Background: Parity-violating elastic electron-nucleon scattering at low momentum transfer allows one to access the nucleon's weak charge, the vector coupling of the $Z$-boson to the nucleon. In the Standard Model and at tree level, the weak charge of the proton is related to the weak mixing angle and accidentally suppressed, $Q_W^{p,\mathrm{tree}}=1-4{sin}^2{\theta }_W\approx 0.07$. Modern experiments aim at extracting $Q_W^p$ at $\sim 1\%$ accuracy. Similarly, parity nonconservation in atoms allows to access the weak charge of atomic nuclei.

Purpose: We consider a novel class of radiative corrections due to the exchange of two photons, with parity violation in the hadronic/nuclear system. These corrections are prone to long-range interactions and may affect the extraction of ${sin}^2{\theta }_W$ from the experimental data at the relevant level of precision.

Methods: The two-photon exchange contribution to the parity-violating electron-proton scattering amplitude is studied in the framework of forward dispersion relations. We address the general properties of the parity-violating forward Compton scattering amplitude and use relativistic chiral perturbation theory to provide the first field-theoretical proof that it obeys a superconvergence relation.

Results: We show that the significance of this new correction increases with the beam energy in parity-violating electron scattering, but the superconvergence relation protects the formal definition of the weak charge as a limit at zero-momentum transfer and zero energy. We evaluate the new correction in a hadronic model with pion loops and the $\mathrm{\Delta }\left(1232\right)$ resonance, supplemented with a high-energy contribution. For the kinematic conditions of existing and upcoming experiments we show that two-photon exchange corrections with hadronic or nuclear parity violation do not pose a problem for the interpretation of the data in terms of the weak mixing angle at the present level of accuracy.

Conclusions: Two-photon exchange in presence of hadronic or nuclear parity violation gives rise to long-range parity-violating interactions. Depending on the kinematic conditions and precision goal, this novel correction may affect the extraction of weak charges from experiments with atoms and electron scattering.

A striking shape was recently observed for the endoplasmic reticulum, a cellular organelle consisting of stacked sheets connected by helical ramps [Terasaki et al., Cell 154, 285 (2013)]. This shape is interesting both for its biological function, to synthesize proteins using an increased surface area for ribosome factories, and its geometric properties that may be insensitive to details of the microscopic interactions. In the present work, we find very similar shapes in our molecular dynamics simulations of the nuclear pasta phases of dense nuclear matter that are expected deep in the crust of neutron stars. There are dramatic differences between nuclear pasta and terrestrial cell biology. Nuclear pasta is 14 orders of magnitude denser than the aqueous environs of the cell nucleus and involves strong interactions between protons and neutrons, while cellular-scale biology is dominated by the entropy of water and complex assemblies of biomolecules. Nonetheless, the very similar geometry suggests both systems may have similar coarse-grained dynamics and that the shapes are indeed determined by geometrical considerations, independent of microscopic details. Many of our simulations self-assemble into flat sheets connected by helical ramps. These ramps may impact the thermal and electrical conductivities, viscosity, shear modulus, and breaking strain of neutron star crust. The interaction we use, with Coulomb frustration, may provide a simple model system that reproduces many biologically important shapes.

In high-energy collisions involving small nuclei ($p+p$ or $x+\text{Au}$ collisions where $x=p, d$, or ${}^3\mathrm{He}$) the fluctuating size, shape, and internal gluonic structure of the nucleon is shown to have a strong effect on the initial size and shape of the fireball of new matter created in the collision. A systematic study of the eccentricity coefficients describing this initial fireball state for several semirealistic models of nucleon substructure and for several practically relevant collision systems involving small nuclei is presented. The key importance of multiplicity fluctuations in such systems is pointed out. Our results show large differences from expectations based on conventional Glauber model simulations of the initial state created in such collisions.

We use linear viscous hydrodynamics to describe the energy and momentum deposited by a fast moving parton in a quark gluon plasma. This energy-momentum is in turn used to compute the probability density for the production of soft partons by means of the Cooper-Frye formula. We use this probability density to render manifest a relation between the average transverse momentum given to the fast moving parton from the medium $\widehat{q}$, the shear viscosity to entropy density ratio $\eta /s$, and the energy lost by the fast moving parton $\mathrm{\Delta }E$ in an expanding medium under similar conditions to those generated in nucleus-nucleus collisions at the CERN Large Hadron Collider. We find that $\widehat{q}$ increases linearly with $\mathrm{\Delta }E$ for both trigger and away side partons that have been produced throughout the medium. On the other hand, $\eta /s$ is more stable with $\mathrm{\Delta }E$. We also study how these transport coefficients vary with the geometrical location of the hard scattering that produces the fast moving partons. The behavior of $\widehat{q}$, with $\mathrm{\Delta }E$, is understood as arising from the length of medium the parton traverses from the point where it is produced. However, since $\eta /s$ is proportional to the ratio of the length of medium traversed by the fast parton and the average number of scatterings it experiences, it has a milder dependence on the energy it loses. This study represents a tool to obtain a direct connection between transport coefficients and the description of in-medium energy loss within a linear viscous hydrodynamical evolution of the bulk.

