We develop a framework that allows us to calculate integrated properties of the nuclear response from first principles. Using the ab initio in-medium similarity renormalization group (IMSRG), we calculate the expectation values of moment operators that are linked to the multipole response of nuclei. This approach is applied to the isoscalar mono- and quadrupole as well as the isovector dipole response of closed-shell nuclei from ${}^4\mathrm{He}$ to ${}^{78}\mathrm{Ni}$ for different chiral two- and three-nucleon interactions. We find that the inclusion of many-body correlations in the nuclear ground state significantly impacts the multipole response when going from the random-phase approximation to the IMSRG level. Our IMSRG calculations lead to an improved description of experimental data in ${}^{16}\mathrm{O}$ and ${}^{40}\mathrm{Ca}$, including a good reproduction of the Thomas-Reiche-Kuhn enhancement factor. These findings highlight the utility of the moment method as a benchmark for other ab initio approaches that describe nuclear response functions through the explicit treatment of excited states.

Converged results for $E2$ observables are notoriously challenging to obtain in ab initio no-core configuration interaction approaches. Matrix elements of the $E2$ operator are sensitive to the large-distance tails of the nuclear wave function, which converge slowly in an oscillator basis expansion. Similar convergence challenges beset ab initio prediction of the nuclear charge radius. However, we exploit systematic correlations between the calculated $E2$ and radius observables to yield meaningful predictions for relations among these observables. In particular, we examine ab initio predictions for dimensionless ratios of the form $Q/r^2$ for nuclei throughout the $p$ shell. Meaningful predictions for electric quadrupole moments may then be made by calibrating to the ground-state charge radius, if experimentally known, or vice versa. Moreover, these dimensionless ratios provide ab initio insight into the nuclear quadrupole deformation.

Converged results for $E2$ observables are notoriously challenging to obtain in ab initio no-core configuration interaction approaches. Matrix elements of the $E2$ operator are sensitive to the large-distance tails of the nuclear wave function, which converge slowly in an oscillator basis expansion. Similar convergence challenges beset ab initio prediction of the nuclear charge radius. However, we exploit systematic correlations between the calculated $E2$ and radius observables to yield meaningful predictions for relations among these observables. In particular, we examine ab initio predictions for dimensionless ratios of the form $B\left(E2\right)/\left(e^2{r}^4\right)$ for nuclei throughout the $p$ shell. Meaningful predictions for $E2$ transition strengths may then be made by calibrating to the ground-state charge radius if experimentally known.

A benchmark study is performed for the excited state of ${}^4\mathrm{He}$. When the Coulomb interaction is switched off, the ${}^4\mathrm{He}$ nucleus exhibits a bound excited state in the vicinity of the $p-{}^3\mathrm{H}$ threshold. As the Coulomb interaction is gradually introduced, the excited state crosses the threshold and eventually becomes a resonant state. Using three numerical methods, we track the evolution of this excited state and determine the resonance energy and width. Comparisons of the theoretical predictions reveal a significant discrepancy with commonly used $R$-matrix values based on the analysis of the experimental data. We explain the origin for this discrepancy. Additionally, the two-level energy spectrum of ${}^4\mathrm{He}$ in the absence of the Coulomb force exhibits characteristics linked to Efimov physics, suggesting a reduced sensitivity to interaction details.

Three new behaviors have been found in comparisons of fusion cross sections for different collision systems. (1) Replacing the energy $E$ with a scaling one, $E_{\mathrm{scal}}=(E-V_g)/\left(\sqrt{2}{W}_g\right)$, is successful for washing out the Coulomb interaction in the spectra of fusion cross sections, where $V_g$ and $W_g$ are barrier height and width of the single-Gaussian barrier distribution model. (2) In a representation of $\sigma E$ vs the scaling energy, $E_{\mathrm{scal}}$, all data sets display in parallel. The ratio for $\sigma E$ from any two fusion systems over the whole range is a constant value. That behavior is also studied in another representation, in which the data sets display as parallel horizontal lines for any heavy-ion fusion system. (3) The constant ratio value is the ratio of parameter products, $R_g^2{W}_g$, of the two systems; where $R_g$ is the barrier radius obtained in the single-Gaussian barrier distribution model. Moreover, when comparing neighboring collision systems at the same $E_{\mathrm{scal}}$, the ratio of $\sigma$ is near a constant value within a few percent over the whole range. Thus a quantitative comparison for the fusion enhancement for neighboring systems is developed. The present finding could be beneficial for predicting unmeasured fusion cross sections.

