The electromagnetic responses obtained from Green's function Monte Carlo (GFMC) calculations are based on realistic treatments of nuclear interactions and currents. The main limitations of this method comes from its nonrelativistic nature and its computational cost, the latter hampering the direct evaluation of the inclusive cross sections as measured by experiments. We extend the applicability of GFMC in the quasielastic region to intermediate momentum transfers by performing the calculations in a reference frame that minimizes nucleon momenta. Additional relativistic effects in the kinematics are accounted for employing the two-fragment model. In addition, we developed a novel algorithm, based on the concept of first-kind scaling, to compute the inclusive electromagnetic cross section of ${}^4\mathrm{He}$ through an accurate and reliable interpolation of the response functions. A very good agreement is obtained between theoretical and experimental cross sections for a variety of kinematical setups. This offers a promising prospect for the data analysis of neutrino-oscillation experiments that requires an accurate description of nuclear dynamics in which relativistic effects are fully accounted for.

The first numerical solution of the five-body Faddeev-Yakubovsky equations is presented. Modern realistic nucleon-nucleon Hamiltonians have been tested when describing low-energy elastic neutron scattering on the ${}^4\mathrm{He}$. Results obtained are compared with those available in the literature and based on the solution of the Schrödinger equation.

Intense electromagnetic fields are created in the quark-gluon plasma by the external ultrarelativistic valence charges. The time evolution and the strength of this field are strongly affected by the electrical conductivity of the plasma. Yet, it has recently been observed that the effect of the magnetic field on the plasma flow is small. We compute the effect of plasma flow on magnetic field and demonstrate that it is less than 10%. These observations indicate that the plasma hydrodynamics and the dynamics of electromagnetic field decouple. Thus, it is a very good approximation, on the one hand, to study QGP in the background electromagnetic field generated by external sources and, on the other hand, to investigate the dynamics of magnetic field in the background plasma. We also argue that the wake induced by the magnetic field in plasma is negligible.

We report the first measurement of the fusion excitation functions for ${}^{39,47}\mathrm{K}+{}^{28}\mathrm{Si}$ at near-barrier energies. Evaporation residues resulting from the fusion process were identified by direct measurement of their energy and time of flight with high geometric efficiency. At the lowest incident energy, the cross section measured for the neutron-rich ${}^{47}\mathrm{K}$-induced reaction is $\approx 6$ times larger than that of the $\beta$-stable system. This experimental approach, both in measurement and in analysis, demonstrates how to efficiently measure fusion with low-intensity re-accelerated radioactive beams, establishing the framework for future studies.

Background: The neutron $\beta$-decay asymmetry parameter $A_0$ defines the angular correlation between the spin of the neutron and the momentum of the emitted electron. Values for $A_0$ permit an extraction of the ratio of the weak axial-vector to vector coupling constants, $\lambda \equiv g_A/g_V$, which under assumption of the conserved vector current hypothesis ($g_V=1$) determines $g_A$. Precise values for $g_A$ are important as a benchmark for lattice QCD calculations and as a test of the standard model.

Purpose: The UCNA experiment, carried out at the Ultracold Neutron (UCN) source at the Los Alamos Neutron Science Center, was the first measurement of any neutron $\beta$-decay angular correlation performed with UCN. This article reports the most precise result for $A_0$ obtained to date from the UCNA experiment, as a result of higher statistics and reduced key systematic uncertainties, including from the neutron polarization and the characterization of the electron detector response.

Methods: UCN produced via the downscattering of moderated spallation neutrons in a solid deuterium crystal were polarized via transport through a 7 T polarizing magnet and a spin flipper, which permitted selection of either spin state. The UCN were then contained within a 3-m long cylindrical decay volume, situated along the central axis of a superconducting 1 T solenoidal spectrometer. With the neutron spins then oriented parallel or anti-parallel to the solenoidal field, an asymmetry in the numbers of emitted decay electrons detected in two electron detector packages located on both ends of the spectrometer permitted an extraction of $A_0$.