The ${}^{14}\mathrm{N}(p,\gamma ){}^{15}\mathrm{O}$ reaction regulates the power generated by the CN cycle and thus impacts the structure and evolution of every star at some point in its life. The lowest positive-energy resonance in this reaction is located at $E_r^{\mathrm{c}.\mathrm{m}.}=259$ keV, too high in energy to strongly influence quiescent stellar burning. However, the strength of this resonance is used as a cross-section normalization for lower-energy measurements of this reaction. We report on new measurements of the energy, strength, and $\gamma$-ray branching ratios for the 259-keV resonance, using different detection and data-analysis schemes. We have also reevaluated previous results, where possible. Our new recommended strength of $\omega \gamma =12.6\left(3\right)$ meV is in agreement with the previous value of 13.1(6) meV, but is more precise and thus provides a more reliable normalization for low-energy $(p,\gamma )$ measurements.

Dilepton production in heavy-ion collisions at collider energies—i.e., for the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC)—is studied within an approach that uses coarse-grained transport simulations to calculate thermal dilepton emission applying in-medium spectral functions from hadronic many-body theory and partonic production rates based on lattice calculations. The microscopic output from the Ultrarelativistic Quantum Molecular Dynamics (UrQMD) model is hereby put on a grid of space-time cells, which makes it possible to extract the local temperature and chemical potential in each cell via an equation of state. The resulting dilepton spectra are in good agreement with the experimental results for the range of energies available at RHIC, $\sqrt{s_{NN}}=19.6–200\mathrm{GeV}$. The comparison of the data with the outcome from the coarse-grained UrQMD simulations shows that the newest measurements by the PHENIX and STAR Collaborations are consistent and that the low-mass spectra can be described by a cocktail of hadronic decay contributions together with thermal emission from broadened vector-meson spectral functions and from the quark-gluon plasma phase. Predictions for dilepton results at LHC energies show no significant change of the spectra as compared to RHIC, but a higher fraction of thermal contribution and harder slopes of the transverse-momentum distributions owing to the higher temperatures and flow obtained.

Background: Lorentz invariance is key in our understanding of nature, yet relatively few experiments have tested Lorentz invariance in weak interactions.

Purpose: Our goal is to obtain limits on Lorentz-invariance violation in weak interactions, in particular rotational invariance in $\beta$ decay.

Method: We search for a dependence of the lifetime of ${}^{20}\mathrm{Na}$ nuclei on the nuclear spin direction. Such directional dependence would be evidence for Lorentz-invariance violation in weak interactions. A difference in lifetime between nuclei that are polarized in the east and west direction is searched for. This difference is maximally sensitive to the rotation of the Earth, while the sidereal dependence is free from most systematic errors.

Results: The experiment sets a limit of $2\times {10}^{-4}$ at 90% C.L. on the amplitude of the sidereal variation of the relative lifetime differences, an improvement by a factor 15 compared to an earlier result.

Conclusions: No significant violation of Lorentz invariance is found. The result sets limits on parameters of theories describing Lorentz-invariance violation.

Research efforts ranging from studies of solid helium to searches for a neutron electric dipole moment require isotopically purified helium with a ratio of ${}^3\mathrm{He}$ to ${}^4\mathrm{He}$ at levels below that which can be measured using traditional mass spectroscopy techniques. We demonstrate an approach to such a measurement using accelerator mass spectroscopy, reaching the ${10}^{-14}$ level of sensitivity, several orders of magnitude more sensitive than other techniques. Measurements of ${}^3\mathrm{He}/{}^4\mathrm{He}$ in samples relevant to the measurement of the neutron lifetime indicate the need for substantial corrections. We also argue that there is a clear path forward to sensitivity increases of at least another order of magnitude.

