We show that observations of the core temperature of transiently accreting neutron stars combined with observations of an accretion outburst give a lower limit to the neutron star core heat capacity. For the neutron stars in the low mass x-ray binaries KS 1731-260, MXB 1659-29, and XTE J1701-462, we show that the lower limit is a factor of a few below the core heat capacity expected if neutrons and protons in the core are paired, so that electrons provide the dominant contribution to the heat capacity. This limit rules out a core dominated by a quark color-flavor-locked phase, which would have a much lower heat capacity. Future observations of or limits on cooling during quiescence will further constrain the core heat capacity.

The $Q_{\text{EC}}$ value of the superallowed ${\beta }^{+}$ emitter ${}^{42}\mathrm{Sc}$ has been measured with the JYFLTRAP Penning-trap mass spectrometer at the University of Jyväskylä to be 6426.350(53) keV. This result is at least a factor of four more precise than all previous measurements, which were also inconsistent with one another. As a byproduct we determine the excitation energy of the $7^{+}$ isomeric state in ${}^{42}\mathrm{Sc}$ to be 616.771(62) keV, which deviates by $8\sigma$ from the previous measurement.

The deformation parameters $(\beta ,\gamma )$ together with two-quasiparticle excitations are taken into account, for the first time, as coordinates within a symmetry conserving (angular momentum and particle number) generator coordinate method. The simultaneous consideration of collective as well as single-particle degrees of freedom allows us to describe soft and rigid nuclei as well as the transition region in between. We apply the new theory to the study of the spectra and transition probabilities of the ${}^{156–172}\mathrm{Er}$ isotopes with a pairing-plus-quadrupole residual interaction. Good agreement with the experimental results is obtained for most of the observables studied and with the same quality for the very soft and the strongly deformed nuclei.

Background: Symmetries are a powerful way to characterize nuclear wave functions. A true dynamical symmetry, where the Hamiltonian is block-diagonal in subspaces defined by the group, is rare. More likely is a quasidynamical symmetry: states with different quantum numbers (i.e., angular momentum) nonetheless sharing similar group-theoretical decompositions.

Purpose: We use group-theoretical decomposition to investigate backbending, an abrupt change in the moment of inertia along the yrast line, in ${}^{48,49,50}\mathrm{Cr}$: prior mean-field calculations of these nuclides suggest a change from strongly prolate to more spherical configurations as one crosses the backbending and increases in angular momentum.

Methods: We decompose configuration-interaction shell-model wave functions using the SU(2) groups $L$ (total orbital angular momentum) and $S$ (total spin), and the groups SU(3) and SU(4). We do not need a special basis but only matrix elements of Casimir operators, applied with a modified Lanczos algorithm.

Results: We find quasidynamical symmetries, albeit often of a different character above and below the backbending, for each group. While the strongest evolution was in SU(3), the decompositions did not suggest a decrease in deformation. We point out with a simple example that mean-field and SU(3) configurations may give very different pictures of deformation.

Conclusions: Persistent quasidynamical symmetries for several groups allow us to identify the members of a band and to characterize how they evolve with increasing angular momentum, especially before and after backbending.

Background: The possible existence of a phase transition to a ferromagnetic state in neutron matter as origin of the extremely high magnetic fields of neutron stars is still an open issue. Whereas many phenomenological interactions predict this transition at densities accessible in neutron stars, microscopic calculations based on realistic interactions show no indication of it. The existence or non-existence of this transition is a consequence of the different role of nucleon-nucleon correlations in polarized and unpolarized neutron matter. Therefore, to give a definite answer to this issue it is necessary to analyze the behavior of these correlations.

Purpose: Using the Hellmann–Feynman theorem we analyze the contribution of the different terms of the nucleon-nucleon interaction to the spin symmetry energy of neutron matter with the purpose of identifying the nature and role of correlations in polarized and unpolarized neutron matter.

Methods: The analysis is performed within the microscopic Brueckner–Hartree–Fock approach using the Argonne V18 realistic potential plus the Urbana IX three-body force.

Results: Our results show no indication of a ferromagnetic transition as the spin symmetry energy of neutron matter is always an increasing function of density. They show also that the main contribution to it comes from the $S=0$ channel, acting only in non-polarized neutron matter, in particular from the ${}^1{S}_0$ and the ${}^1{D}_2$ partial waves. Three-body forces are found to play a secondary role in the determination of the spin symmetry energy.