Results: The UCNA experiment reports a new 0.67% precision result for $A_0$ of $A_0=-0.12054{\left(44\right)}_{\mathrm{stat}}{\left(68\right)}_{\mathrm{syst}}$, which yields $\lambda =g_A/g_V=-1.2783\left(22\right)$. Combination with the previous UCNA result and accounting for correlated systematic uncertainties produces $A_0=-0.12015{\left(34\right)}_{\mathrm{stat}}{\left(63\right)}_{\mathrm{syst}}$ and $\lambda =g_A/g_V=-1.2772\left(20\right)$.

Conclusions: This new result for $A_0$ and $g_A/g_V$ from the UCNA experiment has provided confirmation of the shift in values for $g_A/g_V$ that has emerged in the published results from more recent experiments, which are in striking disagreement with the results from older experiments. Individual systematic corrections to the asymmetries in older experiments (published prior to 2002) were $>10$%, whereas those in the more recent ones (published after 2002) have been of the scale of $<2$%. The impact of these older results on the global average will be minimized should future measurements of $A_0$ reach the 0.1% level of precision with central values near the most recent results.

We explore possible observable signatures of $\alpha$ clustering of light nuclei in ultrarelativistic nuclear collisions involving ${}^{7,9}\mathrm{Be}$, ${}^{12}\mathrm{C}$, and ${}^{16}\mathrm{O}$. The clustering leads to specific spatial correlations of the nucleon distributions in the ground state, which are manifest in the earliest stage of the ultrahigh energy reaction. The formed initial state of the fireball is sensitive to these correlations, and the effect influences, after the collective evolution of the system, the hadron production in the final stage. Specifically, we study effects on the harmonic flow in collisions of light clustered nuclei with a heavy target (${}^{208}\mathrm{Pb}$), showing that measures of the elliptic flow are sensitive to clusterization in ${}^{7,9}\mathrm{Be}$, whereas triangular flow is sensitive to clusterization in ${}^{12}\mathrm{C}$ and ${}^{16}\mathrm{O}$. Specific predictions are made for model collisions at energies available at the CERN Super Proton Synchrotron. In another exploratory development we also examine proton-beryllium collisions, where the $3/2^{-}$ ground state of ${}^{7,9}\mathrm{Be}$ nuclei is polarized by an external magnetic field. Clusterization leads to multiplicity distributions of participant nucleons which depend on the orientation of the polarization with respect to the collision axis, as well as on the magnetic number of the state. The obtained effects on multiplicities reach a factor of a few for collisions with a large number of participant nucleons.

We show that electron time-of-flight can be a useful tool in $\beta$-decay spectroscopy, even in the energy range up to 700 keV relevant for neutron $\beta$ decay. As a first application, we use electron time-of-flight measurements for the precise calibration of an electron detector.

Background: Classical novae are cataclysmic nuclear explosions occurring when a white dwarf in a binary system accretes hydrogen-rich material from its companion star. Novae are partially responsible for the galactic synthesis of a variety of nuclides up to the calcium ($A\sim 40$) region of the nuclear chart. Although the structure and dynamics of novae are thought to be relatively well understood, the predicted abundances of elements near the nucleosynthesis endpoint, in particular Ar and Ca, appear to sometimes be in disagreement with astronomical observations of the spectra of nova ejecta.

Purpose: One possible source of the discrepancies between model predictions and astronomical observations is nuclear reaction data. Most reaction rates near the nova endpoint are estimated only from statistical model calculations, which carry large uncertainties. For certain key reactions, these rate uncertainties translate into large uncertainties in nucleosynthesis predictions. In particular, the ${}^{38}\mathrm{K}\left(p,\gamma \right){}^{39}\mathrm{Ca}$ reaction has been identified as having a significant influence on Ar, K, and Ca production. In order to constrain the rate of this reaction, we have performed a direct measurement of the strengths of three candidate $\ell =0$ resonances within the Gamow window for nova burning, at $386\pm 10$ keV, $515\pm 10$ keV, and $689\pm 10$ keV.

