This work presents a Bayesian inference study for relativistic heavy-ion collisions in the beam energy scan program at the BNL Relativistic Heavy-Ion Collider. The theoretical model simulates event-by-event $(3+1)$-dimensional $\left[(3+1)\text{D}\right]$ collision dynamics using hydrodynamics and hadronic transport theory. We analyze the model's 20-dimensional posterior distributions obtained using three model emulators with different accuracy and demonstrate the essential role of training an accurate model emulator in the Bayesian analysis. Our analysis provides robust constraints on the quark-gluon plasma's transport properties and various aspects of $(3+1)\text{D}$ relativistic nuclear dynamics. By running full model simulations with 100 parameter sets sampled from the posterior distribution, we make predictions for $p_{\mathrm{T}}$-differential observables and estimate their systematic theory uncertainty. A sensitivity analysis is performed to elucidate how individual experimental observables respond to different model parameters, providing useful physics insights into the phenomenological model for heavy-ion collisions.

Nuclear excitation by electron capture (NEEC) is a theoretically envisioned yet experimentally unverified process that intricately links nuclear and electronic degrees of freedom. For nearly five decades, the experimental validation of NEEC has remained elusive, primarily hindered by challenges in excitation and detection methodologies. This study proposes a laser-based strategy for verifying NEEC, exploiting the isomeric excitation of the ${}^{235}\mathrm{U}$ nucleus within a laser-cluster interaction scheme. With proper laser parameters, NEEC overwhelmingly dominates, constituting over 99.9% of the isomeric excitation yield. The long lifetime of the ${}^{235}\mathrm{U}$ isomer allows detection without interfering atomic background. Additionally, distinctive dependencies of isomeric excitation yield on laser parameters provide further confirmation for the NEEC mechanism.

We investigate the dynamics of dilute systems composed of nucleons and light clusters within a linear response approach, taking into account the in-medium Mott effects on cluster appearance, through a density-dependent momentum cut-off. We find that spinodal instabilities and associated growth rates are severely affected by the presence of light clusters and, in particular, by the treatment of in-medium effects, foreshadowing intriguing consequences for fragment formation in heavy-ion collisions and in the broader astrophysical context.

We study dark matter, assumed to be composed of weak interacting massive particles (WIMPs), scattering off ${}^2\mathrm{H}$ and ${}^4\mathrm{He}$ nuclei. In order to parametrize the WIMP-nucleon interaction, the chiral effective field theory approach is used. Considering only interactions invariant under parity, charge conjugation, and time reversal, we examine five interaction types: scalar, pseudoscalar, vector, axial, and tensor. Scattering amplitudes between two nucleons and a WIMP are determined up to second order of chiral perturbation theory. We apply this program to calculate the interaction rate as a function of the WIMP mass and of the magnitude of the WIMP-quark coupling constants. From our study, we conclude that the scalar nuclear response functions is much greater than the others due to their large combination of low energy constants. We verify that the leading order contributions are dominant in these low energy processes. We also provide an estimate for the background due to atmospheric neutrinos.

We report on the observation of the previously unknown isotope ${}^{21}\mathrm{Al}$, the first unbound aluminum isotope located beyond the proton dripline. The ${}^{21}\mathrm{Al}$ nucleus decays by one-proton ($1p$) emission, and its in-flight decays were detected by tracking trajectories of the decay products with silicon microstrip detectors. The $1p$-emission processes were studied by analyzing the measured angular correlations of decay products ${}^{20}\mathrm{Mg}+p$. The $1p$-decay energies of the ground and low-lying excited states of ${}^{21}\mathrm{Al}$, its mass excess, and the proton separation energy value $S_p=-1.{15}_{-0.07}^{+0.10}$ MeV were determined.

