Showing papers on "Nuclear matter published in 2015"

Accurate nuclear radii and binding energies from a chiral interaction

Abstract: With the goal of developing predictive ab initio capability for light and medium-mass nuclei, two-nucleon and three-nucleon forces from chiral effective field theory are optimized simultaneously to low-energy nucleon-nucleon scattering data, as well as binding energies and radii of few-nucleon systems and selected isotopes of carbon and oxygen. Coupled-cluster calculations based on this interaction, named NNLOsat, yield accurate binding energies and radii of nuclei up to Ca-40, and are consistent with the empirical saturation point of symmetric nuclear matter. In addition, the low-lying collective J(pi) = 3(-) states in O-16 and 40Ca are described accurately, while spectra for selected p- and sd-shell nuclei are in reasonable agreement with experiment.

The Equation of State of Hot, Dense Matter and Neutron Stars

Abstract: Recent developments in the theory of pure neutron matter and experiments concerning the symmetry energy of nuclear matter, coupled with recent measurements of high-mass neutron stars, now allow for relatively tight constraints on the equation of state of dense matter. We review how these constraints are formulated and describe the implications they have for neutron stars and core-collapse supernovae. We also examine thermal properties of dense matter, which are important for supernovae and neutron star mergers, but which cannot be nearly as well constrained at this time by experiment. In addition, we consider the role of the equation of state in medium-energy heavy-ion collisions.

Neutrino-driven explosion of a 20 solar-mass star in three dimensions enabled by strange-quark contributions to neutrino–nucleon scattering

Abstract: Interactions with neutrons and protons play a crucial role for the neutrino opacity of matter in the supernova core. Their current implementation in many simulation codes, however, is rather schematic and ignores not only modifications for the correlated nuclear medium of the nascent neutron star, but also free-space corrections from nucleon recoil, weak magnetism, or strange quarks, which can easily add up to changes of several 10% for neutrino energies in the spectral peak. In the Garching supernova simulations with the Prometheus-Vertex code, such sophistications have been included for a long time except for the strange-quark contributions to the nucleon spin, which affect neutral-current neutrino scattering. We demonstrate on the basis of a 20 progenitor star that a moderate strangeness-dependent contribution of to the axial-vector coupling constant can turn an unsuccessful three-dimensional (3D) model into a successful explosion. Such a modification is in the direction of current experimental results and reduces the neutral-current scattering opacity of neutrons, which dominate in the medium around and above the neutrinosphere. This leads to increased luminosities and mean energies of all neutrino species and strengthens the neutrino-energy deposition in the heating layer. Higher nonradial kinetic energy in the gain layer signals enhanced buoyancy activity that enables the onset of the explosion at ~300 ms after bounce, in contrast to the model with vanishing strangeness contributions to neutrino–nucleon scattering. Our results demonstrate the close proximity to explosion of the previously published, unsuccessful 3D models of the Garching group.

Indications for a critical end point in the phase diagram for hot and dense nuclear matter.

Abstract: A finite size scaling analysis of heavy-ion collisions points to the location of the critical end point -- the termination of the first order phase transition line -- in the QCD phase diagram.

Efficient calculation of chiral three-nucleon forces up to N 3 LO for ab initio studies

Abstract: We present a novel framework to decompose three-nucleon forces in a momentum space partial-wave basis. The new approach is computationally much more efficient than previous methods and opens the way to ab initio studies of few-nucleon scattering processes, nuclei and nuclear matter based on higher-order chiral 3N forces. We use the new framework to calculate matrix elements of chiral three-nucleon forces at N2LO and N3LO in large basis spaces and carry out benchmark calculations for neutron matter and symmetric nuclear matter. We also study the size of the individual three-nucleon force contributions for $^3$H. For nonlocal regulators, we find that the sub-leading terms, which have been neglected in most calculations so far, provide important contributions. All matrix elements are calculated and stored in a user-friendly way, such that values of low-energy constants as well as the form of regulator functions can be chosen freely.