Conclusions: By evaluating the kinetic energy difference between the correlated system and the underlying Fermi sea to estimate the importance of correlations in spin-polarized neutron matter, we conclude that non-polarized neutron matter is more correlated than totally polarized one.

Background: In the context of the investigation of the quark gluon plasma produced in heavy-ion collisions, hadrons containing heavy (charm or beauty) quarks play a special role for the characterization of the hot and dense medium created in the interaction. The measurement of the production of charm and beauty hadrons in proton–proton collisions, besides providing the necessary reference for the studies in heavy-ion reactions, constitutes an important test of perturbative quantum chromodynamics (pQCD) calculations. Heavy-flavor production in proton–nucleus collisions is sensitive to the various effects related to the presence of nuclei in the colliding system, commonly denoted cold-nuclear-matter effects. Most of these effects are expected to modify open-charm production at low transverse momenta $\left(p_{\mathrm{T}}\right)$ and, so far, no measurement of $D$-meson production down to zero transverse momentum was available at mid-rapidity at the energies attained at the CERN Large Hadron Collider (LHC).

Purpose: The measurements of the production cross sections of promptly produced charmed mesons in $p$-Pb collisions at the LHC down to $p_{\mathrm{T}}=0$ and the comparison to the results from $pp$ interactions are aimed at the assessment of cold-nuclear-matter effects on open-charm production, which is crucial for the interpretation of the results from Pb-Pb collisions.

Methods: The prompt charmed mesons $D^0,D^{+},D^{*+}$, and $D_s^{+}$ were measured at mid-rapidity in $p$-Pb collisions at a center-of-mass energy per nucleon pair $\sqrt{s_{NN}}=5.02$ TeV with the ALICE detector at the LHC. $D$ mesons were reconstructed from their decays $D^0\to K^{-}{\pi }^{+},D^{+}\to K^{-}{\pi }^{+}{\pi }^{+}, D^{*+}\to D^0{\pi }^{+},D_s^{+}\to \varphi {\pi }^{+}\to K^{-}{K}^{+}{\pi }^{+}$, and their charge conjugates, using an analysis method based on the selection of decay topologies displaced from the interaction vertex. In addition, the prompt $D^0$ production cross section was measured in $pp$ collisions at $\sqrt{s}=7$ TeV and $p$-Pb collisions at $\sqrt{s_{NN}}=5.02$ TeV down to $p_{\mathrm{T}}=0$ using an analysis technique that is based on the estimation and subtraction of the combinatorial background, without reconstruction of the $D^0$ decay vertex.

Results: The production cross section in $pp$ collisions is described within uncertainties by different implementations of pQCD calculations down to $p_{\mathrm{T}}=0$. This allowed also a determination of the total $c\overline{c}$ production cross section in $pp$ collisions, which is more precise than previous ALICE measurements because it is not affected by uncertainties owing to the extrapolation to $p_{\mathrm{T}}=0$. The nuclear modification factor $R_{p\mathrm{Pb}}\left(p_{\mathrm{T}}\right)$, defined as the ratio of the $p_{\mathrm{T}}$-differential $D$ meson cross section in $p$-Pb collisions and that in $pp$ collisions scaled by the mass number of the Pb nucleus, was calculated for the four $D$-meson species and found to be compatible with unity within uncertainties. The results are compared to theoretical calculations that include cold-nuclear-matter effects and to transport model calculations incorporating the interactions of charm quarks with an expanding deconfined medium.

Conclusions: These measurements add experimental evidence that the modification of the $D$-meson transverse momentum distributions observed in Pb–Pb collisions with respect to $pp$ interactions is due to strong final-state effects induced by the interactions of the charm quarks with the hot and dense partonic medium created in ultrarelativistic heavy-ion collisions. The current precision of the measurement does not allow us to draw conclusions on the role of the different cold-nuclear-matter effects and on the possible presence of additional hot-medium effects in $p\text{-Pb}$ collisions. However, the analysis technique without decay-vertex reconstruction, applied on future larger data samples, should provide access to the physics-rich range down to $p_{\mathrm{T}}=0$.