Techniques from artificial intelligence and machine learning are increasingly employed in nuclear theory; however, the uncertainties that arise from the complex parameter manifold encoded by the neural networks are often overlooked. Epistemic uncertainties arising from training the same network multiple times for an ensemble of initial weight sets offer a first insight into the confidence of machine learning predictions, but they often come with a high computational cost. Instead, we apply a single-model uncertainty quantification method called $\mathrm{\Delta }$-UQ that gives epistemic uncertainties with one-time training. We demonstrate our approach on a two-feature model of nuclear binding energies per nucleon with proton and neutron number pairs as inputs. We show that $\mathrm{\Delta }$-UQ can produce reliable and self-consistent epistemic uncertainty estimates and can be used to assess the degree of confidence in predictions made with deep neural networks.

The $\alpha$-decay spectroscopy of ${}^{247}\mathrm{Md}$ was investigated via the fusion-evaporation reaction ${}^{209}\mathrm{Bi}({}^{40}\mathrm{Ar},2n){}^{247}\mathrm{Md}$ by means of $\alpha$-, $\gamma$-, and electron-coincidence measurements. While measuring the previously known $\alpha$-decay properties, a new isomeric state in ${}^{243}\mathrm{Es}$, the daughter nucleus of ${}^{247}\mathrm{Md}$, with a half-life of $5.0\left(5\right)\mu \mathrm{s}$ was observed. Based on the suggested decay scheme of ${}^{247}\mathrm{Md}$, the new isomer in ${}^{243}\mathrm{Es}$ was assigned as the $7/2^{+}\left[633\right]$ Nilsson level, which is the first observation of an isomer in $Z=99$ Es isotopes. The single-particle structure around the $Z=100$ deformed shell gap was discussed.

This paper reports on the measurement of the ground-state spin and nuclear magnetic dipole moment of ${}^{61}\mathrm{Cr}$. The radioactive ion beam was produced at the CERN-ISOLDE facility and was probed using high-resolution resonance ionization laser spectroscopy with the collinear resonance ionization spectroscopy (CRIS) apparatus. The present ground-state spin measurement $I=\frac{1}{2}$, differing from the previously adopted $I=\left(\frac{5}{2}\right)$, has significant consequences on the interpretation of existing beta decay data and nuclear structure in the region. The structure and shape of ${}^{61}\mathrm{Cr}$ is interpreted with state-of-the-art Large-Scale Shell-Model and Discrete NonOrthogonal Shell-Model calculations. From the measured magnetic dipole moment $\mu {(}^{61}\mathrm{Cr})=+0.541\left(6\right){\mu }_N$ and the theoretical findings, its configuration is understood to be driven by two-particle–two-hole neutron excitations with an unpaired $1p_{1/2}$ neutron. This establishes the western border of the $N=40$ island of inversion, characterized by four-particle–four-hole neutron components. We discuss the shape evolution along the Cr isotopic chain as a second-order quantum phase transition at the entrance of the $N=40$ island of inversion.

Background: Electron-capture reactions on iron-group nuclei play a crucial role in the late stages of massive star evolution. Since stellar evolution simulations depend on accurate electron-capture rates—which are highly sensitive to the detailed Gamow-Teller (GT) strength distributions—reliable theoretical models are essential. However, experimental data on GT strength distributions are scarce. High-resolution measurements are therefore vital for benchmarking and improving these theoretical calculations.

Purpose: To provide high-resolution data on Gamow-Teller strength distributions of iron-group nuclei and to compare these results with theoretical calculations within this mass region.

