atom experiments: in a tightly bound system such as ${}^{4}\mathrm{He}$, antiproton annihilation is predominantly peripheral and is therefore sensitive to the nuclear-density tail.

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arXiv2026

Ab Initio Study of $^7$Li with Coupled Mass Partitions

Background: Lithium is of broad interest in nuclear astrophysics, fusion energy research, and nuclear technology. From a theoretical perspective, the nucleus $^7$Li presents a remarkable challenge, as its bound and resonant states can exhibit contributions from both the $^4$He + $^3$H cluster configuration and configurations involving a neutron or proton coupled to a $^6$Li or $^6$He core, respectively. Purpose: We aim to achieve a unified ab initio description of bound-state and continuum properties of $^7$Li by explicitly including simultaneously the coupled mass/charge partitions $^4$He + $^3$H, $^6$Li + $n$, and $^6$He + $p$. Specifically, we investigate the effect of inter-partition coupling on the spectrum of $^7$Li and calculate cross sections for the $^6$Li($n,p)^6$He, $^6$He($p,n)^6$Li, and $^6$He($p,t)^4$He reactions. Method: We employ the no-core shell model with continuum for the first time in a calculation that couples three mass/charge partitions of the aggregate nucleus $^7$Li, using a chiral nucleon-nucleon interaction as input. Results: The calculated spectrum reproduces all the experimentally observed states of $^7$Li in the correct order and predicts additional resonances. The calculation also reproduces the overall energy dependence of the $^6$Li$(n,p)^6$He cross section. Improved agreement with measured cross sections is obtained after phenomenological adjustment of resonance energies. Conclusions: The present results show that coupling the relevant mass/charge partitions is important for a consistent description of the $^7$Li spectrum and reaction cross sections, and offers a useful framework for interpreting existing data and guiding future measurements.

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arXiv2026

Light antiproton-nucleus systems at low energies with the ab initio NCSM/RGM method

The availability of low-energy antiproton beams at the CERN Antiproton Decelerator has renewed interest in using antimatter as a probe of nuclear structure and in forming exotic antiprotonic few-body systems. In this work, we extend the ab initio no-core shell model combined with the resonating group method (NCSM/RGM), which was successfully applied to light-nucleus structure and reactions, to antiproton-nucleus dynamics at low energies. The NCSM/RGM formalism is adapted to antiproton projectiles by removing the requirement of antisymmetrization under exchange of target and projectile constituents, while retaining a fully microscopic description of the nuclear target and the relative motion. We focus on the lightest systems, ${\bar p}+d$, ${\bar p}+{}^3 \mathrm{H}$, and ${\bar p}+{}^3\mathrm{He}$, for which benchmarking against exact solutions of the Schrödinger equation enables stringent validation and helps disentangle methodological uncertainties -- e.g., those associated with the choice of configurations included in the NCSM/RGM expansion -- so that the dominant residual uncertainty can be attributed to the $N\bar{N}$ interaction. We compute phase shifts, scattering lengths, cross sections, antiprotonic-atom level shifts and widths, nuclear quasibound energies, and annihilation densities. We find that the hard short-range components of the meson-exchange-based $N\bar{N}$ interaction lead to slow convergence of the NCSM/RGM kernels expanded in a harmonic-oscillator basis, requiring exceptionally large model spaces and posing significant numerical challenges. We discuss practical strategies to mitigate these limitations and assess the impact of missing closed-channel configurations, which is a significant source of uncertainties in very light systems.

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arXiv2026

Halo Nuclei from Ab Initio Nuclear Theory

A realistic description of halo nuclei, characterized by low-lying breakup thresholds, requires a proper treatment of continuum effects. We have developed an ab initio approach, the no-core shell model with continuum (NCSMC), capable of describing both bound and unbound states in light nuclei in a unified way. With chiral two- and three-nucleon interactions as the only input, we can predict structure and dynamics of halo and other light nuclei and, by comparing to available experimental data, test the quality of chiral nuclear forces. We review NCSMC calculations of weakly bound states and resonances of exotic halo nuclei $^6$He, $^8$B, $^{11}$Be, and $^{15}$C. For the latter, we discuss its production in the capture reaction $^{14}$C(n,$γ$)$^{15}$C. We highlight challenges of a description of $^6$He as a Borromean n-n-$^4$He system. Finally, we present calculations of excited states in $^{10}$Be exhibiting a one-neutron halo structure and a large scale no-core shell model investigation of $^{11}$Li as a precursor of a full n-n-$^9$Li NCSMC study.

