Background: Large-scale computations of fission properties are an important ingredient for nuclear reaction network calculations simulating rapid neutron-capture process (the $r$ process) nucleosynthesis. Due to the large number of fissioning nuclei potentially contributing to the $r$ process, a microscopic description of fission based on nuclear density functional theory (DFT) is computationally challenging.

Purpose: We explore the use of neural networks (NNs) to construct DFT emulators capable of predicting potential energy surfaces and collective inertia tensors across the whole nuclear chart, starting from a minimal set of DFT calculations.

Methods: We use constrained Hartree-Fock-Bogoliubov (HFB) calculations to predict the potential energy and collective inertia tensor in the axial quadrupole and octupole collective coordinates, for a set of nuclei in the $r$-process region. We then employ NNs to emulate the HFB energy and collective inertia tensor across the considered region of the nuclear chart. Least-action pathways characterizing spontaneous fission half-lives and fragment yields are then obtained by means of the nudged elastic band method.

Results: The potential energy predicted by NNs agrees with the DFT value to within a root-mean-square error of 500 keV, and the collective inertia components agree to within an order of magnitude. These results are largely independent of the NN architecture. The exit points on the outer turning line are found to be well emulated. For the spontaneous fission half-lives the NN emulation provides values that are found to agree with the DFT predictions within a factor of ${10}^3$ across more than 70 orders of magnitude.

Conclusions: Neural networks are able to emulate the potential energy and collective inertia well enough to reasonably predict physical observables. Future directions of study, such as the inclusion of additional collective degrees of freedom and active learning, will improve the predictive power of microscopic theory and further enable large-scale fission studies.

Background: When multiple fission modes coexist in a given nucleus, distinct fragment yield distributions appear. Multimodal fission has been observed in a number of fissioning nuclei spanning the nuclear chart, and this phenomenon is expected to affect the nuclear abundances synthesized during the rapid neutron-capture process ($r$-process).

Purpose: In this study, we generalize the previously proposed hybrid model for fission-fragment yield distributions to predict competing fission modes and estimate the resulting yield distributions. Our framework allows for a comprehensive large-scale calculation of fission-fragment yields suited for $r$-process nuclear network studies.

Methods: Nuclear density functional theory is employed to obtain the potential energy and collective inertia tensor on a multidimensional collective space defined by mass multipole moments. Fission pathways and their relative probabilities are determined using the nudged elastic band method. Based on this information, mass and charge fission yields are predicted using the recently developed hybrid model.

Results: Fission properties of fermium isotopes are calculated in the axial quadrupole-octupole collective space for three energy density functionals (EDFs). Disagreement between the EDFs appears when multiple fission modes are present. Within our framework, the ${\mathrm{UNEDF}1}_{\text{HFB}}$ EDF agrees best with experimental data. Calculations in the axial quadrupole-octupole-hexadecapole collective space improve the agreement with the experiment for ${\mathrm{SkM}}^{*}$. We also discuss the sensitivity of fission predictions on the choice of EDF for several superheavy nuclei.

Conclusions: Fission-fragment yield predictions for nuclei with multiple fission modes are sensitive to the underlying EDF. For large-scale calculations in which a minimal number of collective coordinates is considered, ${\mathrm{UNEDF}1}_{\text{HFB}}$ provides the best description of experimental data, though the sensitivity motivates robust quantification of the uncertainties of the theoretical model.

An extensive dataset of cumulative fission product yields has been generated under a joint collaboration between Los Alamos National Laboratory (LANL), Lawrence Livermore National Laboratory (LLNL), and the Triangle Universities Nuclear Laboratory (TUNL). The energy dependence of the cumulative yield for a select number of high-yield fission products has been measured using quasimonoenergetic neutrons with energies between 5.5 and 11.0 MeV. This is in addition to previously published data covering 0.5–4.5 and 14.8 MeV. The absolute number of fissions was determined during the irradiation period using dual-fission ionization chambers, and the fission products were measured postactivation by whole target $\gamma$-ray spectroscopy. This paper presents the absolute cumulative fission product yields as a function of incident neutron energy from the neutron-induced fission of ${}^{235}\mathrm{U}, {}^{238}\mathrm{U}$, and ${}^{239}\mathrm{Pu}$ isotopes at four incident energies in the second-chance fission region and compares them with existing literature values. Corrections relevant to this collaboration's previously published data are also discussed.