Methods: Differential cross sections for the ${}^{59}\mathrm{Co}(t,{}^3\mathrm{He}){}^{59}\mathrm{Fe}$ charge-exchange reaction at 115 MeV/u were measured using the S800 spectrometer. To resolve individual levels that are not distinguishable in the S800 particle singles data, coincident $\gamma$ rays from the ${}^{59}\mathrm{Fe}$ residual nucleus were detected by using the Gamma-Ray Energy Tracking In-beam Nuclear Array $\gamma$-ray tracking array.

Results: The Gamow-Teller transition strength distribution from the ground state of ${}^{59}\mathrm{Co}$ to ${}^{59}\mathrm{Fe}$ was extracted up to an excitation energy of 10 MeV. Additionally, transition strengths for several low-lying states were determined from coincident $\gamma$-ray measurements. Electron-capture rates calculated using the present data indicate that these low-lying states contribute significantly to the overall rates in relevant stellar environments. The experimental results show reasonable agreement with theoretical predictions based on both shell-model and projected shell-model calculations.

Conclusions: High-resolution data on Gamow-Teller strength distributions—particularly for individual low-lying states—are essential for accurately determining electron-capture rates in iron-group nuclei. Coincident $\gamma$-ray measurements provide a powerful tool for obtaining such detailed information. While the present work demonstrates that shell-model calculations successfully reproduce the experimental results, such comparisons are scarce and more experimental data are desirable.

The reaction ${}^{14}\mathrm{N}(n,p){}^{14}\mathrm{C}$ is one of the main neutron poisons during $s$-process nucleosynthesis. In addition, the reaction provides insight into the yields of atmospheric nuclear weapon testing. Because of their high level of sensitivity, total neutron cross sections provide a great deal of constraint on the modeling of reaction cross sections through the $R$-matrix analyses used for nuclear data evaluations. Yet for ${}^{14}\mathrm{N}+n$ only one high sensitivity measurement is available, and it lacks detailed information about its experimental conditions and uncertainties. With these motivations in mind, a new measurement of the ${}^{14}\mathrm{N}+n$ total cross section was performed at the nELBE facility. The cross sections were found to be in good agreement with previous data over much of the energy range, with the key exception of the lowest energy resonance at a neutron energy of 433 keV.

We investigated the nuclear structure of ${}^{31}\mathrm{P}$ near the proton threshold using nuclear resonance fluorescence (NRF) to refine the properties of key resonances in the ${}^{30}\mathrm{Si}(p,\gamma ){}^{31}\mathrm{P}$ reaction, which is critical for nucleosynthesis in stellar environments. Excitation energies and spin-parities were determined for several states, including two unobserved resonances at $E_r=18.7\mathrm{keV}$ and $E_r=50.5\mathrm{keV}$. The angular correlation analysis enabled the first unambiguous determination of the orbital angular momentum transfer for these states. These results provide a significant update to the ${}^{30}\mathrm{Si}(p,\gamma ){}^{31}\mathrm{P}$ thermonuclear reaction rate, with direct implications for models of nucleosynthesis in globular clusters and other astrophysical sites. The revised rate is substantially lower than previous estimates at temperatures below 200 MK, affecting predictions for silicon isotopic abundances in stellar environments. Our work demonstrates the power of NRF in constraining nuclear properties, and provides a framework for future studies of low-energy resonances relevant to astrophysical reaction rates.

Accurately determining the size of the atomic nucleus with realistic nuclear forces is a long-outstanding issue of nuclear physics. The no-core shell model (NCSM), one of the powerful ab initio methods for nuclear structure, can achieve accurate energies of light nuclei. The extraction of converged radii is more difficult. In this work, we present a novel method to effectively extract the radius of light nuclei by restoring the long-range behavior of densities from NCSM calculations. The correct large distance asymptotics of two-body relative densities are deduced based on the NCSM densities in limited basis size. The resulting radii using the corrected densities show a nice convergence. The root-mean-square matter and charge radii of ${}^{4,6,8}\mathrm{He}$ and ${}^{6,7,8}\mathrm{Li}$ can be accurately obtained based on Jacobi-NCSM calculations with the high-precision chiral two-nucleon and three-nucleon forces combined with this new method. Our method can be straightforwardly extended to other ab initio calculations, potentially providing a better description of nuclear sizes with realistic nuclear forces.

