arXiv subjects

Sofia Quaglioni

Publications and source records attributed to Sofia Quaglioni.

At least 37 records · Page 2Linked to original sources

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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arXiv2020

From bound states to the continuum

This white paper reports on the discussions of the 2018 Facility for Rare Isotope Beams Theory Alliance (FRIB-TA) topical program "From bound states to the continuum: Connecting bound state calculations with scattering and reaction theory". One of the biggest and most important frontiers in nuclear theory today is to construct better and stronger bridges between bound state calculations and calculations in the continuum, especially scattering and reaction theory, as well as teasing out the influence of the continuum on states near threshold. This is particularly challenging as many-body structure calculations typically use a bound state basis, while reaction calculations more commonly utilize few-body continuum approaches. The many-body bound state and few-body continuum methods use different language and emphasize different properties. To build better foundations for these bridges, we present an overview of several bound state and continuum methods and, where possible, point to current and possible future connections.

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arXiv2020

Quantifying uncertainties in neutron-alpha scattering with chiral nucleon-nucleon and three-nucleon forces

Background: Modern ab initio theory combined with high-quality nucleon-nucleon (NN) and three-nucleon (3N) interactions from chiral effective field theory (EFT) can provide a predictive description of low-energy light-nuclei reactions relevant for astrophysics and fusion-energy applications. However, the high cost of computations has so far impeded a complete analysis of the uncertainty budget of such calculations. Purpose: Starting from NN potentials up to fifth order (N4LO) combined with leading-order 3N forces, we study how the order-by-order convergence of the chiral expansion and confidence intervals for the 3N contact and contact-plus-one-pion-exchange low-energy constants (cE and cD) contribute to the overall uncertainty budget of many-body calculations of neutron-He elastic scattering. Methods: We compute structure and reaction observables for three-, four- and five-nucleon systems within the ab initio frameworks of the no-core shell model an no-core shell model with continuum. Using a small set of design runs, we construct a Gaussian process model (GPM) that acts as a statistical emulator for the theory. With this, we gain insight into how uncertainties in the 3N low-energy constants propagate throughout the calculation and determine the Bayesian posterior distribution of these parameters with Markov-Chain Monte-Carlo.

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arXiv2019

Optimal Control for the Quantum Simulation of Nuclear Dynamics

We propose a method for enacting the unitary time propagation of two interacting neutrons at leading order of chiral effective field theory by efficiently encoding the nuclear dynamics into a single multi-level quantum device. The emulated output of the quantum simulation shows that, by applying a single gate that draws on the underlying characteristics of the device, it is possible to observe multiple cycles of the nucleons' dynamics before the onset of decoherence. Owing to the signal's longevity, we can then extract spectroscopic properties of the simulated nuclear system. This allows us to validate the encoding of the nuclear Hamiltonian and the robustness of the simulation in the presence of quantum-hardware noise by comparing the extracted spectroscopic information to exact calculations. This work paves the way for transformative calculations of dynamical properties of nuclei on near-term quantum devices.

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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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arXiv2016

${^3{\rm He}}(α,γ){^7{\rm Be}}$ and ${^3{\rm H}}(α,γ){^7{\rm Li}}$ astrophysical $S$ factors from the no-core shell model with continuum

The ${^3{\rm He}}(α,γ){^7{\rm Be}}$ and ${^3{\rm H}}(α,γ){^7{\rm Li}}$ astrophysical $S$ factors are calculated within the no-core shell model with continuum using a renormalized chiral nucleon-nucleon interaction. The ${^3{\rm He}}(α,γ){^7{\rm Be}}$ astrophysical $S$ factors agree reasonably well with the experimental data while the ${^3{\rm H}}(α,γ){^7{\rm Li}}$ ones are overestimated. The seven-nucleon bound and resonance states and the $α+{^3{\rm He}}/{^3{\rm H}}$ elastic scattering are also studied and compared with experiment. The low-lying resonance properties are rather well reproduced by our approach. At low energies, the $s$-wave phase shift, which is non-resonant, is overestimated.

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arXiv2016

Deuteron-induced nucleon transfer reactions within an ab initio framework: First application to p-shell nuclei