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arXiv2025

Ab Initio Complex Scaling and Similarity Renormalization Group for Continuum Properties of Nuclei

We introduce a novel \abinitio many-body method designed to compute the properties of nuclei in the continuum. This approach combines well-established techniques, namely the Complex Scaling (CS) and Similarity Renormalization Group (SRG) methods while employing the translationally invariant No-Core Shell Model (NCSM) as a few-body solver. We demonstrate that this combination effectively overcomes numerical limitations previously encountered in exploring continuum properties of light nuclei with standard many-body techniques, and at the same time makes less imperative the need for a continuous set of basis states for the continuum. To benchmark the method for applications in the many-body sector, we apply it to the \textsuperscript{4}He system, where semi-exact calculations within a finite basis are feasible. Our extrapolated results agree with exact calculations already published in the literature. We argue that different NN parametrizations of chiral EFT Hamiltonians will not permit to reproduce evaluated resonance properties of \textsuperscript{4}He. As an application, we showcase the case of the tetraneutron. This work enables the application of the method to $A>4$-mass systems, providing a reliable representation of the initial Hamiltonian and its continuum properties.

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arXiv2024

Towards Precision Muonic X-Ray Measurements of Charge Radii of Light Nuclei

We propose an experiment to measure the nuclear charge radii of light elements with up to 20~times higher accuracy. These are essential both for understanding nuclear physics at low energies, and for experimental and theoretical applications in simple atomic systems. Such comparisons advance the understanding of bound-state quantum electrodynamics and are useful for searching for new physics beyond the Standard Model. The energy levels of muonic atoms are highly susceptible to nuclear structure, especially to the mean square charge radius. The radii of the lightest nuclei (with the atomic number, $Z=1,2$) have been determined with high accuracy using laser spectroscopy in muonic atoms, while those of medium mass and above were determined using X-ray spectroscopy with semiconductor detectors. In this communication, we present a new experiment, aiming to obtain precision measurements of the radii of light nuclei $3 \leq Z \leq 10$ using single-photon energy measurements with cryogenic microcalorimeters; a quantum-sensing technology capable of high efficiency with outstanding resolution for low-energy X-rays.

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arXiv2023

Comparison of $\bar{\hbox{N}}\hbox{N}$ optical models

We compare the strong part of the $\bar{\hbox{N}}\hbox{N}$ interaction obtained by the Nijmegen partial wave analysis and the results of some of the most popular $\bar{\hbox{N}}\hbox{N}$ optical potentials in configuration space. We have found severe discrepancies in most of the partial waves, especially above $p_{Lab}$=400 MeV/c where the partial wave analysis displays a resonant-like structure in the $^{31}$S$_0$ and $^{33}$P$_0$ waves. Some theoretical difficulties to interpret this behaviour in terms of dynamical resonances are pointed pout and an alternative explanation is suggested. A much better stability is observed in the low energy parameters, apart from some discrepancies due to the presence of near-threshold quasi-bound states in particular waves. Large deviations have also been found between the corresponding potentials, at short and medium-range ($r\gtrsim 1$ fm) distances.

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arXiv2022

Ab initio prediction of the $^4{\rm He}(d,γ)\,^6\rm Li$ big bang radiative capture

The rate at which helium ($^4$He) and deuterium ($d$) fuse together to produce lithium-6 ($^6$Li) and a $γ$ ray, $^4$He$(d,γ)^6$Li, is a critical puzzle piece in resolving the roughly three orders of magnitude discrepancy between big bang predictions and astronomical observations for the primordial abundance of $^6$Li. The accurate determination of this radiative capture rate requires the quantitative and predictive description of the fusion probability across the big bang energy window ($30$ keV $\lesssim E\lesssim 400$ keV), where measurements are hindered by low counting rates. We present first-principles (or, ab initio) predictions of the $^4$He$(d,γ)^6$Li astrophysical S-factor using validated nucleon-nucleon and three-nucleon interactions derived within the framework of chiral effective field theory. By employing the ab initio no-core shell model with continuum to describe $^4{\rm He}$-$d$ scattering dynamics and bound $^6\rm Li$ product on an equal footing, we accurately and consistently determine the contributions of the main electromagnetic transitions driving the radiative capture process. Our results reveal an enhancement of the capture probability below 100 keV owing to previously neglected magnetic dipole (M1) transitions and reduce by an average factor of 7 the uncertainty of the thermonuclear capture rate between $0.002$ and $2$ GK.