The nuclear lattice effective field theory (NLEFT) is an efficient tool for solving nuclear many-body problems, which takes high-fidelity lattice chiral interactions as input and computes nuclear low-energy observables via quantum Monte Carlo techniques. In this work, we present the first next-to-next-to-next-to-leading order chiral forces on the lattice with the isospin-breaking effects fully taken into account. We focus on both the charge-independence breaking and charge-symmetry breaking effects. Specifically, we include the isospin-breaking effect from the mass difference between the charged and neutral pions in the one-pion-exchange potential, the Coulomb force for the $pp$ interaction, and the contribution of two additional charge-dependent contact operators. We also explicitly incorporate the two-pion-exchange potential, which was mostly neglected in previous NLEFT calculations. With these improvements, we are able to accurately reproduce the $np$ and $pp$ scattering phase shifts up to relative momentum $p\approx 200$ MeV as well as the deuteron properties. The construction of these charge-dependent lattice nuclear forces establishes a solid foundation for future high-precision nuclear ab initio calculations within the NLEFT framework.

We investigate the effect of temperature on $\alpha$-particle clustering in the diluted nucleus ${}^{16}\mathrm{O}$ using the multi-constrained finite-temperature relativistic Hartree-Bogoliubov model with the DD-ME2 interaction. At a critical density, the nucleus undergoes a Mott-like transition from a homogeneous to a localized configuration characterized by $\alpha$-particle clustering and the emergence of a finite nonaxial octupole deformation. We study the interplay between the onset of localization under nuclear dilution and the suppression of deformation and $\alpha$-particle clustering due to increasing temperature. Investigating the temperature-density plane, our findings indicate that temperature delays the formation of nonaxial octupole deformation and $\alpha$-particle clustering in dilute environments. Following the transition from homogeneous to clustered configurations, the nonaxial octupole deformation continues to increase with further dilution of the system and becomes nearly independent of temperature. We found that $\alpha$-particle clusters appear at temperatures up to $T=4.10$ MeV and at a corresponding normalized density ${\rho }_{\text{Mott}}/{\rho }_0\approx 0.09$.

Pioneering electroweak measurements of the neutron skin thickness in lead-208 and calcium-48 are challenging our understanding of nuclear dynamics. Many theoretical models suggest that the slope of the symmetry energy controls the development of a neutron skin in neutron-rich nuclei. This led to the expectation that if lead-208 exhibits a large neutron skin, calcium-48 should as well. Given that the PREX Collaboration reported a relatively thick neutron skin in lead, we anticipated that calcium would also have a significant neutron skin. Instead, the CREX Collaboration reported a thin neutron skin in calcium. Although many suggestions have been proposed, the “CREX-PREX dilemma” remains unsolved. Recently, an intriguing scenario has emerged, suggesting that an enhanced isovector spin-orbit interaction could simultaneously account for both results. Following this approach, we performed relativistic mean-field calculations with an increased isovector spin-orbit potential. Our findings indicate that, while this modification significantly affects the structure of calcium-48, it has only a marginal impact on lead-208, thereby bringing the results into better agreement with experiment. However, the strong enhancement required to mitigate the CREX-PREX dilemma destroys the agreement with a successful spin-orbit phenomenology, primarily by modifying the well-known ordering of spin-orbit partners.

Background: A variety of phases in the inner crust of neutron stars are crucial for understanding the pulsar phenomena. However, the three-dimensional coordinate-space calculation of the phases is computationally demanding.

Purpose: We aim to generalize the Fermi operator expansion (FOE) method that is effective for finite-temperature coordinate-space simulation, from the Hartree-Fock theory to Hartree-Fock-Bogoliubov (HFB) theory including the pairing effects. Furthermore, the periodic structure with free neutrons in the inner crust requires us to treat the system with the band theory.

Method: We give a concise proof that the generalized density matrix in the HFB theory can be obtained with the FOE. The Chebyshev polynomial expansion is used for calculations of the HFB band theory.

Results: Using a model for a slab phase of the inner crust, the FOE method produces results in good agreement with those based on the diagonalization of the HFB Hamiltonian.

