The ability to accurately compute the series of coefficients $v_n$ characterizing the momentum space anisotropies of particle production in ultrarelativistic heavy-ion collisions as a function of centrality is widely regarded as a triumph of fluid dynamics as the description of the bulk matter evolution. A key ingredient to fluid dynamical modeling is, however, the initial spatial distribution of matter as created by a not yet completely understood equilibration process. A measurement directly sensitive to this initial state geometry is therefore of high value for constraining models of pre-equilibrium dynamics. Recently, it has been shown that such a measurement is indeed possible in terms of the event-by-event probability distribution of the normalized $v_n$ distribution as a function of centrality, which is to high accuracy independent of the details of the subsequent fluid dynamical evolution and hence directly reflects the primary distribution of spatial eccentricities. We present a study of this observable using a variety of Glauber-based models and argue that the experimental data place very tight constraints on the initial distribution of matter and rule out all simple Glauber-based models.

In reviewing the data that has accumulated about light nuclei, we find that the binding energy plays a critical role in describing the variation in energy of $s$ states relative to other states. The behavior of states with zero angular momentum within a few MeV of threshold is qualitatively different from that of neutron states with any other $\ell$ value or of any proton state. This observation is explored for simple Woods-Saxon potentials and is remarkably successful in describing a wealth of experimental data for nuclei with neutron numbers between 5 and 10. The lingering of neutron $s$ states just below threshold is associated with the increases in radii of the neutron density distributions, the neutron halos, and leads to speculations about possible halos in heavier nuclei.

Background: The ${}_{38}^{88}\text{Sr}$ and ${}_{40}^{90}\text{Zr}$ nuclei have been utilized as closed cores for large-scale shell-model calculations in the $28\le Z\le 50$ region around the $N=50$ shell. Measurements of magnetic moments for nuclei in this region would provide microscopic information about the use of ${}^{88}$Sr and ${}^{90}$Zr as stable closed-core nuclei. While the $g$ factors of the 2${}_1^{+}$ states in the stable Sr isotopes have been previously measured, experimental $g$ factors for the radioactive ${}^{82,90}$Sr have not been obtained to date.

Purpose: The purpose was to measure the $g$ factors of the 2${}_1^{+}$ and 4${}_1^{+}$ states in the unstable ${}^{82}$Sr and ${}^{90}$Sr nuclei in order to extend the systematics along the Sr isotopic chain. A comparison of the structure of the $N=52$ isotopes ${}^{90}$Sr and ${}^{92}$Zr will shed light on the relative robustness of proton subshell closures at $Z=38$ and $Z=40$.

Methods: The pickup reaction of $\alpha$ particles in inverse kinematics together with the transient field technique were applied to beams of ${}^{78}$Kr and ${}^{86}$Kr at the Cyclotron Institute of Texas A&M University.

Results: The values $g{(}^{82}\mathrm{Sr};2_1^{+})=+0.44\left(19\right)$, $g{(}^{82}\mathrm{Sr};4_1^{+})=+0.53\left(39\right)$, $g{(}^{90}\mathrm{Sr};2_1^{+})=-0.12\left(11\right)$, and $g{(}^{90}\mathrm{Sr};4_1^{+})=-0.02\left(17\right)$ were measured for the first time. Simultaneously, the $g$ factors of the low-lying states in the Coulomb-excited beam projectiles were remeasured. The $g\left(4_1^{+}\right)$ = +1.03(14) in ${}^{86}$Kr was also measured for the first time.

Conclusions: For ${}^{82}$Sr both $g$ factors are in agreement with the collective value $Z/A$ expected for nuclei in the middle of a major shell. The $g$ factors in ${}^{90}$Sr are negative but smaller than in the isotone ${}^{92}$Zr. The results also indicate that ${}^{88}$Sr is a proton-soft core nucleus and perhaps even softer than ${}^{90}$Zr.

We apply stochastic hydrodynamics to the study of charge-density fluctuations in QCD matter undergoing Bjorken expansion. We find that the charge-density correlations are given by a time integral over the history of the system, with the dominant contribution coming from the QCD crossover region where the change of susceptibility per entropy, $\chi T/s$, is most significant. We study the rapidity and azimuthal angle dependence of the resulting charge balance function using a simple analytic model of heavy-ion collision evolution. Our results are in agreement with experimental measurements, indicating that hydrodynamic fluctuations contribute significantly to the measured charge correlations in high-energy heavy-ion collisions. The sensitivity of the balance function to the value of the charge diffusion coefficient $D$ allows us to estimate the typical value of this coefficient in the crossover region to be rather small, of the order of ${\left(2\pi T\right)}^{-1}$, characteristic of a strongly coupled plasma.