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arXiv2022

Ab initio informed evaluation of the radiative capture of protons on $^7$Be

The radiative capture of protons by $^7$Be, which is the source of $^8$B that $β$-decays emitting the majority of solar neutrinos measured on earth, has not yet been measured at astrophysically relevant energies. The recommended value for its zero-energy S-factor, $S_{17}$(0) = 20.8$\pm$(0.7)exp$\pm$(1.4)theory eV$\cdot$b, relies on theoretical extrapolations from higher-energy measurements, a process that leads to significant uncertainty. We performed a set of first-principle (or, ab initio) calculations of the $^7$Be($p$, $γ$)$^8$B reaction to provide an independent prediction of the low-energy S-factor with quantified uncertainties. We demonstrate underlying features in the predicted S-factor allowing the combination of theoretical calculations and measurements to produce an evaluated S-factor of $S_{17}$(0) = 19.8$\pm$0.3 eV$\cdot$b. We expect the calculations and uncertainty quantification process described here to set a new standard for the evaluation of light-ion astrophysical reactions.

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arXiv2021

Microscopic investigation of the $^8$Li($n, γ$)$^9$Li reaction

The $^8$Li($n,γ$)$^9$Li reaction plays an important role in several astrophysics scenarios. It cannot be measured directly and indirect experiments have so far provided only cross section limits. Theoretical predictions differ by an order of magnitude. In this work we study the properties of $^9$Li bound states and low-lying resonances and calculate the $^8$Li($n,γ$)$^9$Li cross section within the no-core shell model with continuum (NCSMC) with chiral nucleon-nucleon and three-nucleon interactions as the only input. The NCSMC is an ab initio method applicable to light nuclei that provides a unified description of bound and scattering states well suited to calculate low-energy nuclear scattering and reactions. Our calculations reproduce the experimentally known bound states as well as the lowest $5/2^-$ resonance of $^9$Li. We predict a $3/2^-$ spin-parity assignment for the resonance observed at 5.38 MeV. In addition to the a very narrow $7/2^-$ resonance corresponding presumably to the experimental 6.43 MeV state, we find several other broad low-lying resonances. Our calculated $^8$Li($n,γ$)$^9$Li cross section is within the limits derived from the 1998 National Superconducting Cyclotron Laboratory Coulomb-dissociation experiment [Phys. Rev. C {\bf 57}, 959 (1998)]. However, it is higher than cross sections obtained in recent phenomenological studies. It is dominated by a direct E1 capture to the ground state with a resonant contribution at $\sim0.2$ MeV due to E2/M1 radiation enhanced by the $5/2^-$ resonance.

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arXiv2020

Ab initio framework for nuclear scattering and reactions induced by light projectiles

A quantitative and predictive microscopic theoretical framework that can describe reactions induced by $α$ particles ($^4$He nuclei) and heavier projectiles is currently lacking. Such a framework would contribute to reducing uncertainty in the modeling of stellar evolution and nucleosynthesis and provide the basis for achieving a comprehensive understanding of the phenomenon of nuclear clustering (the organization of protons and neutrons into distinct substructures within a nucleus). We have developed an efficient and general configuration-interaction framework for the description of low-energy reactions and clustering in light nuclei. The new formalism takes full advantage of powerful second-quantization techniques, enabling the description of $α$-$α$ scattering and an exploration of clustering in the exotic $^{12}$Be nucleus. We find that the $^4$He($α$, $α$)$^4$He differential cross section computed with non-locally regulated chiral interactions is in good agreement with experimental data. Our results for $^{12}$Be indicate the presence of strongly mixed helium-cluster states consistent with a molecular-like picture surviving far above the $^6$He+$^6$He threshold, and reveal the strong influence of neutron decay in both the $^{12}$Be spectrum and in the $^6$He($^6$He,$α$)$^8$He cross section. We expect that this approach will enable the description of helium burning cross sections and provide insight on how three-nucleon forces influence the emergence of clustering in nuclei.