Generalized nuclear contacts and momentum distributions

Abstract: The general nuclear contact matrices are defined, taking into consideration all partial waves and finite-range interactions, extending Tan's work on the zero range model. The properties of these matrices are discussed and the relations between the contacts and the one-nucleon and two-nucleon momentum distributions are derived. Using these relations, a new asymptotic connection between the one-nucleon and two-nucleon momentum distributions, describing the two-body short-range correlations in nuclei, is obtained. Using available numerical data, we extract a few connections between the different contacts and verify their relations to the momentum distributions. The numerical data also allows us to identify the main nucleon momentum range affected by two-body short-range correlations. Utilizing these relations and the numerical data, we also verify a previous independent prediction-connecting between the Levinger constant and the contacts. This work provides an important indication for the relevance of the contact formalism to nuclear systems, and should open the path for revealing more useful relations between the contacts and interesting quantities of nuclei and nuclear matter.

Thermodynamics of isospin-asymmetric nuclear matter from chiral effective field theory

Abstract: The density and temperature dependence of the nuclear symmetry free energy is investigated using microscopic two- and three-body nuclear potentials constructed from chiral effective field theory. The nuclear force models and many-body methods are benchmarked to properties of isospin-symmetric nuclear matter in the vicinity of the saturation density as well as the virial expansion of the neutron matter equation of state at low fugacities. The free energy per particle of isospin-asymmetric nuclear matter is calculated assuming a quadratic dependence of the interaction contributions on the isospin asymmetry. The spinodal instability at subnuclear densities is examined in detail.

From holography towards real-world nuclear matter

Abstract: Quantum chromodynamics is notoriously difficult to solve at nonzero baryon density, and most models or effective theories of dense quark or nuclear matter are restricted to a particular density regime and/or a particular form of matter. Here we study dense (and mostly cold) matter within the holographic Sakai-Sugimoto model, aiming at a strong-coupling framework in the wide density range between nuclear saturation density and ultrahigh quark matter densities. The model contains only three parameters, and we ask whether it fulfills two basic requirements of real-world cold and dense matter, a first-order onset of nuclear matter and a chiral phase transition at high density to quark matter. Such a model would be extremely useful for astrophysical applications because it would provide a single equation of state for all densities relevant in a compact star. Our calculations are based on two approximations for baryonic matter---first, an instanton gas and, second, a homogeneous ansatz for the non-Abelian gauge fields on the flavor branes of the model. While the instanton gas shows chiral restoration at high densities but an unrealistic second-order baryon onset, the homogeneous ansatz behaves exactly the other way around. Our study, thus, provides all ingredients that are necessary for a more realistic model and allows for systematic improvements of the applied approximations.

Scaled variance, skewness, and kurtosis near the critical point of nuclear matter

Abstract: The van der Waals (VDW) equation of state predicts the existence of a first-order liquid-gas phase transition and contains a critical point. The VDW equation with Fermi statistics is applied to a description of the nuclear matter. The nucleon number fluctuations near the critical point of nuclear matter are studied. The scaled variance, skewness, and kurtosis diverge at the critical point. It is found that the crossover region of the phase diagram is characterized by the large values of the scaled variance, the almost zero skewness, and the significantly negative kurtosis. The rich structures of the skewness and kurtosis are observed in the phase diagram in the wide region around the critical point, namely, they both may attain large positive or negative values.

