High-energy heavy-ion collisions have been suggested as a favorable environment for the production of $B_c$ mesons, due to a much larger abundance of charm and bottom quarks compared to elementary reactions. Motivated by recent CMS data for $B_c^{+}$ production in Pb-Pb(5.02 TeV) collisions at the Large Hadron Collider (LHC), we deploy a previously developed transport approach for charmonia and bottomonia to evaluate the kinetics of $B_c$ mesons throughout the fireball formed in these reactions. The main inputs to our approach are two transport parameters: the $B_c$'s reaction rate and equilibrium limit. Both quantities are determined by previous calculations via a combination of charm and bottom sectors. In-medium binding energies of $B_c$ mesons are calculated from a thermodynamic $T$ matrix with a lattice-QCD constrained potential, and figure in their inelastic reaction rates. Temperature-dependent equilibrium limits include charm- and bottom-quark fugacities based on their initial production. We compute the centrality dependence of inclusive $B_c$ production and transverse-momentum $\left(p_T\right)$ spectra using two different recombination models: instantaneous coalescence and resonance recombination. The main uncertainty in the resulting nuclear modification factors, $R_{\mathrm{AA}}$, is currently associated with the $B_c$ cross section in elementary $pp$ collisions, caused by the uncertainty in the branching ratio for the $B_c^{-}\to J/\psi {\mu }^{-}\overline{\nu }$ decay. Our results indicate a large enhancement of the $R_{\mathrm{AA}}$ at low $p_T$, with significant regeneration contributions up to $p_T\simeq 20\mathrm{GeV}$. Comparisons to CMS data are carried out but firm conclusions will require a more accurate value of the branching ratio, or alternative channels to measure the $B_c$ production in $pp$ collisions.

Nuclear energy density functionals successfully reproduce properties of nuclei across almost the entire nuclear chart. However, nearly all available functionals are phenomenological in nature and lack a rigorous connection to systematically improvable nuclear forces. This issue might be solved with an energy density functional obtained from first principles. As an intermediate step towards this goal we construct the GUDE family of functionals that is obtained from a hybrid scheme consisting of long-range pion-exchange contributions derived from chiral effective field theory at the Hartree-Fock level and a phenomenological Skyrme part. When including pion contributions beyond next-to-leading order in the chiral expansion, we find significant improvements over a reference Skyrme functional constructed following the same protocol. We analyze the importance of different pion contributions and identify which terms drive the observed improvements. Since pions are incorporated without adding further optimization parameters to the functionals, the improvements can be attributed to the functional form of these terms. Our work therefore suggests that the considered chiral contributions constitute useful ingredients for true ab initio energy density functionals.

We present a detailed study of the contribution from two-body currents to the one-nucleon knockout process induced by electromagnetic interaction. The framework is a relativistic mean-field model in which bound and scattering nucleons are consistently described as solutions of Dirac equation with potentials. We show results obtained with the most general expression of the two-body operator, in which the intermediate nucleons are described by relativistic mean-field bound states; then, we propose two approximations consisting in describing the intermediate states as nucleons in a relativistic Fermi gas, preserving the complexity and consistency in the initial and final states. These approximations simplify the calculations considerably, allowing us to provide outcomes in a reasonable computational time. The results obtained under these approximations are validated by comparing with those from the full model. Additionally, the theoretical predictions are compared with experimental data of the longitudinal and transverse responses of carbon 12. The agreement with data is outstanding for the longitudinal response, where the contribution from the two-body operator is negligible. In the transverse sector, the two-body current increases the response from 30 to 15 %, depending on the approximations and kinematics, in general, improving the agreement with data.

The summation method for the calculation of reactor ${\overline{\nu }}_e$ fluxes and spectra is methodically revised and improved. For the first time, a complete uncertainty budget accounting for all known effects likely to impact these calculations is proposed. Uncertainties of a few percent at low energies and ranging up to 20% at high energies are obtained on the calculation of a typical reactor ${\overline{\nu }}_e$ spectrum. Although huge improvements have been achieved over the past decade, the quality and incompleteness of the present day evaluated nuclear decay data still limit the accuracy of the calculations and therefore dominate by far these uncertainties. Pushing the $\beta$-decay modeling of the thousands of branches making a reactor ${\overline{\nu }}_e$ spectrum to a high level of details comparatively brings modest changes. In particular, including nuclear structure calculations in the evaluation of the nonunique forbidden transitions gives a smaller impact than anticipated in past studies. Finally, this new modeling is challenged against state-of-the-art predictions and measurements. While a good agreement is observed with the most recent inverse $\beta$ decay measurements of reactor ${\overline{\nu }}_e$ fluxes and spectra, it is unable to properly describe the reference aggregate $\beta$ spectra measured at the Institut Laue-Langevin High-Flux Reactor in the 1980s. This result adds to recent suspicions about the reliability of these data and preferentially points toward a misprediction of the ${}^{235}\mathrm{U}$ ${\overline{\nu }}_e$ spectrum.