We explore the use of mean-field models to approximate microscopic nuclear equations of state derived from chiral effective field theory across the densities and temperatures relevant for simulating astrophysical phenomena such as core-collapse supernovae and binary neutron star mergers. We consider both relativistic mean-field theory with scalar and vector meson exchange as well as energy density functionals based on Skyrme phenomenology and compare to thermodynamic equations of state derived from chiral two- and three-nucleon forces in many-body perturbation theory. Quantum Monte Carlo simulations of symmetric nuclear matter and pure neutron matter are used to determine the density regimes in which perturbation theory with chiral nuclear forces is valid. Within the theoretical uncertainties associated with the many-body methods, we find that select mean-field models describe well microscopic nuclear thermodynamics. As an additional consistency requirement, we study as well the single-particle properties of nucleons in a hot/dense environment, which affect e.g., charged-current weak reactions in neutron-rich matter. The identified mean-field models can be used across a larger range of densities and temperatures in astrophysical simulations than more computationally expensive microscopic models.

Recent hydrodynamical simulations of core-collapse supernova (CCSN) evolution have highlighted the importance of thorough control over the microscopic physics responsible for such internal processes as neutrino heating. In particular, it has been suggested that modifications to the neutrino-nucleon elastic cross section can potentially play a crucial role in producing successful CCSN explosions. One possible source of such corrections can be found in a nonzero value for the nucleon's strange helicity content $\mathrm{\Delta }s$. In the present analysis, however, we show that theoretical and experimental progress over the past decade has suggested a comparatively small magnitude for $\mathrm{\Delta }s$, such that its sole effect is not sufficient to provide the physics leading to CCSN explosions.

The Q3D magnetic spectrograph at the Maier-Leibnitz-Laboratorium of the Ludwig-Maximilians-Universität München and the Technische Universität München (Garching, Germany), was used to study the ${}^{208}\mathrm{Pb}\left(\mathit{p},{\mathit{p}}^{\prime }\right)$, ${}^{206,207,208}\mathrm{Pb}\left(\mathit{d},\mathit{p}\right)$, and ${}^{208}\mathrm{Pb}\left(\mathit{d},{\mathit{d}}^{\prime }\right)$ reactions. One hundred fifty-one states at $E_x<6.20\mathrm{MeV}$ in ${}^{208}\mathrm{Pb}$ are identified and spin and parity assigned. Four states are newly identified and new spins and/or parities are assigned to 25 states. Tentative spin assignments are done to five states at $5.90<E_x<6.10\mathrm{MeV}$. Nearly 50 levels below $E_x=6.20\mathrm{MeV}$ listed by the Nuclear Data Sheets as of 2007 are recognized to be nonexistent or doubly placed. The schematic shell model describing one-particle–one-hole configurations without residual interaction is extended by including two-particle–two-hole configurations. The number of configurations thus predicted at $E_x<6.20\mathrm{MeV}$ nearly agrees with the number of states identified. Several states with dominant two-particle–two-hole configurations are identified. New isobaric analog resonances in ${}^{209}\mathrm{Bi}$ with two-particle–one-hole structure are discovered at $E^{\mathrm{res}}=17.6\mathrm{MeV}$. The excitation energies of 70 states with unnatural parity at $E_x<6.20\mathrm{MeV}$ are found to agree within about 200 keV with one-particle–one-hole configurations predicted by the extended schematic shell model. In contrast, the excitation energies of about 20 natural parity states are more than 0.5 MeV lower than predicted, demonstrating the residual interaction among the configurations to be much larger for natural parity than for unnatural parity.

Background: Multinucleon transfer (MNT) and quasifission (QF) processes are dominant processes in low-energy collisions of two heavy nuclei. They are expected to be useful to produce neutron-rich unstable nuclei. Nuclear dynamics leading to these processes depends sensitively on nuclear properties such as deformation and shell structure.

Purpose: We elucidate reaction mechanisms of MNT and QF processes involving heavy deformed nuclei, making detailed comparisons between microscopic time-dependent Hartree-Fock (TDHF) calculations and measurements for the ${}^{64}\text{Ni}+{}^{238}\text{U}$ reaction.

Methods: Three-dimensional Skyrme-TDHF calculations are performed. Particle-number projection method is used to evaluate MNT cross sections from the TDHF wave function after collision.

Results: Fragment masses, total kinetic energy (TKE), scattering angle, contact time, and MNT cross sections are investigated for the ${}^{64}\text{Ni}+{}^{238}\text{U}$ reaction. They show reasonable agreements with measurements. At small impact parameters, collision dynamics depends sensitively on the orientation of deformed ${}^{238}\text{U}$. In tip (side) collisions, we find a larger (smaller) TKE and a shorter (longer) contact time. In tip collisions, we find a strong influence of quantum shells around ${}^{208}\text{Pb}$.

Conclusions: It is confirmed that the TDHF calculations reasonably describe both MNT and QF processes in the ${}^{64}\text{Ni}+{}^{238}\text{U}$ reaction. Analyses of this system indicate the significance of the nuclear structure effects such as deformation and quantum shells in nuclear reaction dynamics at low energies.