Background: Low-energy transfer reactions in which a proton is stripped from a deuteron projectile and dropped into a target play a crucial role in the formation of nuclei in both primordial and stellar nucleosynthesis, as well as in the study of exotic nuclei using radioactive beam facilities and inverse kinematics. Ab initio approaches have been successfully applied to describe the $^3$H$(d,n)^4$He and $^3$He$(d,p)^4$He fusion processes. Purpose: An ab initio treatment of transfer reactions would also be desirable for heavier targets. In this work, we extend the ab initio description of $(d,p)$ reactions to processes with light $p$-shell nuclei. As a first application, we study the elastic scattering of deuterium on $^7$Li and the ${}^{7}$Li($d$,$p$)${}^{8}$Li transfer reaction based on a two-body Hamiltonian. Methods: We use the no-core shell model to compute the wave functions of the nuclei involved in the reaction, and describe the dynamics between targets and projectiles with the help of microscopic-cluster states in the spirit of the resonating group method. Results: The shape of the excitation functions for deuteron impinging on ${}^{7}$Li are qualitatively reproduced up to the deuteron breakup energy. The interplay between $d$-$^7$Li and $p$-$^8$Li particle-decay channels determines some features of the ${}^{9}$Be spectrum above the $d$+${}^{7}$Li threshold. Our prediction for the parity of the 17.298 MeV resonance is at odds with the experimental assignment Conclusions: Deuteron stripping reactions with $p$-shell targets can now be computed ab initio, but calculations are very demanding. A quantitative description of the ${}^{7}$Li($d$,$p$)${}^{8}$Li reaction will require further work to include the effect of three-nucleon forces and additional decay channels, and improve the convergence rate of our calculations.

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arXiv2015

Towards an ab initio description of the light-nuclei radiative captures

The ${^3{\rm He}}(α,γ){^7{\rm Be}}$ and ${^3{\rm H}}(α,γ){^7{\rm Li}}$ astrophysical $S$ factors are evaluated at low collision energies (less than 2.5 MeV in the centre-of-mass frame) within the no-core shell model with continuum approach using a renormalized chiral nucleon-nucleon interaction.

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arXiv2015

Advances in the ab initio description of nuclear three-cluster systems

We introduce the extension of the ab initio no-core shell model with continuum to describe three-body cluster systems. We present results for the ground state of 6He and show improvements with respect to the description obtained within the no-core shell model and the no-core shell model/resonating group methods.

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arXiv2015

Operator evolution for ab initio electric dipole transitions of 4He

A goal of nuclear theory is to make quantitative predictions of low-energy nuclear observables starting from accurate microscopic internucleon forces. A major element of such an effort is applying unitary transformations to soften the nuclear Hamiltonian and hence accelerate the convergence of ab initio calculations as a function of the model space size. The consistent simultaneous transformation of external operators, however, has been overlooked in applications of the theory, particularly for nonscalar transitions. We study the evolution of the electric dipole operator in the framework of the similarity renormalization group method and apply the renormalized matrix elements to the calculation of the 4He total photoabsorption cross section and electric dipole polarizability. All observables are calculated within the ab initio no-core shell model. We find that, although seemingly small, the effects of evolved operators on the photoabsorption cross section are comparable in magnitude to the correction produced by including the chiral three-nucleon force and cannot be neglected.

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arXiv2015

Microscopic Study of $α+N$ Bremsstrahlung from Effective and Realistic Multi-nucleon Interactions

Based on an effective nucleon-nucleon interaction, a microscopic cluster model of the nucleus-nucleus bremsstrahlung, including implicitly a part of the effects of meson-exchange currents via an extension of the Siegert theorem is applied to the $α+p$ and $α+n$ systems. The contributions of the $E1$ and $E2$ transitions to the bremsstrahlung cross sections are evaluated and their relative importance for the mirror systems $α+p$ and $α+n$ is compared. Another approach based on realistic two- and three-nucleon interactions and the No-Core Shell Model/Resonating-Group Method is also investigated. Some preliminary results for the $α+p$ bremsstrahlung are displayed.

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arXiv2014

Unified description of $^6$Li structure and deuterium-$^4$He dynamics with chiral two- and three-nucleon forces

Prototype for the study of weakly bound projectiles colliding on stable targets, the scattering of deuterium ($d$) on $^4$He ($α$) is an important milestone in the search for a fundamental understanding of low-energy reactions. At the same time, it is also important for its role in the Big-bang nucleosynthesis of $^6$Li and applications in the characterization of deuterium impurities in materials. We present the first unified {\em ab initio} study of the $^6$Li ground state and $d$-$^4$He elastic scattering using two- and three-nucleon forces derived within the framework of chiral effective field theory. The six-nucleon bound-state and scattering observables are calculated by means of the no-core shell model with continuum. %and are compared to available experimental data. We analyze the influence of the dynamic polarization of the deuterium and of the chiral three-nucleon force, and examine the role of the continuum degrees of freedom in shaping the low-lying spectrum of $^6$Li. We find that the adopted Hamiltonian correctly predicts the binding energy of $^6$Li, yielding an asymptotic $D$- to $S$-state ratio of the $^6$Li wave function in $d+α$ configuration of $-0.027$ in agreement with the value determined from a phase shift analysis of $^6$Li+$^4$He elastic scattering, but overestimates the excitation energy of the first $3^+$ state by $350$ keV. The bulk of the computed differential cross section is in good agreement with data.

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