Asymmetric nuclear matter in a parity doublet model with hidden local symmetry

Abstract: We construct a model to describe dense hadronic matter at zero and finite temperature, based on the parity doublet model of DeTar and Kunihiro, with including the iso-singlet scalar meson σ as well as ρ and ω mesons. We show that, by including a six-point interaction of σ meson, the model reasonably reproduces the properties of the normal nuclear matter with the chiral invariant nucleon mass m0 in the range from 500 MeV to 900 MeV. Furthermore, we study the phase diagram based on the model, which shows that the value of the chiral condensate drops at the liquid-gas phase transition point and at the chiral phase transition point. We also study asymmetric nuclear matter and find that the first order phase transition for the liquid-gas phase transition disappears in asymmetric matter and that the critical density for the chiral phase transition at non-zero density becomes smaller for larger asymmetry. PACS numbers: 21.65.Cd,21.65.Mn,12.39.Fe I. INTRODUCTION With the advent of next generation radioactive beam facilities isospin asymmetric nuclear matter claims much attention in contemporary nuclear physics. At those facilities we could create terrestrial environment to study dense matter with a large neutron or proton excess through nuclear reactions with radioactive nuclei. Studying nuclear matter is also important to understand the structure of neutron stars [1]. In 2010 and 2013, two neutron stars with twice solar mass were found [2, 3] and many models yielding the soft equation of states (EOS) were excluded. Neutron stars offer very cold and asymmetric dense environment and may have hyperons in the core of the stars. If there are hyperonic degrees of freedom, it is expected that the EOS becomes softer and neutron star mass becomes lighter. Another important astrophysical site for nuclear matter is a hybrid star whose center has quark matter [4]. The properties of asymmetric matter have been investigated in various approaches [5–14]. Very recently liquid-gas and chiral phase transition are studied in a parity doublet model with a six-point scalar interaction in which mesonic fluctuations are included by means of the functional renormalization group [15]. In this work we study isospin asymmetric dense matter in the framework of the parity doublet model (mirror assignment) [16, 17]. The properties of symmetric dense matter such as chiral phase transition were extensively studied in the parity doublet models at zero or finite temperature [18–22]. We extend the parity doublet model by including ρ and ω mesons through the hidden local symmetry and also by adding a six-point interaction of a scalar meson. Here, as a first step, we will not consider hyperonic matter and work within the mean field approximation. We determine our model parameters, except the chiral invariant mass (m0), by performing global fitting to physical inputs (masses and pion decay constant in free space and nuclear matter properties). We then study the equation of state and the phase diagram of dense matter at finite temperature. We find that the predicted slope parameter at the saturation density meets the constraint from heavy ion experiments and neutron star observations (see, e.g. Refs. [23, 24]) and observe that the chiral condensate drops at the chiral and liquid-gas transition points. It is also seen that smaller m0 values prefer smaller critical densities for chiral phase transition. The study of asymmetric matter reveals that the first order nature of the liquid-gas transition disappears in asymmetric matter and the critical densities for the chiral transition become smaller with increasing asymmetries, which are consistent with previous studies. In section II we extend the parity doublet model, and in section III we fix the model parameters. Our results on bulk properties of nuclear matter and density dependence of chiral condensate and nucleon mass are given in section IV. We present the phase diagram of dense (asymmetric) matter in section V. Finally, conclusion and discussion follow in section VI

Nuclear matter equation of state including two-, three-, and four-nucleon correlations

Abstract: Light clusters (mass number $A\ensuremath{\le}4$) in nuclear matter at subsaturation densities are described using a quantum statistical approach to calculate the quasiparticle properties and abundances of light elements. I review the formalism and approximations used and extend it with respect to the treatment of continuum correlations. Virial coefficients are derived from continuum contributions to the partial densities which depend on temperature, densities, and total momentum. The Pauli blocking is modified taking correlations in the medium into account. Both effects of continuum correlations lead to an enhancement of cluster abundances in nuclear matter at higher densities. Based on calculations for $A=2$, estimates for the contributions with $A=3,4$ are given. The properties of light clusters and continuum correlations in dense matter are of interest for nuclear structure calculations, heavy-ion collisions, and astrophysical applications such as the formation of neutron stars in core-collapse supernovae.