The $\mathrm{UCN}\tau$ experiment is designed to measure the lifetime ${\tau }_n$ of the free neutron by trapping ultracold neutrons (UCN) in a magneto-gravitational trap. An asymmetric bowl-shaped NdFeB magnet Halbach array confines low-field-seeking UCN within the apparatus, and a set of electromagnetic coils in a toroidal geometry provides a background “holding” field to eliminate depolarization-induced UCN loss caused by magnetic field nodes. We present a measurement of the storage time ${\tau }_{\text{store}}$ of the trap by storing UCN for various times and counting the survivors. The data are consistent with a single exponential decay, and we find ${\tau }_{\text{store}}=860\pm 19$ s, within $1\sigma$ of current global averages for ${\tau }_n$. The storage time with the holding field deactivated is found to be ${\tau }_{\text{store}}=470\pm 160$ s; this decreased storage time is due to the loss of UCN, which undergo Majorana spin flips while being stored. We discuss plans to increase the statistical sensitivity of the measurement and investigate potential systematic effects.

The hot QCD matter produced in any heavy ion collision with a nonzero impact parameter is produced within a strong magnetic field. We study the imprint that these fields leave on the azimuthal distributions and correlations of the produced charged hadrons. The magnetic field is time dependent and the medium is expanding, which leads to the induction of charged currents owing to the combination of Faraday and Hall effects. We find that these currents result in a charge-dependent directed flow $v_1$ that is odd in rapidity and odd under charge exchange. It can be detected by measuring correlations between the directed flow of charged hadrons at different rapidities, $\langle v_1^{\pm }\left(y_1\right)v_1^{\pm }\left(y_2\right)\rangle$.

Background: Much less is known about neutron structure than that of the proton due to the absence of free neutron targets. Neutron information is usually extracted from data on nuclear targets such as deuterium, requiring corrections for nuclear binding and nucleon off-shell effects. These corrections are model dependent and have significant uncertainties, especially for large values of the Bjorken scaling variable $x$. As a consequence, the same data can lead to different conclusions, for example, about the behavior of the $d$ quark distribution in the proton at large $x$.

Purpose: The Barely Off-shell Nucleon Structure experiment at Jefferson Lab measured the inelastic electron-deuteron scattering cross section, tagging spectator protons in coincidence with the scattered electrons. This method reduces nuclear binding uncertainties significantly and has allowed for the first time a (nearly) model-independent extraction of the neutron structure function $F_2(x,Q^2)$ in the resonance and deep-inelastic regions.

Method: A novel compact radial time projection chamber was built to detect protons with momentum between 70 and 150 MeV$/c$ and over a nearly $4\pi$ angular range. For the extraction of the free-neutron structure function $F_2^n$, spectator protons at backward angles ($>{100}^{\circ }$ relative to the momentum transfer) and with momenta below 100 MeV$/c$ were selected, ensuring that the scattering took place on a nearly free neutron. The scattered electrons were detected with Jefferson Lab's CLAS spectrometer, with data taken at beam energies near 2, 4, and 5 GeV.

Results: The extracted neutron structure function $F_2^n$ and its ratio to the inclusive deuteron structure function $F_2^d$ are presented in both the resonance and the deep-inelastic regions for momentum transfer squared $Q^2$ between 0.7 and 5 GeV${}^2/c^2$, invariant mass $W$ between 1 and 2.7 GeV$/c^2$, and Bjorken $x$ between 0.25 and 0.6 (in the deep-inelastic scattering region). The dependence of the semi-inclusive cross section on the spectator proton momentum and angle is investigated, and tests of the spectator mechanism for different kinematics are performed.

Conclusions: Our data set on the structure function ratio $F_2^n/F_2^d$ can be used to study neutron resonance excitations, test quark-hadron duality in the neutron, develop more precise parametrizations of structure functions, and investigate binding effects (including possible mechanisms for the nuclear EMC effect) and provide a first glimpse of the asymptotic behavior of $d/u$ at $x\to 1$.

Background: Quasi dynamical symmetry (QDS) and partial dynamical symmetry (PDS) play an important role in the understanding of complex systems. Up to now these symmetry concepts have been considered to be unrelated.

Purpose: The goal of this paper is to establish a link between PDS and QDS and find an empirical manifestation.

Methods: Quantum number fluctuations and the intrinsic state formalism are used within the framework of the interacting boson model of nuclei.

Results: A previously unrecognized region of the parameter space of the interacting boson model that has both O(6) PDS (purity) and SU(3) QDS (coherence) in the ground band is established. Many rare-earth nuclei approximately satisfying both symmetry requirements are identified.

Conclusions: PDS is more abundant than previously recognized and can lead to a QDS of an incompatible symmetry.