High-energy backward $\left(u\text{−}\text{channel}\right)$ reactions can involve very large momentum transfers to the target baryons, shifting them by many units of rapidity. These reactions are difficult to understand in conventional models in which baryon number is carried by the valence quarks. Backward Compton scattering is an especially attractive experimental target, because of its simple final state. There is currently limited data on this process, and those data are at low center-of-mass energies. In this paper, we examine the prospects for studying backward Compton scattering at the future Electron Ion Collider (EIC). We model the cross section and kinematics using the limited data on backward Compton scattering and backward meson production, and then simulate Compton scattering at EIC energies in a simple model of the ePIC detector. Generally, the proton is scattered toward midrapidity, while the produced photon is in the far-forward region, visible in a Zero Degree Calorimeter (ZDC). We show that the background from backward ${\pi }^0$ production can be rejected using a high-resolution, well-segmented ZDC.

We employ a microscopic approach that examines the impact of isospin dynamics on the process of low-energy nuclear fusion along an isotope chain and dependence on deformation. Our method utilizes the density constrained time-dependent Hartree-Fock theory (DC-TDHF), where isoscalar and isovector characteristics of the energy density functional (EDF) are examined in turn. This approach is applied to a series of fusion interactions of ${}^{176}\mathrm{Yb}$ with increasingly neutron rich isotopes of calcium. By evaluating the contributions from the isoscalar and isovector components of the EDF, we look to quantify the influence of isospin composition on the conditions under which fusion is most likely to take place. Our findings reveal that, in nonsymmetric systems, the isovector dynamics plays a significant role. Its typical effect is a reduction in the potential barrier, which turns into enhancement for neutron-rich systems.

The fragmentation of a projectile into a number of pieces can lead to the creation of many resonances in different nuclei. We discuss application of the invariant-mass method to the products from such reactions to find some of the most exotic resonances located furthest beyond the proton drip line. We show examples from fragmentation of a fast ${}^{13}\mathrm{O}$ beam including the production of the newly identified ${}^9\mathrm{N}$ resonance. In extracting resonance parameters from invariant-mass spectra, accurate estimates of the background from nonresonant prompt protons are needed. These prompt protons are correlated with the resonances due to long-range Coulomb interactions and will suppress events when resonance decay products and the prompt protons have small invariant masses. The shape of this background is especially important in determining the widths of wide resonances typically found at the edge of the chart of nuclides. An event-mixing recipe is proposed to describe this background, where the mixed events have reduced weighting for the smaller invariant masses to account for the effect of the Coulomb final-state interactions. The weighting is based on the measured correlations of heavier hydrogen isotopes with the resonances or the projectile residues. We also show that the relative magnitude of the background can be reduced in some cases by selecting events where the resonance decay products are accompanied by a deuteron or triton cluster. The deuteron and triton clusters are largely not products of resonant decay and thus events with an accompanying cluster are associated with fewer background protons than events without them. The width of the $5/2^{-}$ state in ${}^9\mathrm{C}$ with the new background prescription was found to be consistent with the value obtained using proton elastic scattering on ${}^8\mathrm{B}$. Improved values for the widths of ${}^{10}\mathrm{C}, {}^{10,11}\mathrm{N}$, and ${}^{12}\mathrm{O}$ resonances were also obtained using the new background prescription.

In the recent Baksan Experiment on Sterile Transitions (BEST), a suppressed rate of neutrino absorption on a gallium target was observed, consistent with earlier results from neutrino source calibrations of the SAGE and GALLEX/GNO solar neutrino experiments. The BEST Collaboration, utilizing a 3.4 MCi ${}^{51}\mathrm{Cr}$ neutrino source, found observed-to-expected counting rates at two very short baselines of $R=0.791\pm 0.05$ and $0.766\pm 0.05$, respectively. Among recent neutrino experiments, BEST is notable for the simplicity of both its neutrino spectrum, line neutrinos from an electron-capture source whose intensity can be measured to a estimated precision of 0.23%, and its absorption cross section, where the precisely known rate of electron capture to the gallium ground state, ${}^{71}\mathrm{Ge}(e^{-},{\nu }_e){}^{71}\mathrm{Ga}\left(\text{g.s.}\right)$, establishes a minimum value. However, the absorption cross section uncertainty is a common systematic in the BEST, SAGE, and GALLEX/GNO neutrino source experiments. Here we update that cross section, considering a variety of electroweak corrections and the role of transitions to excited states, to establish both a central value and reasonable uncertainty, thereby enabling a more accurate assessment of the statistical significance of the gallium anomalies. Results are given for ${}^{51}\mathrm{Cr}$ and ${}^{37}\mathrm{Ar}$ sources. The revised neutrino capture rates are used in a reevaluation of the BEST and gallium anomalies.