Constraining supernova equations of state with equilibrium constants from heavy-ion collisions

Abstract: Cluster formation is a fundamental aspect of the equation of state (EOS) of warm and dense nuclear matter such as can be found in supernovae (SNe). Similar matter can be studied in heavy-ion collisions (HICs). We use the experimental data of Qin et al. [Phys. Rev. Lett. 108, 172701 (2012)] to test calculations of cluster formation and the role of in-medium modifications of cluster properties in SN EOSs. For the comparison between theory and experiment we use chemical equilibrium constants as the main observables. This reduces some of the systematic uncertainties and allows deviations from ideal gas behavior to be identified clearly. In the analysis, we carefully account for the differences between matter in SNe and HICs. We find that, at the lowest densities, the experiment and all theoretical models are consistent with the ideal gas behavior. At higher densities ideal behavior is clearly ruled out and interaction effects have to be considered. The contributions of continuum correlations are of relevance in the virial expansion and remain a difficult problem to solve at higher densities. We conclude that at the densities and temperatures discussed mean-field interactions of nucleons, inclusion of all relevant light clusters, and a suppression mechanism of clusters at high densities have to be incorporated in the SN EOS.

Impact of the equation-of-state-gravity degeneracy on constraining the nuclear symmetry energy from astrophysical observables

Abstract: There is a degeneracy between the equation of state (EOS) of superdense neutron-rich nuclear matter and the strong-field gravity in understanding properties of neutron stars. While the EOS is still poorly known, there are also longstanding ambiguities in choosing Einstein's general relativity (GR) or alternative gravity theories in the not-so-well-tested strong-field regime. Besides the possible appearance of hyperons and new phases, the most uncertain part of the nucleonic EOS is currently the density dependence of nuclear symmetry energy especially at suprasaturation densities. At the same time, the EOS of symmetric nuclear matter (SNM) has been significantly constrained at saturation and suprasaturation densities. To provide information that may help break the EOS-gravity degeneracy, we investigate effects of nuclear symmetry energy within its uncertain range determined by recent terrestrial nuclear laboratory experiments on the gravitational binding energy and space-time curvature of neutron stars within GR and the scalar-tensor subset of alternative gravity models, constrained by recent measurements of the relativistic binary pulsars J1738 + 0333 and J0348 + 0432. In particular, we focus on effects of the following three parameters characterizing the EOS of superdense neutron-rich nucleonic matter: (1) the incompressibility ${K}_{0}$ of SNM, (2) the slope $L$ of nuclear symmetry energy at saturation density, and (3) the high-density behavior of nuclear symmetry energy. We find that the variation of either the density slope $L$ or the high-density behavior of nuclear symmetry energy leads to large changes in both the binding energy and the curvature of neutron stars while effects of varying the more constrained ${K}_{0}$ are negligibly small. The difference in predictions using the GR and the scalar-tensor theory appears only for massive neutron stars, and even then it is significantly smaller than the differences resulting from variations in the symmetry energy. We conclude that, within the scalar-tensor subset of gravity models, the EOS-gravity degeneracy has been broken by the recent relativistic pulsar measurements and that measurements of neutron-star properties sensitive to the compactness constrain mainly the density dependence of the symmetry energy at saturation and suprasaturation densities.

Thermal properties of hot and dense matter with finite range interactions

Abstract: We explore the thermal properties of hot and dense matter using a model that reproduces the empirical properties of isospin symmetric and asymmetric bulk nuclear matter, optical-model fits to nucleon-nucleus scattering data, heavy-ion flow data in the energy range 0.5--2 GeV/$A$, and the largest well-measured neutron star mass of $2{M}_{\ensuremath{\bigodot}}$. This model, which incorporates finite range interactions through a Yukawa-type finite range force, is contrasted with a conventional zero range Skyrme model. Both models predict nearly identical zero-temperature properties at all densities and proton fractions, including the neutron star maximum mass, but differ in their predictions for heavy-ion flow data. We contrast their predictions of thermal properties, including their specific heats, and provide analytical formulas for the strongly degenerate and nondegenerate limits. We find significant differences in the results of the two models for quantities that depend on the density derivatives of nucleon effective masses. We show that a constant value for the ratio of the thermal components of pressure and energy density expressed as ${\mathrm{\ensuremath{\Gamma}}}_{\text{th}}=1+({P}_{\text{th}}/{\ensuremath{\varepsilon}}_{\text{th}})$, often used in simulations of proto-neutron stars and merging compact object binaries, fails to adequately describe results of either nuclear model. The region of greatest discrepancy extends from subsaturation densities to a few times the saturation density of symmetric nuclear matter. Our results suggest alternate approximations for the thermal properties of dense matter that are more realistic.