We consider quantum tunneling between fusing nuclei in the presence of a harmonically time-varying field. The tunneling rate is calculated using Floquet/Volkov (FV) and Kramers-Henneberger (KH) approaches, which are both compared to a first-principles numerical solution of the Crank-Nicolson (CN) type. Numerical validation of the FV approach justifies its use in analytical estimates of laser-enhanced reaction rates, while the KH approach does not reproduce the predictions of the others. For a deuterium-tritium plasma at a temperature of 1 keV, it is found that significant enhancement of fusion reactivity requires field strengths on the order of ${10}^{15}$–${10}^{16}$ V/m and photon energies below 1 keV, which is within reach of next-generation x-ray free electron lasers (XFELs).

We measured the spin-dependent cross section at a neutron-induced $p$-wave resonance using a polarized epithermal neutron beam and a polarized nuclear target. Our study focuses on the 0.75 eV $p$-wave resonance of ${}^{139}\mathrm{La}+n$, where previous experiments have shown a large enhancement of parity violation. The spin dependence of the neutron absorption cross section of polarized ${}^{139}\mathrm{La}$ and polarized neutrons determines the partial neutron width of the $p$-wave resonance. This partial width is a key parameter needed to determine the sensitivity of this system to possible parity-odd and time reversal-odd interactions beyond the standard model. Our findings therefore serve as a foundation for a future search for time reversal violation in this compound nuclear system.

In this work, we have phenomenologically reexamined the $\varepsilon$ dependence of the proton's recoil-polarization ratio $R={\mu }_p{G}_E/G_M$ for possible two-photon-exchange (TPE) corrections beyond the one-photon-exchange (OPE) or Born approximation. High-precision Rosenbluth measurements of ${\sigma }_R(\varepsilon ,Q^2)$ taken at $Q^2=2.64$, 3.20, and $4.10{\mathrm{GeV}}^2$ were used to extract the TPE coefficients needed to provide an estimate of the size of the TPE corrections to the ratio $R$, and construct the two TPE $Y_{(M,E)}(\varepsilon ,Q^2)$ amplitudes. Our results suggest that the ratio $R$ shows very small (negligible) enhancement with $\varepsilon$, less than 0.40% in magnitude, with values consistent with the OPE prediction for the $Q^2$ range studied. We also compare our results to previous phenomenological extractions and several theoretical calculations.

The mass of neutron-deficient nuclide ${}^{58}\mathrm{Zn}$ has been directly measured by using $B\rho$-defined isochronous mass spectrometry, resulting in a more precise proton separation energy of $S_p\left({}^{58}\mathrm{Zn}\right)=2227\left(36\right)$ keV. With this new $S_p$ value, the thermonuclear rate of the ${}^{57}\mathrm{Cu}(p,\gamma ){}^{58}\mathrm{Zn}$ reaction has been re-evaluated to be higher than the most recently published rate by a factor of up to 3 in the temperature range of 0.2 GK $\lesssim T\lesssim$ 1.5 GK. The new rate is used to investigate its astrophysical impact via one-zone post-processing type-I x-ray burst calculations. It shows that the updated rate and new $S_p\left({}^{58}\mathrm{Zn}\right)$ value result in noticeable abundance variations for nuclei with $A=56$–59 and a reduction in $A=57$ abundance by up to $20.7\%$, compared with the results using the recently published rate.

