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 the 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 the exotic halo nuclei 6He, 8B, 11Be, and 15C. For the latter, we discuss its production in the capture reaction 14C(n,γ)15C. We highlight the challenges of a description of 6He as a Borromean n-n-4He system. Finally, we present our calculations of excited states in 10Be exhibiting a one-neutron halo structure and a large scale No-Core Shell Model investigation of 11Li as a precursor of a full n-n-9Li NCSMC study.

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, p̅+d, p̅+^3 H, and p̅+^3He, 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 NN̅ interaction. We compute phase shifts, scattering lengths, cross sections, antiprotonic-atom level shifts and widths, nuclear quasi-bound energies, and annihilation densities. We find that the hard short-range components of the meson-exchange-based NN̅ 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.

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 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 ^4He density, around r≈ 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 ^4He, antiproton annihilation is predominantly peripheral and is therefore sensitive to the nuclear-density tail.

Background: Lithium is of broad interest in nuclear astrophysics, fusion energy research, and nuclear technology. From a theoretical perspective, the nucleus ^7Li presents a remarkable challenge, as its bound and resonant states can exhibit contributions from both the ^4He + ^3H cluster configuration and configurations involving a neutron or proton coupled to a ^6Li or ^6He core, respectively. Purpose: We aim to achieve a unified ab initio description of bound-state and continuum properties of ^7Li by explicitly including simultaneously the coupled mass/charge partitions ^4He + ^3H, ^6Li + n, and ^6He + p. Specifically, we investigate the effect of inter-partition coupling on the spectrum of ^7Li and calculate cross sections for the ^6Li(n,p)^6He, ^6He(p,n)^6Li, and ^6He(p,t)^4He 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 ^7Li, using a chiral nucleon-nucleon interaction as input. Results: The calculated spectrum reproduces all the experimentally observed states of ^7Li in the correct order and predicts additional resonances. The calculation also reproduces the overall energy dependence of the ^6Li(n,p)^6He 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 ^7Li spectrum and reaction cross sections, and offers a useful framework for interpreting existing data and guiding future measurements.

The 3 He(a,y) 7 Be radiative capture reaction plays a key role in the creation of elements in stars as well as in the production of solar neutrinos, the observation of which is one of the main tools to study the properties of our sun. Since accurate experimental measurements of this fusion cross section at solar energies are difficult due to the strong Coulomb repulsion between the reactants, the onus falls on theory to provide a robust means for extrapolating from the region where experimental data is available down to the desired astrophysical regime. We present the first microscopic calculations of 3 He(a, y)7Be with explicit inclusion of three-nucleon forces. Our prediction of the astrophysical S factor qualitatively agrees with experimental data. We further incorporate experimental bound-state and scattering information in our calculation to arrive at a more quantitative description. This process reveals that our current model lacks sufficient repulsion in the 1/2+ channel of our model space to simultaneously reproduce elastic-scattering data. This deficit suggests that 3 He(a,y) 7 Be probes aspects of the nuclear force that are not currently well-constrained.

Observations of anomalies in the electron-positron angular correlations in high-energy decays in $^4$He, $^8$Be, and $^{12}$C have been reported recently by the ATOMKI collaboration. These could be explained by the creation and subsequent decay of a new boson with a mass of ${\sim}17$ MeV. Theoretical understanding of pair creation in the proton capture reactions used in these experiments is important for the interpretation of the anomalies. We apply the ab initio No-Core Shell Model with Continuum (NCSMC) to the proton capture on $^7$Li. The NCSMC describes both bound and unbound states in light nuclei in a unified way with chiral two- and three-nucleon interactions as the only input. We investigate the structure of $^8$Be, the $p+^7$Li elastic scattering, the $^7$Li($p,\gamma$)$^8$Be cross section and the internal pair creation $^7$Li($p,e^+ e^-$)$^8$Be. We discuss the impact of a proper treatment of the initial scattering state on the electron-positron angular correlation spectrum and compare our results to available ATOMKI data sets. Finally, we calculate $^7$Li($p,X$)$^8$Be cross sections for several proposed models of the hypothetical X17 particle.

Simulations of scattering processes are essential in understanding the physics of our universe. Computing relevant scattering quantities from ab initio methods is extremely difficult on classical devices because of the substantial computational resources needed. This work reports the development of an algorithm that makes it possible to obtain phase shifts for generic nonrelativistic elastic scattering processes on a quantum computer. This algorithm is based on extracting phase shifts from the direct implementation of the real-time evolution. The algorithm is improved by a variational procedure making it more accurate and resistant to the quantum noise. The reliability of the algorithm is first demonstrated by means of classical numerical simulations for different potentials and later tested on existing quantum hardware, specifically on IBM quantum processors.