EQUATION of STATE for NEUTRON STARS with HYPERONS and QUARKS in the RELATIVISTIC HARTREE-FOCK APPROXIMATION

Abstract: We construct the equation of state (EoS) for neutron stars explicitly including hyperons and quarks. Using the quark–meson coupling model with the relativistic Hartree–Fock approximation, the EoS for hadronic matter is derived by taking into account the strange (σ* and ϕ) mesons as well as the light non-strange (σ, ω, ${\boldsymbol{\pi }}$, and ${\boldsymbol{\rho }}$) mesons. Relevant coupling constants are determined to reproduce the experimental data of nuclear matter and hypernuclei in SU(3) flavor symmetry. For quark matter, we employ the MIT bag model with a one-gluon-exchange interaction, and Gibbs criteria for chemical equilibrium in the phase transition from hadrons to quarks. We find that the strange vector (ϕ) meson and the Fock contribution make the hadronic EoS stiff, and that the maximum mass of a neutron star can be consistent with the observed mass of heavy neutron stars even if the coexistence of hadrons and quarks takes place in the core. However, in the present calculation, the transition to pure quark matter does not occur in stable neutron stars. Furthermore, the lower bound of the critical chemical potential of the quark–hadron transition at zero temperature turns out to be around 1.5 GeV in order to be consistent with the recent observed neutron-star data.

Universal parametrization of thermal photon rates in hadronic matter

Abstract: Electromagnetic (EM) radiation off strongly interacting matter created in high-energy heavy-ion collisions (HICs) encodes information on the high-temperature phases of nuclear matter. Microscopic calculations of thermal EM emission rates are usually rather involved and not readily accessible to broad applications in models of the fireball evolution which are required to compare with experimental data. An accurate and universal parametrization of the microscopic calculations is thus key to honing the theory behind the EM spectra. Here we provide such a parametrization for photon emission rates from hadronic matter, including the contributions from in-medium $\ensuremath{\rho}$ mesons (which incorporate effects from baryons and antibaryons), as well as bremsstrahlung from $\ensuremath{\pi}\ensuremath{\pi}$ scattering. Individual parametrizations for each contribution are numerically determined through nested fitting functions for photon energies from 0.2 to 5 GeV in chemically equilibrated matter of temperatures 100--180 MeV and baryon chemical potentials 0--400 MeV. Special care is taken to extent the parametrizations to chemical off-equilibrium as encountered in HICs after chemical freeze-out. This provides a functional description of thermal photon rates within a 20% variation of the microscopically calculated values.

Correlated fermions in nuclei and ultracold atomic gases

Abstract: Background: The high momentum distribution of atoms in two spin-state ultra-cold atomic gases with strong short-range interactions between atoms with different spins, which can be described using Tan's contact, are dominated by short range pairs of different fermions and decreases as $k^{-4}$. In atomic nuclei the momentum distribution of nucleons above the Fermi momentum ($k>k_F \approx 250$ Mev/c) is also dominated by short rangecorrelated different-fermion (neutron-proton) pairs. Purpose: Compare high-momentum unlike-fermion momentum distributions in atomic and nuclear systems. Methods: We show that, for $k>k_F$ MeV/c, nuclear momentum distributions are proportional to that of the deuteron. We then examine the deuteron momentum distributions derived from a wide variety of modern nucleon-nucleon potentials that are consistent with $NN$-scattering data. Results: The high momentum tail of the deuteron momentum distribution, and hence of the nuclear momentum distributions appears to decrease as $k^{-4}$. This behavior is shown to arise from the effects of the tensor part of the nucleon-nucleon potential. In addition, when the dimensionless interaction strength for the atomic system is chosen to be similar to that of atomic nuclei, the probability for finding a short range different-fermion pair in both systems is the same. Conclusions: Although nuclei do not satisfy all of the conditions for Tan's contact, the observed similarity of the magnitude and $k^{-4}$ shape of nuclear and atomic momentum distributions is remarkable because these systems differ by about $20$ orders of magnitude in density. This similarity may lead to a greater understanding of nuclei and the density dependence of nuclear systems.