The transition strengths of the first-excited $2^{+}$ states and deformation lengths of the nuclei ${}^{30}\mathrm{Ne}$ and ${}^{36}\mathrm{Mg}$ were determined via Coulomb- and nuclear-force-dominated inelastic scattering at intermediate energies. Beams of these exotic nuclei were produced at the RIKEN Radioactive Isotope Beam Factory and were incident on lead and carbon targets at energies above 200 MeV/$u$. Absolute excitation cross sections on the lead target yielded reduced transition probabilities of 0.0277(79) and 0.0528(121) $e^2{\mathrm{b}}^2$, while the measurements with the carbon target revealed nuclear deformation lengths of ${\delta }_N=1.98$(11) and 1.93(11) fm for ${}^{30}\mathrm{Ne}$ and ${}^{36}\mathrm{Mg}$, respectively. Corresponding quadrupole deformation parameters of ${\beta }_2\sim 0.5$ from the two probes were found comparable in magnitude, showing no indication for a reduction in deformation along isotopic and isotonic chains from ${}^{32}\mathrm{Mg}$ towards the neutron drip-line. Comparisons to shell-model calculations illustrate the importance of neutron excitations across the $N=20$ shell for ${}^{30}\mathrm{Ne}$ and suggest that shallow maximums of collectivity may occur around $N=22$ and 24 along the neon and magnesium isotopic chains, respectively.

The technique of accelerator mass spectrometry (AMS), offering a complementary tool for sensitive studies of key reactions in nuclear astrophysics, was applied for measurements of the ${}^{13}\mathrm{C}\left(n,\gamma \right){}^{14}\mathrm{C}$ and the ${}^{14}\mathrm{N}\left(n,p\right){}^{14}\mathrm{C}$ cross sections, which act as a neutron poison in $s$-process nucleosynthesis. Solid samples were irradiated at Karlsruhe Institute of Technology with neutrons closely resembling a Maxwell-Boltzmann distribution for $kT=25$ keV, and also at higher energies between $E_n=123$ and 182 keV. After neutron irradiation the produced amount of ${}^{14}\mathrm{C}$ in the samples was measured by AMS at the Vienna Environmental Research Accelerator (VERA) facility. For both reactions the present results provide important improvements compared to previous experimental data, which were strongly discordant in the astrophysically relevant energy range and missing for the comparably strong resonances above 100 keV. For ${}^{13}\mathrm{C}\left(n,\gamma \right)$ we find a four times smaller cross section around $kT=25$ keV than a previous measurement. For ${}^{14}\mathrm{N}\left(n,p\right)$, the present data suggest two times lower cross sections between 100 and 200 keV than had been obtained in previous experiments and data evaluations. The effect of the new stellar cross sections on the $s$ process in low-mass asymptotic giant branch stars was studied for stellar models of $2M_{\odot }$ initial mass, and solar and $1/{10}^{\mathrm{th}}$ solar metallicity.

Spectroscopy of a ${}_{\mathrm{\Lambda }}{}^{10}\mathrm{Be}$ hypernucleus was carried out at JLab Hall C using the $(e,e^{\prime }{K}^{+})$ reaction. A new magnetic spectrometer system (SPL+HES+HKS), specifically designed for high resolution hypernuclear spectroscopy, was used to obtain an energy spectrum with a resolution of $\sim 0.78$ MeV (FWHM). The well-calibrated spectrometer system of the present experiment using $p(e,e^{\prime }{K}^{+})\mathrm{\Lambda },{\mathrm{\Sigma }}^0$ reactions allowed us to determine the energy levels; and the binding energy of the ground-state peak (mixture of $1^{-}$ and $2^{-}$ states) was found to be $B_{\mathrm{\Lambda }}=8.55\pm 0.07\left(\mathrm{stat}.\right)\pm 0.11\left(\mathrm{sys}.\right)$ MeV. The result indicates that the ground-state energy is shallower than that of an emulsion study by about 0.5 MeV which provides valuable experimental information on the charge symmetry breaking effect in the $\mathrm{\Lambda }N$ interaction.