The 3He(α,γ)7Be radiative capture reaction plays a key role in the creation of elements in stars as well as in the production of solar neutrinos, the observation of which is one of the main tools to study the properties of our sun. Since accurate experimental measurements of this fusion cross section at solar energies are difficult due to the strong Coulomb repulsion between the reactants, the onus falls on theory to provide a robust means for extrapolating from the region where experimental data is available down to the desired astrophysical regime. We present the first microscopic calculations of 3He(α,γ)7Be with explicit inclusion of three-nucleon forces. Our prediction of the astrophysical S factor qualitatively agrees with experimental data. We further incorporate experimental bound-state and scattering information in our calculation to arrive at a more quantitative description. This process reveals that our current model lacks sufficient repulsion in the 1/2+ channel of our model space to simultaneously reproduce elastic-scattering data. This deficit suggests that 3He(α,γ)7Be probes aspects of the nuclear force that are not currently well-constrained.

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.

Background: The no-core shell model (NCSM) is an ab initio method that solves the nuclear many-body problem by expanding the many-particle wave function into a (typically) harmonic oscillator basis and minimizing the energy to obtain the expansion coefficients. Extensions of the NCSM, such as its coupling with microscopic-cluster basis states, further allow for an ab initio treatment of light-ion nuclear reactions of interest for both astrophysics and nuclear technology applications. A downside of the method is the exponential scaling of the basis size with increasing number of nucleons and excitation quanta, which limits its applicability to mass A <= 16 nuclei, except for variants where the basis is further down-selected via some truncation scheme. Purpose: We consider a basis selection method for the NCSM that was first introduced in the context of the large-scale shell model and captures the essential degrees of freedom of the nuclear wave function leading to a favorable complexity scaling for calculations and enabling ab initio reaction calculations in sd-shell nuclei. Methods: The particle configurations within the NCSM basis are ordered based on their contribution to the first moment of the Hamiltonian matrix that results from the projection onto the many-body basis. The truncation scheme then consists in retaining only the lowest-first-moment configurations, which typically contain only few many-body basis states (Slater determinants). As the energy threshold above which configurations are disregarded is increased, the size of the basis becomes an almost-continuous variable, allowing for tunable fidelity in the obtained wave functions. The resulting wave functions can then be used directly in ab initio reaction calculations. Results: We present calculations for Li-7 and n + C-12 scattering using nucleon-nucleon interactions derived from chiral effective field theory and softened using the similarity renormalization group method. The obtained energy levels invariably demonstrate exponential convergence with the size of the basis, and we find improved convergence in scattering calculations. To demonstrate the possibilities enabled by the approach, we also present a first calculation for the scattering of neutrons from Mg-24. Conclusions: The method presented in this work appears promising for future studies of nuclei with mass A > 16, opening multiple future research directions impacting both nuclear astrophysics and nuclear technology applications.

Indirect methods have become the predominant approach in experimental nuclear astrophysics for studying several low-energy nuclear reactions occurring in stars, as direct measurements of many of these relevant reactions are rendered infeasible due to their low reaction probability. Such indirect methods, however, require theoretical input that in turn can have significant poorly-quantified uncertainties, which can then be propagated to the reaction rates and have a large effect on our quantitative understanding of stellar evolution and nucleosynthesis processes. We present two such examples involving $\alpha$-induced reactions, $^{13}$C($\alpha,n)^{16}$O and $^{12}$C$(\alpha,\gamma)^{16}$O, for which the low-energy cross sections have been constrained with $(^6$Li$,d)$ transfer data. In this Letter, we discuss how a first-principle calculation of $^6$Li leads to a 21% reduction of the $^{12}$C$(\alpha,\gamma)^{16}$O cross sections with respect to a previous estimation. This calculation further resolves the discrepancy between recent measurements of the $^{13}$C$(\alpha,n)^{16}$O reaction and points to the need for improved theoretical formulations of nuclear reactions.

Observations of anomalies in the electron-positron angular correlations in high-energy decays in 4 He, 8 Be, and 12 C have been reported recently by the ATOMKI collaboration.These could be explained by the creation and subsequent decay of a new boson with a mass of ≈17 MeV.Theoretical understanding of pair creation in the proton capture reactions used in these experiments is important for the interpretation of the anomalies.We apply the ab initio No-Core Shell Model with Continuum (NCSMC) to the proton capture on 7 Li.The NCSMC describes both bound and unbound states in light nuclei in a unified way with chiral two-and three-nucleon interactions as the only input.We investigate the structure of 8 Be, the p+ 7 Li elastic scattering, the 7 Li(p, γ) 8 Be cross section and the internal pair creation 7 Li(p, e + e -) 8 Be.We discuss the impact of a proper treatment of the initial scattering state on the electron-positron angular correlation spectrum and compare our results to available ATOMKI data sets.Finally, we calculate 7 Li(p, X) 8 Be cross sections for several proposed models of the hypothetical X17 particle.

The radiative capture of protons by 7Be, which is the source of 8B that β-decays emitting the majority of higher-energy solar neutrinos measured on earth, has not yet been measured at astrophysically relevant energies. The recommended value for its zero-energy S-factor, S17(0)=20.8±(0.7)exp±(1.4)theory eV⋅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 7Be(p,γ)8B 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 S17(0)=19.8±0.3 eV⋅b. We expect the calculations and uncertainty quantification process described here to set a new standard for the evaluation of light-ion astrophysical reactions.