Hexadecapole collectivity and its interplay with quadrupole degrees of freedom is studied in an axial symmetry preserving framework based on the Hartree-Fock-Bogoliubov plus generator coordinate method. Results are obtained for several even-even isotopes of Sm and Gd with various parametrizations of the Gogny force. The analysis of the results indicates the strong coupling between the quadrupole and hexadecapole degrees of freedom. The first two excited states are vibrational in character in most of the cases. The impact of prolate-oblate shape mixing in the properties of hexadecapole states is analyzed.

Over the past decade, machine learning has been successfully applied in various fields of science. In this study, we employ a deep learning method to analyze a Skyrme energy density functional (Skyrme-EDF), which is a Kohn-Sham type functional commonly used in nuclear physics. Our goal is to construct an orbital-free functional that reproduces the results of the Skyrme-EDF. To this end, we first compute energies and densities of a nucleus with the Skyrme Kohn-Sham $+$ Bardeen-Cooper-Schrieffer method by introducing a set of external fields. Those are then used as training data for deep learning to construct a functional which depends only on the density distribution. Applying this scheme to the ${}^{24}\mathrm{Mg}$ nucleus with two distinct random external fields, we successfully obtain a new functional which reproduces the binding energy of the original Skyrme-EDF with an accuracy of about 0.04 MeV. The rate at which the neural network outputs the energy for a given density is about ${10}^5–{10}^6$ times faster than the Kohn-Sham scheme, demonstrating a promising potential for applications to heavy and superheavy nuclei, including the dynamics of fission.

We report the total and differential cross sections for $J/\psi$ photoproduction with the large acceptance GlueX spectrometer for photon beam energies from the threshold at 8.2 GeV up to 11.44 GeV and over the full kinematic range of momentum transfer squared, $t$. Such coverage facilitates the extrapolation of the differential cross sections to the forward ($t=0$) point beyond the physical region. The forward cross section is used by many theoretical models and plays an important role in understanding $J/\psi$ photoproduction and its relation to the $J/\psi$-proton interaction. These measurements of $J/\psi$ photoproduction near threshold are also crucial inputs to theoretical models that are used to study important aspects of the gluon structure of the proton, such as the gluon generalized parton distribution of the proton, the mass radius of the proton, and the trace anomaly contribution to the proton mass. We observe possible structures in the total cross section energy dependence and find evidence for contributions beyond gluon exchange in the differential cross section close to threshold, both of which are consistent with contributions from open-charm intermediate states.

Potassium-40 is a widespread, naturally occurring isotope whose radioactivity impacts estimated geological ages spanning billions of years, nuclear structure theory, and subatomic rare-event searches—including those for dark matter and neutrinoless double-beta decay. The decays of this long-lived isotope must be precisely known for its use as a geochronometer, and to account for its presence in low-background experiments. There are several known decay modes for potassium-40, but a predicted electron-capture decay directly to the ground state of argon-40 has never been observed. The existence of this decay mode impacts several fields, while theoretical predictions span an order of magnitude. Here we report on the first, successful observation of this rare decay mode, obtained by the KDK (potassium decay) Collaboration using a novel combination of a low-threshold x-ray detector surrounded by a tonne-scale, high-efficiency $\gamma$-ray tagger at Oak Ridge National Laboratory. A blinded analysis reveals a distinctly nonzero ratio of intensities of ground-state electron-captures ($I_{{\mathrm{EC}}^0}$) over excited-state ones ($I_{{\text{EC}}^{*}}$) of $I_{{\mathrm{EC}}^0}/I_{{\text{EC}}^{*}}=0.0095\stackrel{\text{stat}}{\pm }0.0022\stackrel{\text{sys}}{\pm }0.0010$ (68% CL), with the null hypothesis rejected at $4\sigma$ [Stukel et al., Phys. Rev. Lett. 131, 052503 (2023)]. In terms of branching ratio, this unambiguous signal yields $I_{{\mathrm{EC}}^0}=0.098\%\stackrel{\text{stat}}{\pm }0.023\%\stackrel{\text{sys}}{\pm }0.010\%$, roughly half of the commonly used prediction. This first observation of a third-forbidden unique electron capture improves our understanding of low-energy backgrounds in dark-matter searches and has implications for nuclear-structure calculations. For example, a shell-model based theoretical estimate for the neutrinoless double-beta decay half-life of calcium-48 is increased by a factor of $7_{-2}^{+3}$. Our nonzero measurement shifts geochronological ages by up to a percent; implications are illustrated for Earth and solar system chronologies.