Conclusions: The FOE method for the HFB band theory is a powerful tool for studying the nontrivial exotic structures in neutron stars. The FOE method is suitable for parallelization and further acceleration is possible with nearsightedness.

The Ichimura-Austern-Vincent (IAV) model provides a powerful theoretical framework for describing inclusive breakup reactions. However, its post-form representation presents significant numerical challenges due to the absence of a natural cutoff in the transition matrix integration. This work presents a systematic assessment of convergence methods for post-form IAV calculations, comparing the bin method and the Vincent-Fortune approach. I demonstrate that while the bin method offers implementation simplicity, it exhibits strong parameter dependence that compromises numerical stability. In contrast, the Vincent-Fortune method, which employs complex contour integration, achieves reliable convergence without arbitrary parameters. I further introduce a novel hybrid approach that integrates finite-range distorted wave Born approximation (DWBA) with the Vincent-Fortune technique, combining the accuracy of finite-range treatment at short distances with the numerical stability of zero-range approximations in the asymptotic region. Numerical results for deuteron and ${}^6\mathrm{Li}$-induced reactions confirm the efficacy of this hybrid method, achieving reliable outcomes while resolving the convergence problems associated with the bin approach. This advancement enables more reliable calculations of nonelastic breakup cross sections and facilitates the extension of the IAV formalism beyond DWBA to incorporate continuum-discretized coupled-channels wave functions for a more comprehensive treatment of breakup processes.

Measurements of the $5p{}^{2}P_{3/2}\to 9s{}^{2}S_{1/2}$ and $5p{}^{2}P_{1/2}\to 8s{}^{2}S_{1/2}$ transitions in the indium atom, combined with new atomic physics calculations, were used to extract the changes in mean-square nuclear charge radii, $\delta \langle r^2\rangle$, of the indium ($Z=49$) isotopes ${}^{101–111}\mathrm{In}, {}^{113–123}\mathrm{In}$, and ${}^{125–131}\mathrm{In}$. With a proton hole in the closed nuclear shell of $Z=50$, indium provides a detailed study of the effect of unpaired nucleons adjacent to the proton-shell closure, allowing investigation into the charge radii for isotopes between the two major neutron-shell closures at $N=50$ and $N=82$. A study of the variations in charge radii between neighboring isotopes with neutron number (the ‘odd-even staggering') is presented and provides further insight and challenges for the theoretical description of the size of proton-hole nuclei. Two nuclear theories, density functional theory and the valence-space in-medium similarity renormalization group method, were employed to interpret the data. The new information obtained in this work provides valuable insights into the successes and shortcomings of the theoretical approaches employed.

Double deeply virtual Compton scattering (DDVCS) is a promising channel for generalized parton distribution (GPD) studies as it is a generalization of the deeply virtual Compton-scattering (DVCS) and timelike Compton-scattering (TCS) processes. In contrast to DVCS and TCS, the GPD phase space accessed through DDVCS is not constrained by on-shell conditions on the incoming and outgoing photons, thus allowing unrestricted GPD extraction from experimental observables. Considering polarized electron and positron beams directed to a polarized proton target, we study the sensitivity of the DDVCS cross-section asymmetries to the chiral-even proton GPDs from different model predictions. The feasibility of such measurements is further investigated in the context of the CLAS and SoLID spectrometers at the Thomas Jefferson National Accelerator Facility and the future Electron-Ion Collider at the Brookhaven National Laboratory.

Experiments are described, and results are provided, for the duplication of the first-ever triton-deuterium (colloquially referred to as DT) fusion experiment accidentally performed by A. J. Ruhlig in 1938, but forgotten in the published scientific literature. We find that Ruhlig overestimated the ratio of the triton-deuterium over deuteron-deuterium neutron yields in his secondary reaction (reaction-in-flight) experiment compared to modern theoretical calculations and our duplication of his experiment using modern neutron detection methods. Nevertheless, Ruhlig's observation provided the motivation for the application of DT fusion after World War II and its more recent importance in peaceful energy production at DT fusion facilities around the world. Additionally, the experimental technique used in the present work provides a novel approach for checking on low-energy triton stopping powers in deuterium containing compounds.