Mass measurements of ^31-33Na and ^31-35Mg using the TITAN MR-TOF-MS at TRIUMF's ISAC facility are presented, with the uncertainty of the ^33Na 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 ^32Na. The precision obtained shows that the binding energy of the normally closed N = 20 neutron shell reaches a minimum for ^32Mg but increases significantly for ^31Na, hinting at the possibility of enhanced shell strength toward the unbound ^28O. We compare the results with new ab initio predictions that raise intriguing questions of nuclear structure beyond the dripline.

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.

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.

The computation of the nuclear equation of state contained a mistake in the normal -ordered approximation of the three-nucleon interaction. The correction affects Fig. 5 of the original Rapid Communication and its description in the text, and a statement about the saturation point in the abstract. All results and conclusions for finite nuclei presented in the original Rapid Communication are unchanged. The equation of state for symmetric nuclear matter using corrected computations with the interaction NNLO sat is shown in Fig. 1. At the saturation point the Fermi momentum is k (f )approximate to 1 . 37 fm (- 1) , the density is rho (0) . 17 fm( - 3) , the energy per nucleon is E / A approximate to - 17 . 0 MeV, and the incompressibility is K approximate to 310 MeV. In contrast to the results published in the original Rapid Communication, the saturation Fermi momentum is now above the empirical range (1 . 30 fm(- 1) less than or similar to k (f ) less than or similar to 1 . 35 fm (- 1) ). The saturation energy has shifted from being underbound by 1.5 MeV to being overbound by about 1 MeV. The incompressibility has moved from the lower end, K approximate to 253 MeV reported in the original Rapid Communication, to the higher end of what one expects for symmetric nuclear matter. At saturation density, coupled -cluster with doubles yields about 8 MeV per particle in correlation energy, while triples corrections (and residual NNN forces beyond the normal -ordered two -body approximation) yield another 1.8 MeV.

The shell closure at N = 32 has been investigated by a first spectroscopy of the N = 31 nucleus 49Ar at the Radioactive Isotope Beam Factory. Using the 50Ar(p, pn) reaction channel in inverse kinematics, 50Ar projectiles at 217 MeV/nucleon impinged on a 150 mm long liquid hydrogen target, part of the MINOS device. Prompt deexcitation gamma rays were measured with the NaI(Tl) array DALI2+. Reaction products were analyzed with the SAMURAI spectrometer, which allowed the measurement of the momentum distributions and angular momentum transfer. Data were compared to state-of-the-art theoretical predictions, including shell -model, energy -density functional, and ab initio calculations. An onset of collectivity is suggested besides the spherical configuration typical of a closed shell nucleus, such as for 52Ca.

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 magnetic dipole transition strength B(M1) of ^{48}Ca is dominated by a single resonant state at an excitation energy of 10.23 MeV. Experiments disagree about B(M1) and this impacts our understanding of spin flips in nuclei. We performed ab initio computations based on chiral effective field theory and found that B(M1:  0^{+}→1^{+}) lies in the range from 7.0 to 10.2  μ_{N}^{2}. This is consistent with a (γ,n) experiment but larger than results from (e,e^{'}) and (p,p^{'}) scattering. Two-body currents yield no quenching of the B(M1) strength and continuum effects reduce it by about 10%. For a validation of our approach, we computed magnetic moments in ^{47,49}Ca and performed benchmark calculations in light nuclei.

Muon capture on nuclei is one of the most promising probes of the nuclear electroweak current driving the yet-hypothetical neutrinoless double-beta ($0\ensuremath{\nu}\ensuremath{\beta}\ensuremath{\beta}$) decay. Both processes involve vector and axial-vector currents at finite momentum transfer, $q\ensuremath{\approx}100$ MeV, as well as the induced pseudoscalar and weak-magnetism currents. Comparing measured muon-capture rates with reliable ab initio nuclear-theory predictions could help us validate these currents. To this end, we compute partial muon-capture rates for $^{6}\mathrm{Li}$, $^{12}\mathrm{C}$, and $^{16}\mathrm{O}$, feeding the ground and excited states in $^{6}\mathrm{He}$, $^{12}\mathrm{B}$, and $^{16}\mathrm{N}$, using ab initio no-core shell model with two- and three-nucleon chiral interactions. We remove the spurious center-of-mass motion by introducing translationally invariant operators and approximate the effect of hadronic two-body currents by Fermi-gas model. We solve the bound-muon wave function from the Dirac wave equations in the Coulomb field created by a finite nucleus. We find that the computed rates to the low-lying states in the final nuclei are in good agreement with the measured counterparts. We highlight sensitivity of some of the transitions to the sub-leading three-nucleon interaction terms. We also compare summed rates to several tens of final states with the measured total capture rates and note that we slightly underestimate the total rate with this simple approach due to limited range of excitation energies.