Equation of state for neutron stars with hyperons and quarks in relativistic Hartree-Fock approximation

Abstract: We construct the equation of state (EoS) for neutron stars explicitly including hyperons and quarks. Using the quark-meson coupling model with relativistic Hartree-Fock approximation, the EoS for hadronic matter is derived by taking into account the strange ($\sigma^{\ast}$ and $\phi$) mesons as well as the light non-strange ($\sigma$, $\omega$, $\vec{\pi}$ and $\vec{\rho}$) mesons. Relevant coupling constants are determined to reproduce the experimental data of nuclear matter and hypernuclei in SU(3) flavor symmetry. For quark matter, we employ the MIT bag model with one-gluon-exchange interaction, and Gibbs criteria for chemical equilibrium in the phase transition from hadrons to quarks. We find that the strange vector ($\phi$) meson and the Fock contribution make the hadronic EoS stiff, and that the maximum mass of a neutron star can be consistent with the observed mass of heavy neutron stars even if the coexistence of hadrons and quarks takes place in the core. However, in the present calculation the transition to pure quark matter does not occur in stable neutron stars. Furthermore, the lower bound of the critical chemical potential of the quark-hadron transition at zero temperature turns out to be around 1.5 GeV in order to be consistent with the recent observed neutron star data.

Many-body Forces in the Equation of State of Hyperonic Matter

Abstract: In this work we introduce an extended version of the formalism proposed originally by Taurines et al. that considers the effects of many-body forces simulated by nonlinear self-couplings and meson–meson interaction contributions. In this extended version of the model, we assume that matter is at zero temperature, charge neutral, and in beta-equilibrium, considering that the baryon octet interacts by the exchange of scalar–isoscalar (σ, ), vector–isoscalar (ω, ϕ), vector–isovector (), and scalar–isovector (δ) meson fields. Using nuclear matter properties, we constrain the parameters of the model that describe the intensity of the indirectly density dependent baryon–meson couplings to a small range of possible values. We then investigate asymmetric hyperonic matter properties. We report that the formalism developed in this work is in reasonable agreement with experimental data and also allows for the existence of massive hyperon stars (with more than ) with small radii, compatible with astrophysical observations.

Appearance of the single gyroid network phase in “nuclear pasta” matter

Abstract: Nuclear matter under the conditions of a supernova explosion unfolds into a rich variety of spatially structured phases, called nuclear pasta. We investigate the role of periodic networklike structures with negatively curved interfaces in nuclear pasta structures, by static and dynamic Hartree-Fock simulations in periodic lattices. As the most prominent result, we identify for the first time the single gyroid network structure of cubic chiral I4123 symmetry, a well-known configuration in nanostructured soft-matter systems, both as a dynamical state and as a cooled static solution. Single gyroid structures form spontaneously in the course of the dynamical simulations. Most of them are isomeric states. The very small energy differences from the ground state indicate its relevance for structures in nuclear pasta.

Neutron stars in the Bogomol'nyi-Prasad-Sommerfield Skyrme model: Mean-field limit versus full field theory

Abstract: Using a solitonic model of nuclear matter, the Bogomol'nyi-Prasad-Sommerfield (BPS) Skyrme model, we compare neutron stars obtained in the full field theory, where gravitational backreaction is completely taken into account, with calculations in a mean-field approximation using the Tolman-Oppenheimer-Volkoff approach. In the latter case, a mean-field-theory equation of state is derived from the original BPS field theory. We show that in the full field theory, where the energy density is nonconstant even at equilibrium, there is no universal and coordinate-independent equation of state of nuclear matter, in contrast to the mean-field approximation. We also study how neutron star properties are modified by going beyond mean-field theory and find that the differences between mean-field theory and exact results can be considerable. Further, we compare both exact and mean-field results with some theoretical and phenomenological constraints on neutron star properties, demonstrating thus the relevance of our model even in its most simple version.

