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.

Four different reactions, ${}^{32}\mathrm{S}+{}^{12}\mathrm{C}$ and ${}^{20}\mathrm{Ne}+{}^{12}\mathrm{C}$ at 25 and 50 MeV/nucleon, have been measured with the FAZIA detector capable of full isotopic identification of most forward emitted reaction products. Fragment multiplicities, angular distributions and energy spectra have been measured and compared with Monte Carlo simulations, i.e., the antisymmetrized molecular dynamics (AMD) and the heavy-ion phase space exploration (HIPSE) models. These models are combined with two different afterburner codes (HF$\ell$ and SIMON) to describe the decay of the excited primary fragments. In the case of AMD, the effect of including the clustering and interclustering processes to form bound particles and fragments is discussed. A clear confirmation of the role of the cluster aggregation in the reaction dynamics and particle production for these light systems, for which the importance of the clustering process increases with bombarding energy, is obtained.

Fission data are essential inputs to reaction networks involved in nucleosynthesis simulations and nuclear forensics. In such applications as well as in the description of multichance fission, the characteristics of fission for odd-mass nuclei are just as important as those for even-even nuclei. The fission theories that aim at explicitly describing fission dynamics are typically based on some variant of the nuclear mean-field theory. In such cases, the treatment of systems with an odd number of particles is markedly more involved, both formally and computationally. In this article, we use the blocking prescription of the Hartree-Fock-Bogoliubov theory with Skyrme energy functionals to compute the deformation properties of odd-mass uranium isotopes. We show that the resulting fission fragment distributions depend quite significantly on the spin of the odd neutron. By direct calculation of the spin distribution of the fissioning nucleus, we propose a methodology to rigorously predict the charge and mass distributions in odd-mass nuclei.

Microscopic theory of the nuclear response based on the relativistic meson-nucleon Lagrangian is applied to the description of the isoscalar giant monopole resonance (ISGMR) in a variety of nuclear systems. It is shown that the parameter-free inclusion of beyond-mean-field correlations of the quasiparticle-vibration coupling (qPVC) type in the leading approximation allows for a simultaneous realistic description of the ISGMR in nuclei of lead, tin, zirconium, and nickel mass regions, which is difficult on the mean-field level. The calculations employ the finite-range effective meson-nucleon interaction, which, in combination with the qPVC, has consistently demonstrated the ability to reliably describe many other nuclear structure phenomena. Systematic calculations of the isoscalar monopole response for nickel isotopes help reveal the central role of the coupling between the ISGMR and the low-energy quadrupole states in the placement of the ISGMR centroids.

The total cross section of the ${}^{82}\mathrm{Kr}{(p,\gamma )}^{83}\mathrm{Rb}$ reaction was measured for the first time at effective center-of-mass energies between 2.4 and 3.0 MeV, within the relevant Gamow window for the astrophysical $\gamma$ process. The experiment took place at the National Superconducting Cyclotron Laboratory at Michigan State University using the ReA facility. A ${}^{82}\mathrm{Kr}$ beam was directed onto a hydrogen gas cell located at the center of the Summing NaI(Tl) (SuN) detector. The obtained spectra were analyzed using the $\gamma$-summing technique and the extracted cross section was compared to standard statistical model calculations using the non-smoker and talys codes. The comparison indicates that standard statistical model calculations tend to overproduce the cross section of the ${}^{82}\mathrm{Kr}(p,\gamma$)${}^{83}\mathrm{Rb}$ reaction relative to the experimentally measured values. Furthermore, the experimental data were used to provide additional constraints on the nuclear level density and the $\gamma$-ray strength function used in the statistical model calculations.

We analyze the axial $\gamma W$-box diagram for $I\left(J^P\right)=1\left(0^{+}\right)$ nuclei and provide a dispersion representation of the nuclear-structure correction ${\delta }_{\text{NS}}$ including its energy-dependent part. We also summarize useful isospin rotation formula and representations in nuclear theory that could facilitate the calculation of the parity-odd nuclear structure function $F_3(\nu ,Q^2)$. They provide a rigorous theory framework for the future, high-precision calculation of the nuclear structure correction ${\delta }_{\text{NS}}$ necessary for the extraction of the Cabibbo-Kobayashi-Maskawa matrix element $|V_{ud}|$ from superallowed nuclear $\beta$ decays.

We develop a general decomposition of an ensemble of initial density profiles in terms of an average state and a basis of modes that represent the event-by-event fluctuations of the initial state. The basis is determined such that the probability distributions of the amplitudes of different modes are uncorrelated. Based on this decomposition, we quantify the different types and probabilities of event-by-event fluctuations in Glauber and Saturation models and investigate how the various modes affect different characteristics of the initial state. We perform simulations of the dynamical evolution with KøMPøST and music to investigate the impact of the modes on final-state observables and their correlations.