The possibility that nuclear matter might be quarkyonic is considered. Quarkyonic matter is high baryon density matter that is confined but can be approximately thought of as a filled Fermi sea of quarks surrounded by a shell of nucleons. Here, nuclear matter is described by the IdylliQ sigma model for quarkyonic matter, generalizing the noninteracting IdylliQ model [Y. Fujimoto et al., Phys. Rev. Lett. 132, 112701 (2024)] to include interactions with a $\sigma$ meson and a pion. When such interactions are included, we find that isospin-symmetric nuclear matter binds with acceptable values of the compressibility and other parameters for nuclear matter at saturation. The energy per nucleon and sound velocity of such matter is computed, and the isospin dependence is determined. Nuclear matter is formed at a density close to but slightly above the density at which quarkyonic matter forms. Quarkyonic matter predicts a strong depletion of nucleons in normal nuclear matter at low momentum. Such a depletion for nucleon momenta $k\lesssim 120$ MeV is shown to be consistent with electron scattering data.

The spectral $\gamma$ ray emission from the reaction ${}^3\mathrm{H}({}^2\mathrm{H},\gamma ){}^5\mathrm{He}$ has been measured for the first time in a magnetic confinement deuterium-tritium plasma experiment at the Joint European Torus. A custom developed gamma ray spectrometer system based on a $\mathrm{La}{\mathrm{Br}}_3$ scintillator combined to a LiH neutron attenuator and a zero dead time fast digital data acquisition allowed to measure the weak $\gamma$ ray emission under the $\approx {10}^5$ more intense $14\mathrm{MeV}$ neutron field. The $R$-matrix analysis of the ${}^5\mathrm{He}$ nucleus has been used to predict the expected gamma ray spectrum which has been compared with the measurement, but cannot predict the relative intensity of the $\gamma$ lines. The data analysis has identified the energy and width of the known 16.75 MeV $\gamma$ ray emission (${\gamma }_0$), from the second excited state to the ground state of the formed ${}^5\mathrm{He}$ nucleus, and confirmed the presence of a second emission (${\gamma }_1$) at $\approx 14\mathrm{MeV}$ due to the transition from the second to the first excited state. The analysis has shown that the ${\gamma }_1$ emission is broad and has assessed for the first time in a magnetic confinement experiment the relative yield ${\gamma }_1$ to ${\gamma }_0$ equal to $1.09\pm 0.25$.

For light nuclei, ab initio many-body methods such as the no-core shell model are the tools of choice for predictive, high-precision nuclear structure calculations. The applicability and the level of precision of these methods, however, is limited by the model-space truncation that has to be employed to make such computations feasible. We present a universal framework based on artificial neural networks to predict the value of observables for an infinite model-space size based on finite-size no-core shell model data. Expanding upon our previous ansatz of training the neural networks to recognize the observable-specific convergence pattern with data from few-body nuclei, we improve the results obtained for ground-state energies and show a way to handle excitation energies within this framework. Furthermore, we extend the framework to the prediction of converged root-mean-square radii, which are more difficult due to the much less constrained convergence behavior. For all observables robust and statistically significant uncertainties are extracted via the sampling over a large number of network realizations and evaluation data samples.

Nuclei around $N\approx Z\approx 40$ are strongly deformed and exhibit coexistence of shapes. These phenomena have challenged nuclear models. Here, we perform ab initio coupled-cluster computations of low-lying collective states and electromagnetic quadrupole transitions of the even-even nuclei ${}^{72}\mathrm{Kr}, {}^{76,78}\mathrm{Sr}, {}^{78,80}\mathrm{Zr}$, and ${}^{84}\mathrm{Mo}$ starting from chiral nucleon-nucleon and three-nucleon forces. Our calculations reproduce the coexistence of oblate and prolate shapes in these nuclei, yield rotational bands and strong electromagnetic transitions, but are not accurate for some observables and nuclei. These results highlight the advances and challenges of ab initio computations of heavy deformed nuclei.

Observations of anomalies in the electron-positron angular correlations in high-energy decays in ${}^4\mathrm{He}, {}^8\mathrm{Be}$, and ${}^{12}\mathrm{C}$ 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 $\approx 17\mathrm{MeV}$. 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 ${}^7\mathrm{Li}$. 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 ${}^8\mathrm{Be}$, the $p+{}^7\mathrm{Li}$ elastic scattering, the ${}^7\mathrm{Li}(p,\gamma ){}^8\mathrm{Be}$ cross section, and the internal pair creation ${}^7\mathrm{Li}(p,e^{+}{e}^{-}){}^8\mathrm{Be}$. 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 ${}^7\mathrm{Li}(p,X){}^8\mathrm{Be}$ cross sections for several proposed models of the hypothetical X17 particle.