Nuclear neutron-proton contact and the photoabsorption cross section.

Abstract: The nuclear neutron-proton contact is introduced, generalizing Tan's work, and evaluated from medium energy nuclear photodisintegration experiments. To this end we reformulate the quasideuteron model of nuclear photodisintegration and establish the bridge between the Levinger constant and the contact. Using experimental evaluations of Levinger's constant, we extract the value of the neutron-proton contact in finite nuclei and in symmetric nuclear matter. Assuming isospin symmetry we propose to evaluate the neutron-neutron contact through the measurement of photonuclear spin correlated neutron-proton pairs.

From asymmetric nuclear matter to neutron stars: A functional renormalization group study

Abstract: A previous study of nuclear matter in a chiral nucleon-meson model is extended to isospin-asymmetric matter. Fluctuations beyond mean-field approximation are treated in the framework of the functional renormalization group. The nuclear liquid-gas phase transition is investigated in detail as a function of the proton fraction in asymmetric matter. The equations of state at zero temperature of both symmetric nuclear matter and pure neutron matter are found to be in good agreement with realistic many-body computations. We also study the density dependence of the pion mass in the medium. The question of chiral symmetry restoration in neutron matter is addressed; we find a stabilization of the phase with spontaneously broken chiral symmetry once fluctuations are included. Finally, neutron-star matter including $\ensuremath{\beta}$ equilibrium is discussed. The model satisfies the constraints imposed by the existence of two-solar mass neutron stars.

Effects of the kinetic symmetry energy reduced by short-range correlations in heavy-ion collisions at intermediate energies

Abstract: Besides earlier predictions based on both phenomenological models and modern microscopic many-body theories, circumstantial evidence was recently found for a reduced kinetic symmetry energy of isospin-asymmetric nucleonic matter compared to the free Fermi gas model prediction due to the short-range correlation of high-momentum neutron-proton pairs. While keeping the total symmetry energy near the saturation density of nuclear matter consistent with existing experimental constraints, we examine the correspondingly enhanced role of the isospin degree of freedom in heavy-ion collisions at intermediate energies due to the reduced (enhanced) kinetic (potential) symmetry energy. Important observable consequences are investigated.