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.

This white paper was submitted to the 2022 Fundamental Symmetries, Neutrons, and Neutrinos (FSNN) Town Hall Meeting in preparation for the next NSAC Long Range Plan. We advocate to support current and future theoretical and experimental searches for physics beyond the Standard Model using nuclear β decay

We apply the No-Core Shell Model with Continuum (NCSMC) that is 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. The NCSMC can predict structure and dynamics of light nuclei and, by comparing to available experimental data, test the quality of chiral nuclear forces. We discuss applications of NCSMC to the α–α scattering and the structure of 8 Be, the p+ 7 Be and p+ 7 Li radiative capture and the production of the hypothetical X17 boson claimed in ATOMKI experiments. The 7 Be(p, γ) 8 B reaction plays a role in Solar nucleosynthesis and Solar neutrino physics and has been subject of numerous experimental investigations. We also highlight our investigation of the neutron rich exotic 8 He that has been recently studied experimentally at TRIUMF with an unexpected deformation reported.

We discuss some of the challenges that future nuclear modeling may face in order to improve the description of the nuclear structure. One challenge is related to the need for A-body nuclear interactions justified by various contemporary nuclear physics studies. Another challenge is related to the discrepancy in the NNN contact interaction parameters for 3He and 3H that suggests the need for accurate proton and neutron masses in the future precision calculations. MSC2010 Classification: 17B81 Applications to physics, 17B80 Applications to integrable systems, 81R12 Relations with integrable systems, 81V70 Many-body theory, 81V35 Nuclear physics, 81U15 Exactly and quasi-solvable systems, 82B23 Exactly solvable models; Bethe ansatz.

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_{sat}, which allows an assessment of their accuracy. We find agreement with experiment in differential radii δ⟨r_{c}^{2}⟩ for all employed ab initio methods and interactions, while the absolute radii are consistent with data only for NNLO_{sat}. Within nuclear density functional theory, the Skyrme functional SV-min matches experiment more closely than the Fayans functional Fy(Δr,HFB).

We propose a novel storage scheme for three-nucleon (3N) interaction matrix elements relevant for the normal-ordered two-body approximation used extensively in ab initio calculations of atomic nuclei. This scheme reduces the required memory by approximately two orders of magnitude, which allows the generation of 3N interaction matrix elements with the standard truncation of E3max = 28, well beyond the previous limit of 18. We demonstrate that this is sufficient to obtain the ground-state energy of 132Sn converged to within a few MeV with respect to the E3max truncation. In addition, we study the asymptotic convergence behavior and perform extrapolations to the un-truncated limit. Finally, we investigate the impact of truncations made when evolving free-space 3N interactions with the similarity renormalization group. We find that the contribution of blocks with angular momentum Jrel > 9/2 to the ground-state energy is dominated by a basis-truncation artifact, which vanishes in the large-space limit, so these computationally expensive components can be neglected. For the two sets of nuclear interactions employed in this work, the resulting binding energy of 132Sn agrees with the experimental value within theoretical uncertainties. This work enables converged ab initio calculations of heavy nuclei.

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 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≲E≲400 keV), where measurements are hindered by low counting rates. We present first-principle (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}He-d scattering dynamics and bound ^{6}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.

States in the N=35 and 37 isotopes 55,57Ca have been populated by direct proton-induced nucleon removal reactions from 56,58Sc and 56Ca beams at the RIBF. In addition, the (p,2p) quasi-free single-proton removal reaction from 56Ca was studied. Excited states in 55K, 55Ca, and 57Ca were established for the first time via in-beam γ-ray spectroscopy. Results for the proton and neutron removal reactions from 56Ca to states in 55K and 55Ca 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 57Ca suggests a transition in the calcium isotopic chain from single-particle dominated states at N=35 to collective excitations at N=37.

The exotic beta-delayed proton emission is calculated in Be-11 from first principles using chiral two- and three- nucleon forces. To investigate the unexpectedly large branching ratio measured in Ayyad et al. [Phys. Rev. Lett. 123, 082501 (2019)], we calculate the proposed (1/2(+), 1/2) proton resonance in B-11 using the no-core shell model with continuum. This calculation helps to address whether this enhancement is caused by unknown dark decay modes or an unobserved proton resonance. We report a branching ratio of b(p) = (1.3 +/- 0.5) x 10(-6), suggesting that its unexpectedly large value is caused by an unobserved proton resonance in B-11.