Since He et al. [J. J. He et al., Phys. Lett. B 725, 287 (2013)] postulated the existence of a new, low-lying, broad resonance in the ${}^6\mathrm{Li}(p,\gamma ){}^7\mathrm{Be}$ reaction, there have been several studies that have looked to further investigate this assertion. However, none of these approaches has utilized the stringent constraint imposed by the unitarity of the scattering matrix. That is, the corresponding compound nucleus state must produce a corresponding resonance in ${}^3\mathrm{He}(\alpha ,\alpha ){}^3\mathrm{He}$ and ${}^6\mathrm{Li}(p,p){}^7\mathrm{Li}$ scattering and the ${}^6\mathrm{Li}(p,\alpha ){}^3\mathrm{He}$ reaction. Since precision data are readily available in the literature, the level of consistency can be tested using a phenomenological $R$-matrix analysis. In addition, remaining discrepancies with the description of the ${}^6\mathrm{Li}(p,\gamma ){}^7\mathrm{Be}$ reaction are discussed.

A dedicated isochronous storage ring, named the Rare-RI Ring, was constructed at the RI Beam Factory of RIKEN, aiming at precision mass measurements of nuclei located in uncharted territories of the nuclear chart. The Rare-RI Ring employs the isochronous mass spectrometry technique with the goal to achieve a relative mass precision of ${10}^{-6}$ within a measurement time of less than 1 ms. The performance of the facility was demonstrated through mass measurements of neutron-rich nuclei with well-known masses. Velocity or magnetic rigidity is measured for every particle prior to its injection into the ring, wherein its revolution time is accurately determined. The latter quantity is used to determine the mass of the particle, while the former one is needed for nonisochronicity corrections. Mass precisions on the order of ${10}^{-5}$ were achieved in the first commissioning, which demonstrates that the Rare-RI Ring is a powerful tool for mass spectrometry of short-lived nuclei.

The interplay between the ${}^{22}\mathrm{Ne}(\alpha ,\gamma ){}^{26}\mathrm{Mg}$ reaction and the competing ${}^{22}\mathrm{Ne}(\alpha ,n){}^{25}\mathrm{Mg}$ reaction determines the efficiency of the latter as a neutron source at the temperatures of stellar helium burning. In both cases, the rates are dominated by the $\alpha$-cluster resonance at 830 keV. This resonance plays a particularly important role in determining the strength of the neutron flux for both the weak and main $s$ process as well as the $n$ process. Recent experimental studies based on transfer reactions suggest that the neutron and $\gamma$-ray strengths for this resonance are approximately equal. In this study, the ${}^{22}\mathrm{Ne}(\alpha ,n){}^{25}\mathrm{Mg}$ resonance strength has been remeasured and found to be similar to the previous direct studies. This reinforces an 830 keV resonance strength that is approximately a factor of 3 larger for the ${}^{22}\mathrm{Ne}(\alpha ,n){}^{25}\mathrm{Mg}$ reaction than for the ${}^{22}\mathrm{Ne}(\alpha ,\gamma ){}^{26}\mathrm{Mg}$ reaction.

The exploration of nuclear mass or binding energy, a fundamental property of atomic nuclei, remains at the forefront of nuclear physics research due to limitations in experimental studies and uncertainties in model calculations, particularly when moving away from the stability line. In this work, we employ two machine learning (ML) models, support vector regression (SVR) and Gaussian process regression (GPR), to assess their performance in predicting nuclear mass excesses using available experimental data and a physics-based feature space. We also examine the extrapolation capabilities of these models using newly measured nuclei from AME2020 and by extending our calculations beyond the training and test set regions. Our results indicate that both SVR and GPR models perform quite well within the training and test regions when informed with a physics-based feature space. Furthermore, these ML models demonstrate the ability to make reasonable predictions away from the available experimental data, offering results comparable to the model calculations. Through further refinement, these models can be used as reliable and efficient ML tools for studying nuclear properties in the future.