We present a many-channel microscopic model that extends the three-cluster model previously formulated by Vasilevsky et al. [Nucl. Phys. A 824, 37 (2009)]. This extended model incorporates multiple three-cluster configurations, which are subsequently reduced to a comprehensive set of binary channels. These channels dictate the dynamics of various nuclear processes and the resonance structure of a compound nucleus across a broad energy spectrum. The application of this model focuses on investigating the nature of high-energy resonance states in ${}^9\mathrm{Be}$ and ${}^9\mathrm{B}$, as well as the astrophysical $S$ factors for the reactions ${}^7\mathrm{Li}(d,n)\alpha \alpha$ and ${}^7\mathrm{Be}(d,p)\alpha \alpha$, particularly pertinent to the cosmological lithium problem. Parametrization of resonance states is performed across a wide range of total angular momenta and includes states of both positive and negative parity. Dominant decay channels are identified for each resonance state. Detailed analysis of astrophysical $S$ factors resulting from deuteron interactions with ${}^7\mathrm{Li}$ and ${}^7\mathrm{Be}$ is conducted within an energy range from zero to 2 MeV. Four exit channels in ${}^9\mathrm{Be}$ $[{}^8\mathrm{Be}\left(0^{+}\right)+n, {}^8\mathrm{Be}\left(2^{+}\right)+n, {}^5\mathrm{He}(3/2^{-})+\alpha , {}^5\mathrm{He}(1/2^{-})+\alpha ]$ and four in ${}^9\mathrm{B}$ $[{}^8\mathrm{Be}\left(0^{+}\right)+p, {}^8\mathrm{Be}\left(2^{+}\right)+p, {}^5\mathrm{Li}(3/2^{-})+\alpha , {}^5\mathrm{Li}(1/2^{-})+\alpha ]$ are considered. A clear hierarchy of reactions is established for the energy range $0\le E<1.0$ MeV. Notably, reactions ${}^7\mathrm{Li}+d={}^8\mathrm{Be}\left(0^{+}\right)+n$ and ${}^7\mathrm{Be}+d={}^8\mathrm{Be}\left(0^{+}\right)+p$ substantially dominate over all other reactions within this energy range. The model satisfactorily describes the experimental astrophysical $S$ factors for these reactions.

The reflection asymmetric relativistic mean-field (RASRMF) theory was applied to investigate the static fission properties of ${}^{240}\mathrm{Pu}$. The potential energy curves with different spatial symmetries were obtained, and the calculated fission barrier heights are in good agreement with available experimental data. Compared with other theoretical calculations, better consistency with experiments is achieved. The matter density distributions clearly show the shape evolution along the fission path. Notably, the outer barrier is significantly reduced due to the reflection asymmetric deformations, making mass-asymmetric fission more favorable. These results indicate that the reflection asymmetric degree of freedom plays an important role in regulating the fission barrier and fission path of ${}^{240}\mathrm{Pu}$. Furthermore, the sensitivity of fission barriers to pairing correlations was examined. The use of the BCS approximation with a constant pairing gap led to unphysical results; therefore, a variable pairing strength was used to evaluate its influence on the fission barriers. It was found that a slight increase in pairing strength enhanced ground state stability but reduced fission barrier heights, promoting fission through more effective state repopulation during nuclear elongation. This conclusion remains consistent across RASRMF calculations employing both the NL3 and PK1 parameters.

We propose a theory framework to study the isospin-symmetry breaking correction ${\delta }_{\text{C}}$ in superallowed nuclear $\beta$ decays, crucial for the precise determination of $|V_{ud}|$. Based on a general assumptions of the isovector dominance in isospin-symmetry breaking interactions, we construct a set of functions $F_{T_z}$ which involve nuclear matrix elements of isovector monopole operators and the nuclear Green's function. Via the functions $F_{T_z}$, a connection of ${\delta }_{\text{C}}$ to measurable electroweak nuclear radii is established, providing an experimental gauge of the theory accuracy of ${\delta }_{\text{C}}$. We outline a strategy to perform ab initio calculations of $F_{T_z}$ based on the Lanczos algorithm, and discuss its similarity with other nuclear-structure-dependent inputs in nuclear $\beta$ decays.

High-spin states in ${}^{76}\mathrm{As}$ have been observed for the first time by using the ${}^4\mathrm{He}+{}^{74}\mathrm{Ge}$ reaction at beam energies of 58.6 and 62.6 MeV. Two positive-parity and three negative-parity bands have been found in ${}^{76}\mathrm{As}$. The two positive-parity bands with $\pi g_{9/2}\otimes \nu g_{9/2}$ configurations are tentatively interpreted as chiral doublet bands, which are supported by triaxial particle rotor model calculations. The electric dipole transitions linking the yrast positive- and negative-parity bands are also observed in this work, implying the possible presence of octupole correlations in ${}^{76}\mathrm{As}$.