Method: The experiment was performed in inverse kinematics using a beam of unstable ${}^{38}\mathrm{K}$ impinged on a windowless hydrogen gas target. The ${}^{39}\mathrm{Ca}$ recoils and prompt $\gamma$ rays from ${}^{38}\mathrm{K}\left(p,\gamma \right){}^{39}\mathrm{Ca}$ reactions were detected in coincidence using a recoil mass separator and a bismuth-germanate scintillator array, respectively.

Results: For the 689 keV resonance, we observed a clear recoil-$\gamma$ coincidence signal and extracted resonance strength and energy values of ${120}_{-30}^{+50}{(\mathrm{stat}.)}_{-60}^{+20}(\mathrm{sys}.)\mathrm{meV}$ and ${679}_{-1}^{+2}(\mathrm{stat}.)\pm 1(\mathrm{sys}.)\mathrm{keV}$, respectively. We also performed a singles analysis of the recoil data alone, extracting a resonance strength of $120\pm 20(\mathrm{stat}.)\pm 15(\mathrm{sys}.)$ meV, consistent with the coincidence result. For the 386 keV and 515 keV resonances, we extract $90\%$ confidence level upper limits of 2.54 meV and 18.4 meV, respectively.

Conclusions: We have established a new recommended ${}^{38}\mathrm{K}(p,\gamma ){}^{39}\mathrm{Ca}$ rate based on experimental information, which reduces overall uncertainties near the peak temperatures of nova burning by a factor of $\sim 250$. Using the rate obtained in this work in model calculations of the hottest oxygen-neon novae reduces overall uncertainties on Ar, K, and Ca synthesis to factors of 15 or less in all cases.

Bose-Einstein correlations between identified charged pions are measured for $p$+Pb collisions at $\sqrt{s_{\mathit{NN}}}=5.02\mathrm{TeV}$ using data recorded by the ATLAS detector at the CERN Large Hadron Collider corresponding to a total integrated luminosity of $28{\mathrm{nb}}^{-1}$. Pions are identified using ionization energy loss measured in the pixel detector. Two-particle correlation functions and the extracted source radii are presented as a function of collision centrality as well as the average transverse momentum ($k_{\mathrm{T}}$) and rapidity ($y_{\pi \pi }^{★}$) of the pair. Pairs are selected with a rapidity $-2<y_{\pi \pi }^{★}<1$ and with an average transverse momentum $0.1<k_{\mathrm{T}}<0.8\mathrm{GeV}$. The effect of jet fragmentation on the two-particle correlation function is studied, and a method using opposite-charge pair data to constrain its contributions to the measured correlations is described. The measured source sizes are substantially larger in more central collisions and are observed to decrease with increasing pair $k_{\mathrm{T}}$. A correlation of the radii with the local charged-particle density is demonstrated. The scaling of the extracted radii with the mean number of participating nucleons is also used to compare a selection of initial-geometry models. The cross term $R_{\mathrm{ol}}$ is measured as a function of rapidity, and a nonzero value is observed with $5.1\sigma$ combined significance for $-1<y_{\pi \pi }^{★}<1$ in the most central events.