A global Bayesian analysis of relativistic $\mathrm{Pb}+\mathrm{Pb}$ collisions at $\sqrt{s_{NN}}=2.76\mathrm{TeV}$ is performed, using a multistage model consisting of an ip-glasma initial state, a viscous fluid dynamical evolution, and a hadronic transport final state. The observables considered are from the soft sector hadronic final state. Posterior and maximum a posteriori parameter distributions that pertain to the ip-glasma and hydrodynamic phases are obtained, including the shear and bulk specific viscosity of strong interacting matter. The first use of inference with transfer learning in heavy-ion analyses is presented, together with Bayes model averaging.

We sample the posterior probability distributions of the low-energy constants (LECs) in $\mathrm{\Delta }$-full chiral effective field theory $\left(\chi \mathrm{EFT}\right)$ up to third order. We use eigenvector continuation for fast and accurate emulation of the likelihood and Hamiltonian Monte Carlo to draw effectively independent samples from the posteriors. Our Bayesian inference is conditioned on the Granada database of neutron-proton ($np$) cross sections and polarizations. We use priors grounded in $\chi \mathrm{EFT}$ assumptions and a Roy-Steiner analysis of pion-nucleon scattering data. We model correlated EFT truncation errors using a two-feature Gaussian process, and find correlation lengths for $np$ scattering energies and angles in the ranges 45–83 MeV and 24–39 degrees, respectively. These correlations yield a nondiagonal covariance matrix and reduce the number of independent scattering data with factors of 8 and 4 at the second and third chiral orders, respectively. The relatively small difference between the second- and third-order predictions in $\mathrm{\Delta }$-full $\chi \mathrm{EFT}$ suppresses the marginal variance of the truncation error and the effects of its correlation structure. Our results are particularly important for analyzing the predictive capabilities in ab initio nuclear theory.

From the observation of both heavy neutron stars and light ones with small radii, one anticipates a steep rise in the speed of sound of nuclear matter as a function of baryon density up to values close to the causal limit. A question follows whether such behavior of the speed of sound in neutron-rich matter is compatible with the equation of state extracted from low-energy heavy-ion collisions. In this work, we consider a family of neutron-star equations of state characterized by a steep rise in the speed of sound, and use the symmetry energy expansion to obtain equations of state applicable to the almost-symmetric nuclear matter created in heavy-ion collisions. We then compare collective flow data from low-energy heavy-ion experiments with results of simulations obtained using the hadronic transport code smash with the mean-field potential reproducing the density-dependence of the speed of sound. We show that equations of state featuring a peak in the speed of sound squared occurring at densities between $2\text{–}3$ times the saturation density of normal nuclear matter, producing neutron stars of nearly $M_{\text{max}}\approx 2.5M_{\odot }$, are consistent with heavy-ion collision data.

Background: The inner crust of neutron stars consists of a Coulomb lattice of neutron-rich nuclei, immersed in a sea of superfluid neutrons with background relativistic electron gas. A proper quantum-mechanical treatment for such a system under a periodic potential is the band theory of solids. The effect of band structure on the effective mass of dripped neutrons, the so-called entrainment effect, is currently in a debatable situation, and it has been highly desired to develop a microscopic nuclear band theory taking into account neutron superfluidity in a fully self-consistent manner.

Purpose: The main purpose of the present work is twofold: (1) to develop a formalism of the time-dependent self-consistent band theory, taking fully into account nuclear superfluidity, based on time-dependent density-functional theory (TDDFT) extended for superfluid systems, and (2) to quantify the effects of band structure and superfluidity on the entrainment phenomenon, applying the formalism to the slab phase of the inner crust of neutron stars.

Methods: The fully self-consistent time-dependent band theory, proposed in a previous work [Phys. Rev. C 105, 045807 (2022)], is extended to superfluid systems. To this end, a superfluid TDDFT with a local treatment of pairing, known as time-dependent superfluid local density approximation, is formulated in the coordinate space with a Skyrme-type energy density functional, adopting the Bloch's boundary condition. A real-time method is employed to extract the collective masses of a slab and of protons, which in turn quantify the conduction neutron number density and the neutron effective mass, i.e., the entrainment effect.