A semiclassical approach combined with adiabatic approximation is applied to the particle+triaxial rotor (PTR) model in order to derive the collective potential and mass parameters of a collective Hamiltonian (CH) that depends only on the total angular momentum for the wobbling motion. The CH approximates very well the energies and $E2$ transition probabilities associated with the lowest wobbling states in both even-even and odd-mass systems. The spin squeezed state (SSS) representation with the character of a wave function in orientation angle of the total angular momentum is introduced to illustrate on the structure of the states. The probability distribution for the lowest states of the PTR and the CH agree well. This demonstrates that the method can be used to incorporate collective wobbling into the microscopic tilted axis cranking approach. A classification scheme for the wobbling mode based on the SSS states is discussed.

For the neutron-induced fission of nuclei ${}^{235,238}\mathrm{U}$, the evolution of the shape of fission-fragment neutron multiplicity distribution with increasing excitation energy is explored within the improved scission-point model. For a wide range of incident neutron energies, the dependence of an average number of neutrons emitted per a fission event on the excitation energy is studied.

New measurements of photofission and photoneutron reactions on ${}^{238}\mathrm{U}$ and ${}^{232}\mathrm{Th}$ in the giant dipole resonance (GDR) energy region were performed at the laser Compton-scattering $\gamma$-ray source of the NewSUBARU synchrotron radiation facility using a high-and-flat efficiency moderated ${}^3\mathrm{He}$ detection array. The neutron-multiplicity sorting of high-multiplicity fission neutron coincidence events was performed using a dedicated energy dependent, multiple firing statistical treatment. The photoneutron reactions $(\gamma ,in)$ with $i=1–3$ and photofission reaction $(\gamma ,F)$ were discriminated by considering a Gaussian distribution of prompt-fission neutron (PFN) multiplicities predicted by the theory of evaporation in sequential neutron emission from excited fission fragments. We report experimental $(\gamma ,n)$, $(\gamma ,2n)$, $(\gamma ,3n)$, and $(\gamma ,F)$ cross sections, average energies of PFNs and of $(\gamma ,in)$ photoneutrons, as well as the mean number of PFNs per fission and the width of the PFN multiplicity distribution. Based on these primary experimental results and combined with reasonable assumptions, we extract also the first- and second-chance fission contributions. The new experimental results are compared with statistical-model calculations performed with the empire 3.2 Malta and talys 1.964 codes on the present data and with prompt fission emission calculations obtained with the Los Alamos model in the frame of the most probable fragmentation approach with and without sequential emission.

This paper represents a continuation of our investigation into the limitations on total, proton, and neutron particle number densities, as well as the asymmetry of proton and neutron density distributions achievable in central heavy-ion collisions. We explore these aspects at low and medium energies within the framework of the Boltzmann-Uhlenbeck-Uehling (pBUU) transport and time-dependent Hartree-Fock (TDHF) models. Previous studies, focusing on symmetric and asymmetric collisions of Ca and Sn nuclei [1], and initial results on Pb-nuclei collisions [2], emphasized the role of the Coulomb interaction in these events. Our findings indicated that: (i) the highest total densities predicted at $E_{\text{beam}}=800$ MeV/nucleon were on the order of $\approx 2.5{\rho }_0$ (${\rho }_0=0.16{\text{fm}}^{-3}$), (ii) the proton-neutron asymmetry for maximal densities, $\delta =({\rho }_{\mathrm{n}}^{\max }-{\rho }_{\mathrm{p}}^{\max })/({\rho }_{\mathrm{n}}^{\max }+{\rho }_{\mathrm{p}}^{\max })$, did not generally exceed the asymmetry in the initial state of the collision at all beam energies and tended to decrease during the reaction, and (iii) a significant portion of this asymmetry had its microscopic origin in Coulomb forces, masking the pure nuclear contribution. These new findings, particularly relevant in the astrophysical context, are further examined in this work, focusing on the heaviest target-projectile combination ${}^{212,208}\mathrm{Pb}$ accessible in an experiment. We introduce the SkT3 Skyrme force model, not previously used for the Pb system, and compare it to the SV-bas and SV-sym34 models to explore the symmetry-energy dependence of the results. Contour plots of nucleonic densities are presented, contrasting the time evolution of the density distributions in low (TDHF) and high (pBUU) models. We also present the evolution of normalized maximal proton, neutron, and total nucleon number density with increasing beam energy in the full pBUU model and the Vlasov approximation, aiming to explore the impact of correlations in the reaction. The time evolution of the proton and neutron density distributions in the plane transverse to the beam direction is illustrated at both low and high beam energy. In conclusion, our detailed examination of the Pb system in this work provides further essential evidence that the aforementioned findings (i)–(iii) are only weakly dependent on system size and a symmetry-energy model, and thus, they are of more general importance.