Microscopic positive-energy potential based on the Gogny interaction

Abstract: We present nucleon elastic scattering calculation based on Green's function formalism in the Random-Phase Approximation. For the first time, the Gogny effective interaction is used consis- tently throughout the whole calculation to account for the complex, non-local and energy-dependent optical potential. Effects of intermediate single-particle resonances are included and found to play a crucial role in the account for measured reaction cross section. Double counting of the particle- hole second-order contribution is carefully addressed. The resulting integro-differential Schrodinger equation for the scattering process is solved without localization procedures. The method is ap- plied to neutron and proton elastic scattering from 40 Ca. A successful account for differential and integral cross sections, including analyzing powers, is obtained for incident energies up to 30 MeV. Discrepancies at higher energies are related to much too high volume integral of the real potential for large partial waves. Moreover, this works opens the way for future effective interactions suitable simultaneously for both nuclear structure and reaction. Nuclear structure and nuclear reactions are two aspects of the same many-body problem, although in practice they are often addressed as different phenomena. A con- sistent, quantitative and predictive account for both is still a challenging open problem in nuclear physics. The description of nucleon-nucleus elastic scattering based solely on the nucleon-nucleon (NN) interaction is an im- portant step forward toward this unification. Depending on projectile energy and target mass, vari- ous strategies have been adopted in order to treat micro- scopically elastic scattering. Nuclear matter models (1) provide reasonable descriptions of nucleon elastic scat- tering at incident energies above 50 MeV (2), even up to �1 GeV (3). The Resonating Group Method within the No-Core Shell Model, has successfully described nucleon and deuteron scattering from light nuclei (4). These mod- els have recently been extended to include three-nucleon forces for nucleon scattering from 4 He (5). The Green's Function Monte Carlo method has been used to describe elastic scattering from 4 He (6). These models yield en- couraging results but are still restricted to light targets at low energies. The Self-Consistent Green's Function (SCGF) method has been applied to microscopic calcu- lation of the optical potentials for proton scattering from 16 O (7, 8). The coupled-cluster theory has been applied to proton elastic scattering from 40 Ca (9). These last two methods are limited to closed-shell nuclei. Work on Gorkov-Green's function theory is in progress to ex- tend SCGF to nuclei around closed-shell nuclei (10, 11). An alternative method consists of using microscopic ap- proaches based on the self-consistent mean-field theory and its extensions beyond mean-field. In nuclear physics, they are usually based on energy density functionals built from phenomenological parametrizations of the NN effec- tive interaction, such as Skyrme (12, 13) or Gogny forces (14-17). These approaches have successfully predicted a broad body of nuclear structure observables for nuclei ranging from medium to heavy masses. This wealth of developments can be extended to reaction calculations based on NN effective interaction. The so-called Nuclear Structure Method (NSM) for scattering (18-22) relies on the self-consistent Hartree-Fock (HF) and Random- Phase Approximations (RPA) of the microscopic opti- cal potential. The former is a mean-field potential and the latter is a polarization potential built from target- nucleus excitations. A strictly equivalent method, the continuum particle-vibration coupling using a Skyrme in- teraction, has been recently applied to neutron scattering from 16 O (23), but neglecting part of the residual inter- action in the coupling vertices. Other approaches are in progress, where optical potential is approximated as the HF term plus the imaginary part of the uncorrelated particle-hole potential neglecting the collectivity of tar- get excited states (24, 25). We report on optical potential calculations using NSM (18). Here the optical potential V consists of two compo- nents, V = V HF + �V.

Characteristics of hybrid compact stars with a sharp hadron-quark interface

Abstract: We describe two aspects of the physics of hybrid stars that have a sharp interface between a core of quark matter and a mantle of nuclear matter. Firstly, we analyze the mass-radius relation. We describe a generic "Constant Speed of Sound" (CSS) parameterization of the quark matter equation of state (EoS), in which the speed of sound is independent of density. In terms of the three parameters of the CSS EoS we obtain the phase diagram of possible forms of the hybrid star mass-radius relation, and we show how observational constraints on the maximum mass and typical radius of neutron stars can be expressed as constraints on the CSS parameters. Secondly, we propose a mechanism for the damping of density oscillations, including r-modes, in hybrid stars with a sharp interface. The dissipation arises from the periodic conversion between quark matter and nuclear matter induced by the pressure oscillations in the star. We find the damping grows nonlinearly with the amplitude of the oscillation and is powerful enough to saturate an r-mode at very low saturation amplitude, of order $10^{-10}$, which is compatible with currently-available observations of neutron star spin frequencies and temperatures.

Twist-averaged boundary conditions for nuclear pasta Hartree-Fock calculations

Abstract: Nuclear pasta phases, present in the inner crust of neutron stars, are associated with nucleonic matter at subsaturation densities arranged in regular shapes. Those complex phases, residing in a layer which is approximately 100-m thick, impact many features of neutron stars. Theoretical quantum-mechanical simulations of nuclear pasta are usually carried out in finite three-dimensional boxes assuming periodic boundary conditions. The resulting solutions are affected by spurious finite-size effects. To remove spurious finite-size effects, it is convenient to employ twist-averaged boundary conditions (TABC) used in condensed matter, nuclear matter, and lattice quantum chromodynamics applications. In this work, we study the effectiveness of TABC in the context of pasta phase simulations within nuclear density functional theory. We demonstrated that by applying TABC reliable results can be obtained from calculations performed in relatively small volumes. By studying various contributions to the total energy, we gain insights into pasta phases in mid-density range. Future applications will include the TABC extension of the adaptive multiresolution 3D Hartree-Fock solver and Hartree-Fock-Bogoliubov TABC applications to superfluid pasta phases and complex nucleonic topologies as in fission.