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arXiv2019

$^7$Be and $^7$Li nuclei within the no-core shell model with continuum

The production of $^7$Be and $^7$Li nuclei plays an important role in primordial nucleosynthesis, nuclear astrophysics, and fusion energy generation. The $^3\mathrm{He}(α, γ) ^7\mathrm{Be}$ and $^3\mathrm{H}(α, γ) ^7\mathrm{Li}$ radiative-capture processes are important to determine the $^7$Li abundance in the early universe and to predict the correct fraction of pp-chain branches resulting in $^7$Be versus $^8$B neutrinos. In this work we study the properties of $^7$Be and $^7$Li within the no-core shell model with continuum (NCSMC) method, using chiral nucleon-nucleon interactions as the only input, and analyze all the binary mass partitions involved in the formation of these systems. The NCSMC is an ab initio method applicable to light nuclei that provides a unified description of bound and scattering states and thus is well suited to investigate systems with many resonances and pronounced clustering like $^7$Be and $^7$Li. Our calculations reproduce all the experimentally known states of the two systems and provide predictions for several new resonances of both parities. Some of these new possible resonances are built on the ground states of $^6$Li and $^6$He, and thus represent a robust prediction. We do not find any resonance in the p${+}^6$Li mass partition near the threshold. On the other hand, in the p${+}^6$He mass partition of $^7$Li we observe an $S$-wave resonance near the threshold producing a very pronounced peak in the calculated S factor of the $^6\mathrm{He} (\mathrm{p},γ) ^7\mathrm{Li}$ radiative-capture reaction, which could be relevant for astrophysics and its implications should be investigated.

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arXiv2019

Ab initio predictions for polarized DT thermonuclear fusion

The fusion of deuterium (D) with tritium (T) is the most promising of the reactions that could power the thermonuclear reactors of the future. Already favored for its low activation energy and high yield, it may lead to even more efficient energy generation if obtained in a polarized state, i.e. with the spin of the reactants aligned. While the DT fusion rate has been measured extensively, very little is known of the effects of polarization. Meanwhile, arriving at a fundamental understanding of the fusion process in terms of the laws of quantum mechanics and the underlying theory of the strong force has been a daunting challenge. We use nuclear forces derived from chiral effective field theory and apply the ab initio reaction method known as no-core shell model with continuum to predict, for the first time from first principles, the enhancement factor of the polarized DT fusion rate and anisotropy of the emitted neutron and α particle.

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arXiv2017

Structure of the exotic $^9$He nucleus from the no-core shell model with continuum

The exotic $^9$He nucleus, which presents one of the most extreme neutron-to-proton ratios, belongs to the $N=7$ isotonic chain famous for the phenomenon of ground-state parity inversion with decreasing number of protons. Consequently, it would be expected to have an unnatural (positive) parity ground state similar to $^{11}$Be and $^{10}$Li. Despite many experimental and theoretical investigations, its structure remains uncertain. Apart from the fact that it is unbound, other properties including the spin and parity of its ground state and the very existence of additional low-lying resonances are still a matter of debate. In this work we study the properties of $^9$He by analyzing the $n+^8$He continuum in the context of the ab initio no-core shell model with continuum (NCSMC) formalism with chiral interactions as the only input. The NCSMC is a state-of-the-art approach for the ab initio description of light nuclei. With its capability to predict properties of bound states, resonances, and scattering states in a unified framework, the method is particularly well suited for the study of unbound nuclei such as $^9$He. Our analysis produces an unbound $^9$He nucleus. Two resonant states are found at the energies of ${\sim}1$ and ${\sim}3.5$ MeV, respectively, above the $n+^8$He breakup threshold. The first state has a spin-parity assignment of $J^π = {1/2}^-$ and can be associated with the ground state of $^9$He, while the second, broader state has a spin-parity of ${3/2}^-$. No resonance is found in the ${1/2}^+$ channel, only a very weak attraction. We find that the $^9$He ground-state resonance has a negative parity and thus breaks the parity-inversion mechanism found in the $^{11}$Be and $^{10}$Li nuclei of the same $N=7$ isotonic chain.