Background: Heavy-flavor modification in relativistic $p\left(d\right)+A$ collisions are sensitive to different kinds of strong-interaction physics ranging from modifications of the nuclear wave function to initial- and final-state energy loss. Modifications to single heavy-flavor particles and their decay leptons at midrapidity and forward rapidity are well established at the Relativistic Heavy Ion Collider (RHIC).

Purpose: This paper presents measurements of azimuthal correlations of electron-muon pairs produced from heavy-flavor decays, primarily $c\overline{c}$, in $\sqrt{s_{{}_{NN}}}=200$ GeV $p+p$ and $d+\mathrm{Au}$ collision using the PHENIX detector at RHIC. The electrons are measured at midrapidity while the muons in the pair are measured at forward rapidity, defined as the direction of the deuteron beam, in order to utilize the deuteron to probe low-$x$ partons in the gold nucleus.

Methods: This analysis uses the central spectrometer arms for electron identification and forward spectrometer arms for muon identification. Azimuthal correlations are built in all sign combinations for $e\text{−}\mu$ pairs. Subtracting the like-sign yield from the unlike-sign yield removes the correlations from light flavor decays and conversions.

Results: Comparing the $p+p$ results with several different Monte Carlo event generators, we find the results are consistent with a total charm cross section ${\sigma }_{c\overline{c}}=538\pm 46$ (stat) $\pm$ 197 (data syst) $\pm$ 174 (model syst) $\mu$b. These generators also indicate that the back-to-back peak at $\Delta \varphi =\pi$ is dominantly from the leading-order contributions (gluon fusion), while higher-order processes (flavor excitation and gluon splitting) contribute to the yield at all $\Delta \varphi$. We observe a suppression in the pair yield per collision in $d+\mathrm{Au}$. We find the pair yield suppression factor for $2.7<\Delta \varphi <3.2$ rad is $J_{dA}=0.433\pm 0.087$ (stat) $\pm$ 0.135 (syst).

Conclusions: The $e\text{−}\mu$ pairs result from partons at $x_{\mathrm{Au}}\sim {10}^{-2}$ at $Q^2=10$ GeV/$c^2$ at the edge of the shadowing region. The pair suppression indicates modification to $c\overline{c}$ pairs for these kinematics in the cold nuclear medium at RHIC.

In the present work we were able to synthesize and measure with high accuracy the production cross sections of more than 190 heavy neutron-rich nuclei by the in-flight fragmentation of relativistic ${}^{208}$Pb projectiles, 26 of which were produced for the first time. This work has shown that the $N=126$ region far below the doubly magic ${}^{208}$Pb has become accessible experimentally and represents a step further towards the study of heavy neutron-rich nuclei approaching the r-process waiting point at $A=195$.

A systematic analysis of low-lying quadrupole and octupole collective states is presented based on the microscopic energy density functional framework. By mapping the deformation constrained self-consistent axially symmetric mean-field energy surfaces onto the equivalent Hamiltonian of the $sdf$ interacting boson model (IBM), that is, onto the energy expectation value in the boson condensate state, the Hamiltonian parameters are determined. The study is based on the global relativistic energy density functional DD-PC1. The resulting IBM Hamiltonian is used to calculate excitation spectra and transition rates for the positive- and negative-parity collective states in four isotopic chains characteristic for two regions of octupole deformation and collectivity: Th, Ra, Sm, and Ba. Consistent with the empirical trend, the microscopic calculation based on the systematics of ${\beta }_2$-${\beta }_3$ energy maps, the resulting low-lying negative-parity bands and transition rates show evidence of a shape transition between stable octupole deformation and octupole vibrations characteristic for ${\beta }_3$-soft potentials.

Background: Above the nucleon resonance region, the $N(e,e^{\prime }{\pi }^{\pm })N^{\prime }$ data cannot be explained by conventional hadronic models. For example, the observed magnitude of the transverse cross section is significantly underestimated in a framework with Reggeized background amplitudes.

Purpose: Develop a phenomenological framework for the $N(e,e^{\prime }{\pi }^{\pm })N^{\prime }$ reaction at high invariant mass $W$ and deep photon virtuality $Q^2$.

Method: Building on the work of Kaskulov and Mosel [Phys. Rev. C 81, 045202 (2010)], a gauged pion-exchange current is introduced with a running cutoff energy for the proton electromagnetic transition form factor. A new transition form factor is proposed. It respects the correct on-shell limit, has a simple physical interpretation, and reduces the number of free parameters by one.

Results: A study of the $W$ dependence of the $N(e,e^{\prime }{\pi }^{\pm })N^{\prime }$ lends support for the newly proposed transition form factor. In addition, an improved description of the separated and unseparated cross sections at $-t\lesssim 0.5{\text{GeV}}^2$ is obtained. The predictions overshoot the measured unseparated cross sections for $-t>0.5{\text{GeV}}^2$. Introducing a strong hadronic form factor in the Reggeized background amplitudes brings the calculations considerably closer to the high $-t$ data.