We present the results of our final analysis of the full data set of $g_1^p\left(Q^2\right)$, the spin structure function of the proton, collected using CLAS at Jefferson Laboratory in 2000–2001. Polarized electrons with energies of 1.6, 2.5, 4.2, and 5.7 GeV were scattered from proton targets (${}^{15}\mathrm{NH}_3$ dynamically polarized along the beam direction) and detected with CLAS. From the measured double spin asymmetries, we extracted virtual photon asymmetries $A_1^p$ and $A_2^p$ and spin structure functions $g_1^p$ and $g_2^p$ over a wide kinematic range (0.05 ${\mathrm{GeV}}^2<Q^2<$ 5 ${\mathrm{GeV}}^2$ and 1.08 GeV $<W<$ 3 GeV) and calculated moments of $g_1^p$. We compare our final results with various theoretical models and expectations, as well as with parametrizations of the world data. Our data, with their precision and dense kinematic coverage, are able to constrain fits of polarized parton distributions, test pQCD predictions for quark polarizations at large $x$, offer a better understanding of quark-hadron duality, and provide more precise values of higher twist matrix elements in the framework of the operator product expansion.

Background: Modern applications of nuclear time-dependent density functional theory (TDDFT) are often capable of providing quantitative description of heavy ion reactions. However, the structures of precompound (preequilibrium, prefission) states produced in heavy ion reactions are difficult to assess theoretically in TDDFT as the single-particle density alone is a weak indicator of shell structure and cluster states.

Purpose: We employ the time-dependent nucleon localization function (NLF) to reveal the structure of precompound states in nuclear reactions involving light and medium-mass ions. We primarily focus on spin saturated systems with $N=Z$. Furthermore, we study reactions with oxygen and carbon ions, for which some experimental evidence for $\alpha$ clustering in precompound states exists.

Method: We utilize the symmetry-free TDDFT approach with the Skyrme energy density functional UNEDF1 and compute the time-dependent NLFs to describe ${}^{16}\mathrm{O}$ + ${}^{16}\mathrm{O},{}^{40}\mathrm{Ca}$ + ${}^{16}\mathrm{O},{}^{40}\mathrm{Ca}$ + ${}^{40}\mathrm{Ca}$, and ${}^{16,18}\mathrm{O}$ + ${}^{12}\mathrm{C}$ collisions at energies above the Coulomb barrier.

Results: We show that NLFs reveal a variety of time-dependent modes involving cluster structures. For instance, the ${}^{16}\mathrm{O}$ + ${}^{16}\mathrm{O}$ collision results in a vibrational mode of a quasimolecular $\alpha \text{−}{}^{12}\mathrm{C}\text{−}{}^{12}\mathrm{C}\text{−}\alpha$ state. For heavier ions, a variety of cluster configurations are predicted. For the collision of ${}^{16,18}\mathrm{O}$ + ${}^{12}\mathrm{C}$, we showed that the precompound system has a tendency to form $\alpha$ clusters. This result supports the experimental findings that the presence of cluster structures in the projectile and target nuclei gives rise to strong entrance channel effects and enhanced $\alpha$ emission.

Conclusion: The time-dependent nucleon localization measure is a very good indicator of cluster structures in complex precompound states formed in heavy-ion fusion reactions. The localization reveals the presence of collective vibrations involving cluster structures, which dominate the initial dynamics of the fusing system.

Local chiral effective field theory interactions have recently been developed and used in the context of quantum Monte Carlo few- and many-body methods for nuclear physics. In this work, we go over detailed features of local chiral nucleon-nucleon interactions and examine their effect on properties of the deuteron, paying special attention to the perturbativeness of the expansion. We then turn to three-nucleon interactions, focusing on operator ambiguities and their interplay with regulator effects. We then discuss the nuclear Green's function Monte Carlo method, going over both wave-function correlations and approximations for the two- and three-body propagators. Following this, we present a range of results on light nuclei: Binding energies and distribution functions are contrasted and compared, starting from several different microscopic interactions.