We reexamine the deuteron elastic breakup cross sections on ${}^{12}\mathrm{C}$ and ${}^{10}\mathrm{Be}$ at low incident energies, for which a serious discrepancy between the continuum-discretized coupled-channels (CDCC) method and the Faddeev-Alt-Grassberger-Sandhas (FAGS) theory was pointed out. We show the closed channels neglected in the preceding study affect significantly the breakup cross section calculated with the CDCC method, resulting in good agreement with the result of FAGS theory.

Background: The ${}^{22}\mathrm{Ne}(p,\gamma ){}^{23}\mathrm{Na}$ reaction is the most uncertain process in the neon-sodium cycle of hydrogen burning. At temperatures relevant for nucleosynthesis in asymptotic giant branch stars and classical novae, its uncertainty is mainly due to a large number of predicted but hitherto unobserved resonances at low energy.

Purpose: A new direct study of low-energy ${}^{22}\mathrm{Ne}(p,\gamma ){}^{23}\mathrm{Na}$ resonances has been performed at the Laboratory for Underground Nuclear Astrophysics (LUNA), in the Gran Sasso National Laboratory, Italy.

Method: The proton capture on ${}^{22}\mathrm{Ne}$ was investigated in direct kinematics, delivering an intense proton beam to a ${}^{22}\mathrm{Ne}$ gas target. $\gamma$ rays were detected with two high-purity germanium detectors enclosed in a copper and lead shield suppressing environmental radioactivity.

Results: Three resonances at 156.2 keV [$\omega \gamma =(1.48\pm 0.10)\times {10}^{-7}$ eV], 189.5 keV [$\omega \gamma =(1.87\pm 0.06)\times {10}^{-6}$ eV] and 259.7 keV [$\omega \gamma =(6.89\pm 0.16)\times {10}^{-6}$ eV] proton beam energy, respectively, have been observed for the first time. For the levels at $E_{\mathrm{x}}=8943.5$, 8975.3, and 9042.4 keV excitation energy corresponding to the new resonances, the $\gamma$-decay branching ratios have been precisely measured. Three additional, tentative resonances at 71, 105, and 215 keV proton beam energy, respectively, were not observed here. For the strengths of these resonances, experimental upper limits have been derived that are significantly more stringent than the upper limits reported in the literature.

Conclusions: Based on the present experimental data and also previous literature data, an updated thermonuclear reaction rate is provided in tabular and parametric form. The new reaction rate is significantly higher than previous evaluations at temperatures of 0.08–0.3 GK.

Background: Competing interpretations of the results of a ${}^{30}\mathrm{Mg}$($t,p$)${}^{32}\mathrm{Mg}$ measurement populating the ground state and $0_2^{+}$ state in ${}^{32}\mathrm{Mg}$, both limited to a two-state mixing description, have left an open question regarding the nature of the ${}^{32}\mathrm{Mg}$ ground state.

Purpose: Inspired by recent shell-model calculations, we explore the possibility of a consistent interpretation of the available data for the low-lying $0^{+}$ states in ${}^{32}\mathrm{Mg}$ by expanding the description from two-level to three-level mixing.

Methods: A phenomenological three-level mixing model of unperturbed 0p0h, 2p2h, and 4p4h states is applied to describe both the excitation energies in ${}^{32}\mathrm{Mg}$ and the transfer reaction cross sections.

Results: Within this approach, self-consistent solutions exist that provide good agreement with the available experimental information obtained from the ${}^{30}\mathrm{Mg}$($t,p$)${}^{32}\mathrm{Mg}$ reaction.

Conclusion: The inclusion of the third state, namely the 4p4h configuration, resolves the “puzzle” that results from a two-levelmodel interpretation of the same data. In our analysis, the ${}^{32}\mathrm{Mg}$ ground state emerges naturally as dominated by intruder (2p2h and 4p4h) configurations, at the 95% level.

A quantitative description of the change in ground-state neutron occupancies between ${}^{136}\mathrm{Xe}$ and ${}^{136}\mathrm{Ba}$, the initial and final state in the neutrinoless double-$\beta$ decay of ${}^{136}\mathrm{Xe}$, has been extracted from precision measurements of the cross sections of single-neutron-adding and -removing reactions. Comparisons are made to recent theoretical calculations of the same properties using various nuclear-structure models. These are the same calculations used to determine the magnitude of the nuclear matrix elements for the process, which at present disagree with each other by factors of 2 or 3. The experimental neutron occupancies show some disagreement with the theoretical calculations.

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.