Bayesian statistical inference is a powerful tool for model-data comparisons and extractions of physical parameters that are often unknown functions of system variables. Existing Bayesian analyses often rely on explicit parametrizations of the unknown function. It can introduce long-range correlations that impose fictitious constraints on physical parameters in regions of the variable space that are not probed by the experimental data. We develop an information field (IF) approach to modeling the prior distribution of the unknown function that is free of long-range correlations. We apply the IF approach to the first global Bayesian inference of the jet transport coefficient $\widehat{q}$ as a function of temperature ($T$) from all existing experimental data on single-inclusive hadron, dihadron, and $\gamma$-hadron spectra in heavy-ion collisions at the BNL Relativistic Heavy Ion Collider and CERN Large Hadron Collider energies. The extracted $\widehat{q}/T^3$ exhibits a strong $T$ dependence as a result of the progressive constraining power when data from more central collisions and at higher colliding energies are incrementally included. The IF method guarantees that the extracted $T$ dependence is not biased by a specific functional form.

The study of open quantum systems (OQSs), i.e., systems interacting with an environment, impacts our understanding of exotic nuclei in low-energy nuclear physics, hadrons, cold-atom systems, or even noisy intermediate-scale quantum computers. Such systems often exhibit resonance states characterized by energy positions and dispersions (or decay widths), the properties of which can be difficult to predict theoretically due to their coupling to the continuum of scattering states. Dealing with this phenomenon poses challenges both conceptually and numerically. For that reason, we investigate how the reduced basis method known as eigenvector continuation (EC), which has emerged as a powerful tool to emulate bound and scattering states in closed quantum systems, can be used to study resonance properties. In particular, we present a generalization of EC that we call conjugate-augmented eigenvector continuation, which is based on the complex-scaling method and designed to predict Gamow-Siegert states, and thus resonant properties of OQSs, using only bound-state wave functions as input.

Based on the microscopic descriptions of the ground state with static Hartree-Fock calculations and reaction dynamic using time-dependent Hartree-Fock (TDHF) theory, in combination with coupled-channel and fusion-by-diffusion models the capture and fusion processes of the hot-fusion reaction ${}^{48}\mathrm{Ca}+{}^{238}\mathrm{U}$ are investigated. Considering the survival of the compound nucleus with statistical models, our calculations reproduce the experimental evaporation-residue cross sections. The orientation effects of ${}^{238}\mathrm{U}$ are self-consistently included in the capture and fusion processes. With the internuclear potentials from the density-constrained frozen HF methods, the calculated capture cross sections agree well with the experimental data. The TDHF evolutions with different orientations and incident energies are used to extract the injection distance, which is the only input of the fusion-by-diffusion model for fusion probabilities. The fusion probabilities are strongly dependent on the orientations and the present calculations without any free parameters show that the tip-orientation collision is favorable for both the capture process and the formation of compound nucleus.

A unitary transformation allows to remove redundant terms in the two-nucleon ($2N$) contact interaction at the fourth order (N3LO) in the low-energy expansion of chiral effective field theory. In so doing a three-nucleon ($3N$) interaction is generated. We express its short-range component in terms of five combinations of low-energy constants (LECs) parametrizing the N3LO $2N$ contact Lagrangian. Within a hybrid approach, in which this interaction is considered in conjunction with the phenomenological AV18 $2N$ potential, we show that the involved LECs can be used to fit very accurate data on polarization observables of low-energy $p\text{−}d$ scattering, in particular the $A_y$ asymmetry. The resulting interaction is of the right order of magnitude for a N3LO contribution.

We present a comprehensive theoretical study of low-energy few-nucleon scattering for systems with $A\le 4$. To this end, we utilize pionless effective field theory, which we employ at next-to-leading order. We show that at this level the theory yields accurate predictions for the low-energy scattering parameters in all studied channels. These predictions are on par with the best experimental evaluations and the available theoretical calculations. We confirm the recent observation that a four-body force is needed at next-to-leading-order and find that for nuclear systems it only appears in a single spin-isospin channel.