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arXiv2017

Three-cluster dynamics within the ab initio no-core shell model with continuum: How many-body correlations and $α$-clustering shape $^6$He

We realize the treatment of bound and continuum nuclear systems in the proximity of a three-body breakup threshold within the ab initio framework of the no-core shell model with continuum. Many-body eigenstates obtained from the diagonalization of the Hamiltonian within the harmonic-oscillator expansion of the no-core shell model are coupled with continuous microscopic three-cluster states to correctly describe the nuclear wave function both in the interior and asymptotic regions. We discuss the formalism in detail and give algebraic expressions for the case of core+$n$+$n$ systems. Using similarity-renormalization-group evolved nucleon-nucleon interactions, we analyze the role of $^4$He+$n$+$n$ clustering and many-body correlations in the ground and low-lying continuum states of the Borromean $^6$He nucleus, and study the dependence of the energy spectrum on the resolution scale of the interaction. We show that $^6$He small binding energy and extended radii compatible with experiment can be obtained simultaneously, without recurring to extrapolations. We also find that a significant portion of the ground-state energy and the narrow width of the first $2^+$ resonance stem from many-body correlations that can be interpreted as core-excitation effects.

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arXiv2016

Can Ab Initio Theory Explain the Phenomenon of Parity Inversion in ${}^{11}$Be?

The weakly bound exotic ${}^{11}$Be nucleus, famous for its ground-state parity inversion and distinct n+ ${}^{10}$Be halo structure, is investigated from first principles using chiral two- and three-nucleon forces. An explicit treatment of continuum effects is found to be indispensable. We study the sensitivity of the ${}^{11}$Be spectrum to the details of the three-nucleon force and demonstrate that only certain chiral interactions are capable of reproducing the parity inversion. With such interactions, the extremely large E1 transition between the bound states is reproduced. We compare our photodisintegration calculations to conflicting experimental data and predict a distinct dip around the $3/2^-_1$ resonance energy. Finally, we predict low-lying $3/2^+$ and $9/2^+$ resonances that are not or not sufficiently measured in experiments.

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arXiv2016

How many-body correlations and $α$-clustering shape $^6$He

The Borromean $^6$He nucleus is an exotic system characterized by two `halo' neutrons orbiting around a compact $^4$He (or $α$) core, in which the binary subsystems are unbound. The simultaneous reproduction of its small binding energy and extended matter and point-proton radii has been a challenge for {\em ab initio} theoretical calculations based on traditional bound-state methods. Using soft nucleon-nucleon interactions based on chiral effective field theory potentials, we show that supplementing the model space with $^4$He+$n$+$n$ cluster degrees of freedom largely solves this issue. We analyze the role played by the $α$-clustering and many-body correlations, and study the dependence of the energy spectrum on the resolution scale of the interaction.

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arXiv2016

Unified ab initio approaches to nuclear structure and reactions

The description of nuclei starting from the constituent nucleons and the realistic interactions among them has been a long-standing goal in nuclear physics. In addition to the complex nature of the nuclear forces, with two-, three- and possibly higher many-nucleon components, one faces the quantum-mechanical many-nucleon problem governed by an interplay between bound and continuum states. In recent years, significant progress has been made in ab initio nuclear structure and reaction calculations based on input from QCD-employing Hamiltonians constructed within chiral effective field theory. After a brief overview of the field, we focus on ab initio many-body approaches - built upon the No-Core Shell Model - that are capable of simultaneously describing both bound and scattering nuclear states, and present results for resonances in light nuclei, reactions important for astrophysics and fusion research. In particular, we review recent calculations of resonances in the $^6$He halo nucleus, of five- and six-nucleon scattering, and an investigation of the role of chiral three-nucleon interactions in the structure of $^9$Be. Further, we discuss applications to the $^7$Be$(p,γ)^8$B radiative capture. Finally, we highlight our efforts to describe transfer reactions including the $^3$H$(d,n)^4$He fusion.

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