We introduce an event-by-event perturbative-QCD + saturation + hydro (“EKRT”) framework for ultrarelativistic heavy-ion collisions, where we compute the produced fluctuating QCD-matter energy densities from next-to-leading-order perturbative QCD using a saturation conjecture to control soft-particle production and describe the space-time evolution of the QCD matter with dissipative fluid dynamics, event by event. We perform a simultaneous comparison of the centrality dependence of hadronic multiplicities, transverse momentum spectra, and flow coefficients of the azimuth-angle asymmetries against the LHC and RHIC measurements. We compare also the computed event-by-event probability distributions of relative fluctuations of elliptic flow and event-plane angle correlations with the experimental data from $\mathrm{Pb}+\mathrm{Pb}$ collisions at the LHC. We show how such a systematic multienergy and multiobservable analysis tests the initial-state calculation and the applicability region of hydrodynamics and, in particular, how it constrains the temperature dependence of the shear viscosity-to-entropy ratio of QCD matter in its different phases in a remarkably consistent manner.

Recent work has provided the means to rigorously determine properties of superhadronic matter from experimental data through the application of broad scale modeling of high-energy nuclear collisions within a Bayesian framework. These studies have provided unprecedented statistical inferences about the physics underlying nuclear collisions by virtue of simultaneously considering a wide range of model parameters and experimental observables. Notably, this approach has been used to constrain both the QCD equation of state and the shear viscosity above the quark-hadron transition. Although the inferences themselves have a clear meaning, the complex nature of the relationships between model parameters and observables has remained relatively obscure. We present here a novel extension of the standard Bayesian Markov-chain Monte Carlo approach that allows for the quantitative determination of how inferences of model parameters are driven by experimental measurements and their uncertainties. This technique is then applied in the context of heavy-ion collisions in order to explore previous results in greater depth. The resulting relationships are useful for identifying model weaknesses, prioritizing future experimental measurements, and, most importantly, developing an intuition for the roles that different observables play in constraining our understanding of the underlying physics.

We report on the measurement of freeze-out radii for pairs of identical-charge pions measured in Pb-Pb collisions at $\sqrt{s_{\mathit{NN}}}=2.76$ TeV as a function of collision centrality and the average transverse momentum of the pair $k_{\mathrm{T}}$. Three-dimensional sizes of the system (femtoscopic radii), as well as direction-averaged one-dimensional radii are extracted. The radii decrease with $k_{\mathrm{T}}$, following a power-law behavior. This is qualitatively consistent with expectations from a collectively expanding system, produced in hydrodynamic calculations. The radii also scale linearly with ${\langle d{N}_{\mathrm{ch}}/d\eta \rangle }^{1/3}$. This behavior is compared to world data on femtoscopic radii in heavy-ion collisions. While the dependence is qualitatively similar to results at smaller $\sqrt{s_{\mathit{NN}}}$, a decrease in the ratio $R_{\mathrm{out}}/R_{\mathrm{side}}$ is seen, which is in qualitative agreement with a specific prediction from hydrodynamic models: a change from inside-out to outside-in freeze-out configuration. The results provide further evidence for the production of a collective, strongly coupled system in heavy-ion collisions at the CERN Large Hadron Collider.

${}^{82}\mathrm{Rb}$ is a positron-emitting isotope used in cardiac positron emission tomography (PET) imaging which has been reported to deliver a significantly lower effective radiation dose than analogous imaging isotopes like ${}^{201}\mathrm{Tl}$ and ${}^{99m}\mathrm{Tc}$ sestamibi. High-quality $\beta$-decay data are essential to accurately appraise the total dose received by the patients. A source of ${}^{82}\mathrm{Sr}$ was produced at the Brookhaven Linac Isotope Producer (BLIP), transported to Argonne National Laboratory, and studied with the Gammasphere facility. Significant revisions have been made to the level scheme of ${}^{82}\mathrm{Kr}$ including 12 new levels, 50 new $\gamma$-ray transitions, and the determination of many new spin assignments through angular correlations. These new high-quality data allow a precise reappraisal of the $\beta$-decay strength function and thus the consequent dose received by patients.

Background: Besides their intrinsic nuclear-structure value, nuclear mass models are essential for astrophysical applications, such as $r\text{-process}$ nucleosynthesis and neutron-star structure.

Purpose: To overcome the intrinsic limitations of existing “state-of-the-art” mass models through a refinement based on a Bayesian neural network (BNN) formalism.

Methods: A novel BNN approach is implemented with the goal of optimizing mass residuals between theory and experiment.

Results: A significant improvement (of about 40%) in the mass predictions of existing models is obtained after BNN refinement. Moreover, these improved results are now accompanied by proper statistical errors. Finally, by constructing a “world average” of these predictions, a mass model is obtained that is used to predict the composition of the outer crust of a neutron star.

Conclusions: The power of the Bayesian neural network method has been successfully demonstrated by a systematic improvement in the accuracy of the predictions of nuclear masses. Extension to other nuclear observables is a natural next step that is currently under investigation.