Nonlocal nucleon-nucleus optical potential with rotational quadrupole deformation enabling the excitation of the ${}^{24}\mathrm{Mg}\left(2^{+}\right)$ state is developed; it fits well the proton-${}^{24}\mathrm{Mg}$ elastic and inelastic differential cross section in the beam energy range from 30 to 45 MeV per nucleon. The inelastic deuteron-${}^{24}\mathrm{Mg}$ scattering leading to the excited ${}^{24}\mathrm{Mg}\left(2^{+}\right)$ state is studied in the same energy regime by solving the three-body Faddeev-type equations for transition operators. Effects of the optical potential nonlocality are evaluated by comparison with local models. Significant effects on the inelastic differential cross section are found at forward angles up to the first peak and at larger angles beyond the second peak. Nonlocal optical potential provides a simultaneous reasonable reproduction of the experimental data for the elastic and inelastic proton-${}^{24}\mathrm{Mg}$ and deuteron-${}^{24}\mathrm{Mg}$ scattering not achieved using local potentials.

Studies of the correlations of the two highest transverse momentum (leading) jets in individual Pb+Pb collision events can provide information about the mechanism of jet quenching by the hot and dense matter created in such collisions. In Pb+Pb and $pp$ collisions at $\sqrt{s_{NN}}=5.02\mathrm{TeV}$, measurements of the leading dijet transverse momentum ($p_{\mathrm{T}}$) correlations are presented. Additionally, measurements in Pb+Pb collisions of the dijet pair nuclear modification factors projected along leading and subleading jet $p_{\mathrm{T}}$ are made. The measurements are performed using the ATLAS detector at the LHC with 260 ${\mathrm{pb}}^{-1}$ of $pp$ data collected in 2017 and 2.2 ${\mathrm{nb}}^{-1}$ of Pb+Pb data collected in 2015 and 2018. An unfolding procedure is applied to the two-dimensional leading and subleading jet $p_{\mathrm{T}}$ distributions to account for experimental effects in the measurement of both jets. Results are provided for dijets with leading jet $p_{\mathrm{T}}$ greater than 100 $\mathrm{GeV}$. Measurements of the dijet-yield-normalized $x_{\mathrm{J}}$ distributions in Pb+Pb collisions show an increased fraction of imbalanced jets compared to $pp$ collisions; these measurements are in agreement with previous measurements of the same quantity at 2.76 $\mathrm{TeV}$ in the overlapping kinematic range. Measurements of the absolutely normalized dijet rate in Pb+Pb and $pp$ collisions are also presented, and show that balanced dijets are significantly more suppressed than imbalanced dijets in Pb+Pb collisions. It is observed in the measurements of the pair nuclear modification factors that the subleading jets are significantly suppressed relative to leading jets with $p_{\mathrm{T}}$ between 100 and 316 $\mathrm{GeV}$ for all centralities in Pb+Pb collisions.

Results of a measurement of dimuon photoproduction in nonultraperipheral $\mathrm{Pb}+\mathrm{Pb}$ collisions at $\sqrt{s_{NN}}=5.02$ TeV are presented. The measurement uses ATLAS data from the 2015 and 2018 $\mathrm{Pb}+\mathrm{Pb}$ data-taking periods at the LHC with an integrated luminosity of $1.94{\mathrm{nb}}^{-1}$. The $\gamma \gamma \to {\mu }^{+}{\mu }^{-}$ pairs are identified via selections on pair momentum asymmetry and acoplanarity. Differential cross sections for dimuon production are measured in different centrality, average muon momentum, and pair rapidity intervals as functions of acoplanarity and $k_{\perp }$, the transverse momentum kick of one muon relative to the other. Measurements are also made as a function of the rapidity separation of the muons and the angle of the muon pair relative to the second-order event plane to test whether magnetic fields generated in the quark-gluon plasma affect the measured muons. A prior observation of a centrality-dependent broadening of the acoplanarity distribution is confirmed. Furthermore, the improved precision of the measurement reveals a depletion in the number of pairs having small acoplanarity or $k_{\perp }$ values in more central collisions. The acoplanarity distributions in a given centrality interval are observed to vary with the mean $p_{\mathrm{T}}$ of the muons in the pair, but the $k_{\perp }$ distributions do not. Comparisons with recent theoretical predictions are made. The predicted trends associated with effects of magnetic fields on the dimuons are not observed.