Description Of Nuclear Matter Research Articles

Neutron stars and phase diagram from a double hard-wall

Description of nuclear matter in the core of neutron stars eludes the main tools of investigation of QCD, such as perturbation theory and the lattice formulation of the theory. Recently, the application of the holographic paradigm (both via top-down and bottom-up models) to this task has led to many encouraging results, both qualitatively and quantitatively. We present our approach to the description of neutron star cores, relying on a simple model of the (double) hard-wall type: we discuss results concerning the nature of homogeneous nuclear matter at high density emerging from the model including a quarkyonic phase, the mass-radius relation for neutron stars, as well as the rather stiff equation of state we have found. We show how, despite the very simple model employed, for an appropriate calibration we are able to obtain neutron stars that only slightly fall short of the observational bounds on radius and tidal deformability.

Equation of state effects on gravitational waves from rotating core collapse

Gravitational waves (GWs) generated by axisymmetric rotating collapse, bounce, and early postbounce phases of a galactic core-collapse supernova will be detectable by current-generation gravitational wave observatories. Since these GWs are emitted from the quadrupole-deformed nuclear-density core, they may encode information on the uncertain nuclear equation of state (EOS). We examine the effects of the nuclear EOS on GWs from rotating core collapse and carry out 1824 axisymmetric general-relativistic hydrodynamic simulations that cover a parameter space of 98 different rotation profiles and 18 different EOS. We show that the bounce GW signal is largely independent of the EOS and sensitive primarily to the ratio of rotational to gravitational energy, and at high rotation rates, to the degree of differential rotation. The GW frequency of postbounce core oscillations shows stronger EOS dependence that can be parameterized by the core's EOS-dependent dynamical frequency $\sqrt{G\bar{\rho}_c}$. We find that the ratio of the peak frequency to the dynamical frequency follows a universal trend that is obeyed by all EOS and rotation profiles and that indicates that the nature of the core oscillations changes when the rotation rate exceeds the dynamical frequency. We find that differences in the treatments of low-density nonuniform nuclear matter, of the transition from nonuniform to uniform nuclear matter, and in the description of nuclear matter up to around twice saturation density can mildly affect the GW signal. We find that approximations and uncertainties in electron capture rates can lead to variations in the GW signal that are of comparable magnitude to those due to different nuclear EOS. This emphasizes the need for reliable nuclear electron capture rates and for self-consistent multi-dimensional neutrino radiation-hydrodynamic simulations of rotating core collapse.

Towards a theoretical description of dense QCD

The properties of matter at finite baryon densities play an important role for the astrophysics of compact stars as well as for heavy ion collisions or the description of nuclear matter. Because of the sign problem of the quark determinant, lattice QCD cannot be simulated by standard Monte Carlo at finite baryon densities. I review alternative attempts to treat dense QCD with an effective lattice theory derived by analytic strong coupling and hopping expansions, which close to the continuum is valid for heavy quarks only, but shows all qualitative features of nuclear physics emerging from QCD. In particular, the nuclear liquid gas transition and an equation of state for baryons can be calculated directly from QCD. A second effective theory based on strong coupling methods permits studies of the phase diagram in the chiral limit on coarse lattices.

Dirac-Brueckner mean fields and the effective Dirac-Hartree-Fock interaction in nuclear matter

Our objective is to include Fock exchange terms, and thus the pion, in a Dirac description of nuclear matter and to determine a density-dependent Dirac-Hartree-Fock meson exchange interaction that is hopefully less density-dependent than a typical Hartree one. We begin by performing extended self-consistent nuclear matter Dirac-Brueckner calculations over a wide range of densities and asymmetries. We maintain the momentum dependence of the Brueckner mean fields in the hope of fitting it with the momentum dependence of the effective Dirac-Hartree-Fock mean fields. To improve the description of the Brueckner mean fields within a Hartee-Fock approximation, we include contact terms in the interaction.

Modification of nucleon properties in nuclear matter and finite nuclei

We present a model for the description of nuclear matter and finite nuclei, and at the same time, for the study of medium modifications of nucleon properties. The nucleons are described as nontopological solitons which interact through the self-consistent exchange of scalar and vector mesons. The model explicitly incorporates quark degrees of freedom into nuclear many-body systems and provides satisfactory results on the nuclear properties. The present model predicts a significant increase of the nucleon radius at normal nuclear matter density. It is very interesting to see the nucleon properties change from the nuclear surface to the nuclear interior.

STRANGE MATTER AND STRANGE STARS WITH TSALLIS STATISTICS

We investigate the properties of β-equilibrated electrically charged neutral strange matter and strange stars at finite temperature in the framework of Tsallis statistics [C. Tsallis, J. Stat. Phys.52 (1988) 479]. As the main result of our study we find out that a QHD description of nuclear matter combined with Tsallis statistics may open new possibilities for nuclear matter models.

Two-scale scalar mesons in nuclei

We generalize the linear σ model in order to develop a chiral-invariant model of the nuclear structure. The model is natural, and contains not only the usual σ-meson which is the chiral partner of the pion but also a new chiral singlet that is responsible for the medium-range nucleon-nucleon attraction. This approach provides significant advantages in terms of its description of nuclear matter and finite nuclei in comparison with conventional models based on the linear σ model.

NUCLEAR MATTER SYMMETRY ENERGY AND MATTER–ANTI-MATTER STATES

In this work we discuss points of interest for a general phase diagram of nuclear matter at alternative conditions (such as isospin broken phase and existence of anti-matter state(s)) which are relevant for the description of astrophysics. The inclusion of light isovector spin zero mesons in relativistic models of nuclear systems is discussed and deviation from the usual results for the symmetry energy description is found in agreement with a development done for a non-relativistic description of nuclear matter.