The polarizations of $\mathrm{\Lambda }$ and $\overline{\mathrm{\Lambda }}$ hyperons are important quantities in extracting the fluid vorticity of the strongly coupled quark gluon plasma and the magnitude of the magnetic field created in off-central heavy-ion collisions, through the spin-vorticity and spin-magnetic coupling. We computed the energy dependence of the global $\mathrm{\Lambda }$ polarization in off-central Au+Au collisions in the energy range $\sqrt{s_{NN}}=7.7–200$ GeV using a multiphase transport model. The observed polarizations with two different impact parameters agree quantitatively with recent STAR measurements. The energy dependence of the global $\mathrm{\Lambda }$ polarization is decomposed as energy dependence of the $\mathrm{\Lambda }$ distribution at hadronization and the space-time distribution of the fluid-vorticity field. The visualization of both the $\mathrm{\Lambda }$ distribution and the fluid-vorticity field show a smaller tilt at higher collisional energies, which indicates that the smaller global polarization at higher collisional energies is caused by a smaller angular momentum deposition at midrapidity.

The Compton scattering cross section from ${}^4\mathrm{He}$ has been measured with high statistical accuracy over a scattering angle range of ${40}^{\circ }-{159}^{\circ }$ using a quasimonoenergetic 61-MeV photon beam at the High Intensity Gamma-Ray Source. The data are interpreted using a phenomenological model sensitive to the dipole isoscalar electromagnetic polarizabilities (${\alpha }_s$ and ${\beta }_s$) of the nucleon. These data can be fit with the model using values of ${\alpha }_s$ and ${\beta }_s$ that are consistent with the currently accepted values. These data will serve as benchmarks of future calculations from effective field theories and lattice quantum chromodynamics.

We present measurements of bulk properties of the matter produced in Au+Au collisions at $\sqrt{s_{NN}}=7.7,11.5,19.6,27$, and 39 GeV using identified hadrons (${\pi }^{\pm }$, $K^{\pm }$, $p$, and $\overline{p}$) from the STAR experiment in the Beam Energy Scan (BES) Program at the Relativistic Heavy Ion Collider (RHIC). Midrapidity ($\left|y\right|<0.1$) results for multiplicity densities $dN/dy$, average transverse momenta $\langle p_T\rangle$, and particle ratios are presented. The chemical and kinetic freeze-out dynamics at these energies are discussed and presented as a function of collision centrality and energy. These results constitute the systematic measurements of bulk properties of matter formed in heavy-ion collisions over a broad range of energy (or baryon chemical potential) at RHIC.

Non-zero-range interactions are often incorporated into mean field theories through gradient terms. Here, a formalism is developed to incorporate such terms into hydrodynamics. These terms alter expressions for the entropy, chemical potential, temperature, and the stress-energy tensor. The formalism respects local conservation of energy, charge, and entropy. The formalism leads to static solutions where the temperature, chemical potential, and hydrodynamic acceleration all vanish, even when the density profile might be nonuniform. Profiles for a phase boundary and for correlation functions are calculated to illustrate the gradient modifications for various thermodynamic quantities. Also, for hydrodynamic calculations that add thermal noise to generate density-density correlations of the desired strength, an additional noise term is derived so that, at equilibrium, correlations are generated with both the correct size and length scale.

Background: Exotic nuclei, particularly those near the drip lines, are at the core of one of the fundamental questions driving nuclear structure and astrophysics today: What are the limits of nuclear binding? Exotic nuclei play a critical role in both informing theoretical models as well as in our understanding of the origin of the heavy elements.

Purpose: Our aim is to refine existing mass models through the training of an artificial neural network that will mitigate the large model discrepancies far away from stability.

Methods: The basic paradigm of our two-pronged approach is an existing mass model that captures as much as possible of the underlying physics followed by the implementation of a Bayesian neural network (BNN) refinement to account for the missing physics. Bayesian inference is employed to determine the parameters of the neural network so that model predictions may be accompanied by theoretical uncertainties.