The measurement of direct photons from $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{{}_{NN}}}=39$ and 62.4 GeV in the transverse-momentum range $0.4<p_T<3\mathrm{Gev}/c$ is presented by the PHENIX collaboration at the BNLRelativistic Heavy Ion Collider. A significant direct-photon yield is observed in both collision systems. A universal scaling is observed when the direct-photon $p_T$ spectra for different center-of-mass energies and for different centrality selections at $\sqrt{s_{{}_{NN}}}=62.4$ GeV is scaled with ${(d{N}_{\mathrm{ch}}/d\eta )}^{\alpha }$ for $\alpha =1.21\pm 0.04$. This scaling also holds true for direct-photon spectra from $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{{}_{NN}}}=200$ GeV measured earlier by PHENIX, as well as the spectra from $\mathrm{Pb}+\mathrm{Pb}$ at $\sqrt{s_{{}_{NN}}}=2760$ GeV published by ALICE. The scaling power $\alpha$ seems to be independent of $p_T$, center of mass energy, and collision centrality. The spectra from different collision energies have a similar shape up to $p_T$ of 2 $\mathrm{Gev}/c$. The spectra have a local inverse slope $T_{\mathrm{eff}}$ increasing with $p_T$ of $0.174\pm 0.018\mathrm{Gev}/c$ in the range $0.4<p_T<1.3\mathrm{Gev}/c$ and increasing to $0.289\pm 0.024\mathrm{Gev}/c$ for $0.9<p_T<2.1\mathrm{Gev}/c$. The observed similarity of low-$p_T$ direct-photon production from $\sqrt{s_{{}_{NN}}}=39$ to 2760 GeV suggests a common source of direct photons for the different collision energies and event centrality selections, and suggests a comparable space-time evolution of direct-photon emission.

In this work, we calculate the ground state energy of pure neutron matter using the renormalization group based low-momentum effective interaction $V_{\text{low-}k}$ in Bogoliubov many-body perturbation theory, which is a perturbative expansion around the Hartree-Fock-Bogoliubov (HFB) ground state. In order to capture the low-density behavior of neutron matter, it turns out to be better to use a density dependent cutoff in the $V_{\text{low-}k}$ interaction. Perturbative corrections to the HFB energy up to third order are included. We find that at low densities corresponding to the inner crust of neutron stars, the HFB state that includes pairing is a better starting point for perturbation expansion. It is observed that including the higher order perturbative corrections, the cutoff dependence of the ground state energy is reduced.

The microscopic origin of the $\gamma$ softness (fluctuations in the triaxiality parameter $\gamma$ of the nuclear shape) observed in atomic nuclei is studied in the framework of the triaxial projected shell model approach, which is based on a multiquasiparticle configuration space generated from a deformed mean field. It is demonstrated that the coupling to quasiparticle excitations drives the system from a $\gamma$-rigid to a $\gamma$-soft pattern. As an illustrative example for a $\gamma$-soft nucleus, a detailed study has been performed for the ${}^{104}\mathrm{Ru}$ nucleus. The experimental energies and a large sample of measured $E2$ matrix elements available for this nucleus are reproduced quite accurately. The shape invariant analysis of the calculated $E2$ matrix elements elucidates the $\gamma$-soft nature of ${}^{104}\mathrm{Ru}$.

We report a measurement of cumulants and correlation functions of event-by-event proton multiplicity distributions from fixed-target Au+Au collisions at $\sqrt{s_{NN}}$ = 3 GeV measured by the STAR experiment. Protons are identified within the rapidity ($y$) and transverse momentum ($p_{\mathrm{T}}$) region $-0.9<y<0$ and $0.4<p_{\mathrm{T}}<2.0$ GeV/$c$ in the center-of-mass frame. A systematic analysis of the proton cumulants and correlation functions up to sixth order as well as the corresponding ratios as a function of the collision centrality, $p_{\mathrm{T}}$, and $y$ are presented. The effect of pileup and initial volume fluctuations on these observables and the respective corrections are discussed in detail. The results are compared to calculations from the hadronic transport UrQMD model as well as a hydrodynamic model. In the most central 5% collisions, the value of proton cumulant ratio $C_4/C_2$ is negative, drastically different from the values observed in Au+Au collisions at higher energies. Compared to model calculations including lattice QCD, a hadronic transport model, and a hydrodynamic model, the strong suppression in the ratio of $C_4/C_2$ at 3 GeV Au+Au collisions indicates an energy regime dominated by hadronic interactions.