A study of prompt ${\mathrm{\Xi }}_c^{+}$ production in proton-lead collisions is performed with the LHCb experiment at a center-of-mass energy per nucleon pair of 8.16 $\text{TeV}$ in 2016 in $p\mathrm{Pb}$ and $\mathrm{Pb}p$ collisions with an estimated integrated luminosity of approximately 12.5 and 17.4 ${\text{nb}}^{-1}$, respectively. The ${\mathrm{\Xi }}_c^{+}$ production cross section, as well as the ${\mathrm{\Xi }}_c^{+}$ to ${\mathrm{\Lambda }}_c^{+}$ production cross-section ratio, are measured as a function of the transverse momentum and rapidity and compared to the latest theory predictions. The forward-backward asymmetry is also measured as a function of the ${\mathrm{\Xi }}_c^{+}$ transverse momentum. The results provide strong constraints on theoretical calculation and are a unique input for hadronization studies in different collision systems.

We present a detailed discussion of the thermodynamics of the parity-doublet nucleon-meson model within a mean-field theory, at finite temperature and baryon-chemical potential, with special emphasis on the chiral transition at large baryon densities and vanishing temperature. We consider isospin-symmetric matter. We systematically compare the parity-doublet model to a related singlet model obtained by disregarding the chiral partner of the nucleon. After studying the ground-state properties of nuclear matter, the nuclear liquid-gas transition, and the density modifications of the nucleon sigma term which govern the low-density regime, we give new insight into the underlying mechanisms of the zero-temperature chiral transition occurring at several times the nuclear saturation density. We show that the chiral transition is driven by a kind of symmetry energy that tends to equilibrate the populations of opposite-parity baryons. This symmetry energy dictates the composition of matter at large baryon densities, once the phase space for the appearance of the negative-parity partner is opened. We furthermore highlight the characteristic role within the thermodynamics of the chiral-invariant mass of the parity-doublet model. We include the chiral limit in all of our discussions in order to provide a complete picture of the chiral transition.

We study how the nuclear symmetry energy slope ($L$) can affect the hadron-quark phase transition and neutron star properties. We show that the main physical quantities as the critical chemical potential and pressure are strongly influenced by the symmetry energy slope. In extreme cases, the total amount of deconfined quarks can reach up to $99\%$ of the hybrid star mass.

We present new maximally local two-nucleon interactions derived in $\mathrm{\Delta }$-less chiral effective field theory up to next-to-next-to-next-to-leading order (${\mathrm{N}}^3\mathrm{LO}$), which include all contact and pion-exchange contributions to the nuclear Hamiltonian up to this order. Our interactions are fit to nucleon-nucleon phase shifts using a Bayesian statistical approach, and explore a wide cutoff range from $0.6–0.9$ fm ($\approx 660–440\mathrm{MeV}$). These interactions can be straightforwardly employed in quantum Monte Carlo methods, such as the auxiliary field diffusion Monte Carlo method. Together with local three-nucleon forces, calculations with these new interactions will provide improved benchmarks for the structure of atomic nuclei and serve as crucial input to analyses of astrophysical phenomena of neutron stars, such as binary neutron-star mergers.