Results: Static calculations have been performed for a range of baryon number density ($n_{\text{b}}=0.04\text{–}0.07{\mathrm{fm}}^{-3}$) under the $\beta$-equilibrium condition with and without superfluidity, for various interslab spacings. From the results, we find that the system gains energy through the formation of Cooper pairs for all densities examined, which supports the existence of superfluidity in the inner crust of cold neutron stars. From a response of the system to an external potential, we dynamically extract the collective masses of a slab and of protons immersed in neutron superfluid. The obtained results show the collective mass of a slab is substantially reduced by 57.5%–82.5% for $n_{\text{b}}=0.04\text{–}0.07{\mathrm{fm}}^{-3}$, which corresponds to an enhancement of conduction neutron number density and, thus, to a reduction of the neutron effective mass, which we call the anti-entrainment effect. A comparison of the results with and without superfluidity reveals that superfluidity slightly enhances the anti-entrainment effects for the slab phase of neutron-star matter. We discuss a novel phenomenon associated with superfluidity, that is, quasiparticle resonances in the inner crust, which are absent in normal systems.

Conclusions: Our fully self-consistent, microscopic, superfluid band theory calculations based on (TD)DFT showed that the effective mass of dripped neutrons is reduced by about 20%–40% for $n_{\text{b}}=0.04\text{–}0.07{\mathrm{fm}}^{-3}$ because of the band-structure effects, and superfluidity slightly $\mathit{enhances}$ the reduction.

The presence of ${}^{240}\mathrm{Pu}$ in nuclear fuels for reactors has resulted in high uncertainties in the results of reactor and nuclear transmutation calculations because of deficiencies in ${}^{240}\mathrm{Pu}$-related nuclear data. Specifically for the prompt fission neutron spectrum (PFNS) of ${}^{240}\mathrm{Pu}$, there is only one neutron-induced, ($n,f$), measurement at 0.85 MeV incident neutron energy and only one complete spontaneous fission, (sf), measurement. This limited availability of data does not sufficiently guide nuclear data evaluations of these quantities. Here, we report on a measurement of both the ${}^{240}\mathrm{Pu}$(sf) and the ${}^{240}\mathrm{Pu}(n,f$) PFNS, both over the emitted neutron energy range of 0.79–10.0 MeV, and from incident neutron energies of 1.0–20.0 MeV for the ($n,f$) reaction. Measurements were made with a hemispherical array of liquid scintillators at the high-energy Los Alamos Neutron Science Center white neutron source at the Weapons Neutron Research facility as part of the joint LANL-LLNL Chi-Nu experimental campaign to measure actinide fission neutron spectra. These measurements are the first of their kind, and provide clear experimental evidence for second-chance fission, third-chance fission, and pre-equilibrium neutron emission processes in neutron-induced fission of ${}^{240}\mathrm{Pu}$, and are the first ever measurements above 1 MeV incident neutron energy.

A relativistic microscopic optical model potential for nucleon-nucleus scattering is developed based on the ab initio relativistic Brueckner-Hartree-Fock (RBHF) theory with the improved local density approximation, which is abbreviated as the RBOM potential. Both real and imaginary parts of the single-particle potentials in symmetric and asymmetric nuclear matter at various densities are determined uniquely in the full Dirac space. The density distributions of the target nuclei are calculated by the covariant energy density functional theory with the density functional PC-PK1. The central and spin-orbit terms of the optical potentials are quantitatively consistent with the relativistic phenomenological optical potentials. The performance of the RBOM potential is evaluated by considering proton scattering with incident energy $E\le 200$ MeV on five target nuclei, ${}^{208}\mathrm{Pb}, {}^{120}\mathrm{Sn}, {}^{90}\mathrm{Zr}, {}^{48}\mathrm{Ca}$, and ${}^{40}\mathrm{Ca}$. Scattering observables including the elastic scattering angular distributions, analyzing powers, spin rotation functions, and reaction cross sections are analyzed. Theoretical predictions show good agreements with the experimental data and the results derived from phenomenological optical potentials. We anticipate that the RBOM potential can provide reference for other phenomenological and microscopic optical model potentials, as well as reliable descriptions for nucleon scattering on exotic nuclei in the era of rare-isotope beams.