Results: Despite the undeniable quality of the mass models adopted in this work, we observe a significant improvement (of about 40%) after the BNN refinement is implemented. Indeed, in the specific case of the Duflo-Zuker mass formula, we find that the rms deviation relative to experiment is reduced from ${\sigma }_{\mathrm{rms}}=0.503$ MeV to ${\sigma }_{\mathrm{rms}}=0.286$ MeV. These newly refined mass tables are used to map the neutron drip lines (or rather “drip bands”) and to study a few critical $r$-process nuclei.

Conclusions: The BNN approach is highly successful in refining the predictions of existing mass models. In particular, the large discrepancy displayed by the original “bare” models in regions where experimental data are unavailable is considerably quenched after the BNN refinement. This lends credence to our approach and has motivated us to publish refined mass tables that we trust will be helpful for future astrophysical applications.

We calculate the three-hole-line contributions to the binding energy of symmetric nuclear matter in the Brueckner-Bethe-Goldstone expansion using various modern nucleon-nucleon potentials of high precision. The relation with the correlation parameter $\kappa =\rho V_{\mathrm{core}}$ is examined. In all cases the three-hole-line contributions turn out to be sufficiently small, but no satisfactory saturation is obtained. This means that three-nucleon forces are essential for all considered potentials.

Chiral effective field theory (EFT) predictions are necessarily truncated at some order in the EFT expansion, which induces an error that must be quantified for robust statistical comparisons to experiment. In previous work, a Bayesian model for truncation errors of perturbative expansions was adapted to EFTs. The model yields posterior probability distribution functions (pdfs) for these errors based on expectations of naturalness encoded in Bayesian priors and the observed order-by-order convergence pattern of the EFT. A first application was made to chiral EFT for neutron-proton scattering using the semilocal potentials of Epelbaum, Krebs, and Meißner (EKM). Here we extend this application to consider a larger set of regulator parameters, energies, and observables as a general example of a statistical approach to truncation errors. The Bayesian approach allows for statistical validations of the assumptions and enables the calculation of posterior pdfs for the EFT breakdown scale. The statistical model is validated for EKM potentials whose convergence behavior is not distorted by regulator artifacts. For these cases, the posterior for the breakdown scale is consistent with EKM assumptions.

Sulphur isotopic ratio measurements may help to establish the astrophysical sites in which certain presolar grains were formed. Nova model predictions of the ${}^{34}\mathrm{S}/{}^{32}\mathrm{S}$ ratio are, however, unreliable due to the lack of an experimental ${}^{34}\mathrm{S}(p,\gamma ){}^{35}\mathrm{Cl}$ reaction rate. To this end, we have measured the ${}^{34}\mathrm{S}({}^3\mathrm{He},d){}^{35}\mathrm{Cl}$ reaction at 20 MeV using a high resolution quadrupole-dipole-dipole-dipole magnetic spectrograph. Twenty-two levels over 6.2 MeV $<E_x\left({}^{35}\mathrm{Cl}\right)<7.4$ MeV were identified, ten of which were previously unobserved. Proton-transfer spectroscopic factors have been measured for the first time over the energy range relevant for novae. With this new spectroscopic information a new ${}^{34}\mathrm{S}(p,\gamma ){}^{35}\mathrm{Cl}$ reaction rate has been determined using a Monte Carlo method. Hydrodynamic nova model calculations have been performed using this new reaction rate. These models show that remaining uncertainties in the ${}^{34}\mathrm{S}(p,\gamma )$ rate affect nucleosynthesis predictions by less than a factor of 1.4, and predict a ${}^{34}\mathrm{S}/{}^{32}\mathrm{S}$ isotopic ratio of 0.014–0.017. Since recent type II supernova models predict ${}^{34}\mathrm{S}/{}^{32}\mathrm{S}=0.026-0.053$, the ${}^{34}\mathrm{S}/{}^{32}\mathrm{S}$ isotopic ratio may be used, in conjunction with other isotopic signatures, to distinguish presolar grains from oxygen-neon nova and type II supernova origin. Our results address a key nuclear physics uncertainty on which recent considerations discounting the nova origin of several grains depend.