Field-theoretical description of nuclear matter with only the pion-nucleon interaction.

A relativistic Lagrangian for the nuclear matter consisting of nucleons and pions with a pseudoscalar interaction term is considered. It is shown that a nonrelativistic reduction of the problem automatically introduces a Lorentz scalar, isoscalar coupling of nucleons with two correlated pions. The corresponding Hamiltonian is used to study the properties of infinite nuclear matter nonperturbatively, treating both the nucleons and pions as quantized fields. The model is shown to reproduce the characteristic nuclear matter properties very nicely without the necessity of {sigma} and {omega} fields, as is usually done in the mean field Walecka model. The corresponding equation of state for zero temperature nuclear matter is calculated and is shown to be consistent with the known phenomenological equations of state. The binding energy of nuclear matter is calculated to be 15.3 MeV at a saturation density of 0.153 fm{sup {minus}3}. The model reproduces a softer nuclear matter with the incompressibility of 134 MeV. {copyright} {ital 1996 The American Physical Society.}

Relativistic RPA response functions for quasielastic electron scattering with a density-dependent NN interaction

Abstract The longitudinal and transverse response functions as well as the Coulomb sum for quasielastic electron scattering on 12C, 40Ca, 48Ca, 56Fe and 208Pb have been calculated taking into account relativistic RPA correlations. For these calculations a density-dependent relativistic NN interaction reproducing the saturation curve of a Dirac-Brueckner calculation providing a good description of nuclear matter has been used. The agreement with experiment is considerably improved for nuclei ranging from 40Ca to 208Pb, but not for a relatively light nucleus like 12C.

A microscopic approach to self-consistent calculations in nuclei

A self-consistent second RPA (SRPA) calculation of the giant monopole resonance in several nuclei in a wide mass range is presented. Ground states and excited states are treated consistently on the basis of a microscopic approach. The effective interaction to be used in the Hartree-Fock calculations of the ground state and the particle-hole interaction for the SRPA are extracted from a meson exchange potential via G-matrix theory. After inclusion of appropriate corrections to take into account higher-order and/or relativistic effects, satisfactory results are obtained for both ground and excited states. From the requirement of overall consistency in the description of nuclear matter and finite nuclei an evaluation of the value of nuclear matter incompressibility and the symmetry energy is also obtained.

Partial restoration of chiral symmetry in nuclear matter.

Recent work of Cohen, Furnstahl, and Griegel has advanced our understanding of the behavior of quark and gluon condensates in nuclear matter. We make use of their analysis to discuss the role of chiral condensates as they appear in relativistic Brueckner-Hartree-Fock theory. We find some support for assumptions we used to discuss the properties of nuclear matter in our earlier work. We also find that a rather consistent picture emerges from these studies, when we relate the parameters of the boson-exchange model of nuclear forces to an underlying field-theoretic description of nuclear matter.

GENESIS AND EVOLUTION OF THE SKYRME MODEL FROM 1954 TO THE PRESENT

Not widely known facts on the genesis of the Skyrme model are presented in a historical survey, based on Skyrme's earliest papers and on his own published remembrance. We consider the evolution of Skyrme's model description of nuclear matter from the "Mesonic Fluid" model up to its final version, known as the baryon model. We pay special tribute to some well-known ideas in contemporary particle physics which one can find in Skyrme's earlier papers, such as: Nuclear Democracy, the Solitonic Mechanism, the Nonlinear Realization of Chiral Symmetry, Topological Charges, Fermi–Bose Transmutations, etc. It is curious to note in the final version of the Skyrme model gleams of Kelvin's "Vortex Atoms" theory. In conclusion we make a brief analysis of the validity of Skyrme's conjectures in view of recent results and pinpoint some questions which still remain.

Mean free path in the relativistic mean field.

An exact expression for the mean free path in a relativistic description of nuclear matter is given. Corrections due to the spatial nonlocality are calculated. While the latter play a significant role in giving reasonable values for the mean free path in nonrelativistic models, they have minor effects in the relativistic description of the nuclear matter mean field.

Boost-invariant description of nuclear matter

We present a self-consistent mean-field description of nuclear matter making use of light front dynamics and focus our attention on a model with nucleons coupled to scalar mesons. We derive the mean-field Dirac equation for nucleon separation energies. The nucleon-meson seagull interactions are included in the calculations. In the mean-field analysis, the nucleon momentum sum rule, which plays an important role in the description of deep-inelastic scattering from heavy nuclei, results from the thermodynamical consistency condition that there is no pressure at zero temperature. We obtain the same energy density of nuclear matter at rest as one derives in the instant form of dynamics. Our light front mean-field formalism describes nuclear matter in uniform motion at any possible velocity.

Finite-temperature system of strongly interacting baryons

A fully relativistic finite temperature many body theory is constructed and used to examine the bulk properties of a system of strongly interacting baryons. The strong interactions are described by a two parameter phenomenological model fit to a simple description of nuclear matter at T = 0. The zero temperature equation of state for such a system which has already been discussed in the literature was developed to give a realistic description of nuclear matter. The model presented here is the exact finite temperature extension of that model. The effect of the inclusion of baryon pairs for T greater than or equal to 2mc/sup 2//k is discussed in detail. The phase transition identified with nuclear matter vanishes for system temperatures in excess of T/sub C/ = 1.034 x 10/sup 11/ /sup 0/K. All values of epsilon (P,T) correspond to systems that are causal in the sense that the locally determined speed of sound never exceeds the speed of light.

On the superfluid state of strongly-interacting fermions

A model two-particle interaction hamiltonian is proposed to overcome the difficulties arising from the presence of strong short-range potentials (hard cores) in N-N and He/sup 3/--He/sup 3/ interactions in the superfluid description of nuclear matter and liquid He/sup 3/. (C.E.S.)