Conclusions: Hadronic models corrected for resonance-parton duality describe the separated pion electroproduction cross sections above the resonance region reasonably well at low $-t$. In order to validate the applicability of these models at high $-t$, separated cross sections are needed. These are expected to provide a more profound insight into the relevant reaction mechanisms.

Background: Presolar meteoritic graphite grains from supernovas show spatially correlated excesses in ${}^{15}\mathrm{N}$ and ${}^{18}\mathrm{O}$. These excesses signal the helium-rich layers of supernova ejecta as important source material for the grains.

Purpose: Elucidate the explosive nucleosynthesis of ${}^{15}\mathrm{N}$ in massive stars, especially during shock passage through the helium-rich stellar layers.

Method: A simple but realistic model of shock passage through the outer layers of exploding massive stars is used to follow the important ${}^{15}\mathrm{N}$ nucleosynthesis production pathways and their sensitivity to explosion energy and governing reaction rates in a particular stellar model. All calculations are performed with open-source, freely available codes.

Results: Recent reaction rate updates tend to decrease by $\sim 4\times$ the explosive helium-burning yield of ${}^{15}\mathrm{N}$ relative to some commonly used stellar model outputs.

Conclusions: Neutron-capture reactions on ${}^{18}\mathrm{F}$ play an important role in the explosive production of ${}^{15}\mathrm{N}$ in helium-rich stellar layers. This neutron-induced nucleosynthesis is likely connected to that of other isotopic signatures in presolar supernova grains. The Supplemental Material provides instructions that interested readers can follow for their own calculations of explosive nucleosynthesis and nuclear reaction rate sensitivities.

Background: The sub-barrier elastic scattering of heavy ions is discussed.

Purpose: We investigate the sensitivity of the Mott cross section to the short-range nuclear interaction.

Method: The conventional scattering theory of identical particles is used to discuss the disappearance of Mott oscillations at a certain critical value of the Sommerfeld parameter. This, universal, phenomenon is called “transverse isotropy”.

Results: It is found that a significant modification of the transverse isotropy arises from the inclusion of the short-range nuclear interaction even at the sub-barrier energies considered. Comparison with existing data on the $\alpha +\alpha$ system is made.

Conclusions: We suggest that the study of this lack of Mott oscillation, namely the “transverse isotropy” is a potentially useful mean to study the nuclear interaction.

Background: The response of neutron star matter to weak probes determines the neutrino mean free path, the knowledge of which is required for the description of neutron star cooling.

Purpose: We compute the response of cold neutron matter to neutral-current interactions, to determine the mean free path of low-energy neutrinos.

Methods: Our calculations have been carried out using an effective interaction and effective operators consistently derived within the formalism of correlated basis functions. To check the accuracy of the calculation, we have also employed the Landau theory of normal Fermi liquids and the auxiliary field diffusion Monte Carlo approach.

Results: The neutrino mean free path obtained from the calculated responses turns out to be strongly affected by both short- and long-range correlations, leading to a sizable increase with respect to the prediction of the Fermi gas model.

Conclusions: Our results show that for a realistic description of the neutron matter response and of the neutrino mean free path both long- and short-range correlations need to be taken into account.

Background: Momentum distributions of individual nucleons and nucleon pairs reflect features of the short-range structure of nuclei and provide useful insights into various reactions on nuclei, such as $(e,e^{\prime }p)$ and $(e,e^{\prime }pp/pn)$ electrodisintegration processes or neutrino-nucleus interaction experiments.

Purpose: To provide the nuclear physics community with the results (available online) of a systematic study of single-nucleon momentum distributions and nucleon-pair and nucleon-cluster momentum distributions for $A\le 12$ nuclei.

Method: The realistic Argonne $v_{18}$ two-nucleon and Urbana X three-nucleon potentials are used to generate accurate variational Monte Carlo wave functions for the $A\le 12$ nuclei; quantum Monte Carlo methods are used to calculate the momentum distributions.

Results: Single-nucleon distributions are given, broken down into proton and neutron components and spin-up and spin-down components where appropriate. Nucleon-pair momentum distributions are given either in pair spin and isospin ST projection or for $pp$, $pn$, and $nn$ pairs. Nucleon-cluster momentum distributions include $dp$ in ${}^3$He, $tp$ and $dd$ in ${}^4$He, $\alpha d$ in ${}^6$Li, $\alpha t$ in ${}^7$Li, and $\alpha \alpha$ in ${}^8$Be.

Conclusions: The momentum distributions exhibit common characteristic shapes, with tensor correlations (or lack thereof) playing a dominant role in the 1.5–3 fm${}^{-1}$ range, while spin-isospin correlations dominate at higher momenta.