Showing papers on "Nuclear matter published in 1995"

Microscopic mass formulas

Abstract: By assuming the existence of a pseudopotential smooth enough to do Hartree-Fock variations and good enough to describe nuclear structure, we construct mass formulas that rely on general scaling arguments and on a schematic reading of shell model calculations. Fits to 1751 known binding energies for N,Z\ensuremath{\ge}8 lead to rms errors of 375 keV with 28 parameters. Tests of the extrapolation properties are passed successfully. The Bethe-Weizs\"acker formula is shown to be the asymptotic limit of the present one(s). The surface energy of nuclear matter turns out to be probably smaller than currently accepted.

Phase transitions in warm, asymmetric nuclear matter.

Abstract: A relativistic mean-field model of nuclear matter with arbitrary proton fraction is studied at finite temperature. An analysis is performed of the liquid-gas phase transition in a system with two conserved charges (baryon number and isospin) using the stability conditions on the free energy, the conservation laws, and Gibbs' criteria for phase equilibrium. For a binary system with two phases, the coexistence surface (binodal) is two dimensional. The Maxwell construction through the phase-separation region is discussed, and it is shown that the stable configuration can be determined uniquely at every density. Moreover, because of the greater dimensionality of the binodal surface, the liquid-gas phase transition is continuous (second order by Ehrenfest's definition), rather than discontinuous (first order), as in familiar one-component systems. Using a mean-field equation of state calibrated to the properties of nuclear matter and finite nuclei, various phase-separation scenarios are considered. The model is then applied to the liquid-gas phase transition that may occur in the warm, dilute matter produced in energetic heavy-ion collisions. In asymmetric matter, instabilities that produce a liquid-gas phase separation arise from fluctuations in the proton concentration (chemical instability), rather than from fluctuations in the baryon density (mechanical instability).

Formation of Superdense Hadronic Matter in High-Energy Heavy-Ion Collisions

Abstract: We present the detail of a newly developed relativistic transport model (ART 1.0) for high energy heavy-ion collisions. Using this model, we first study the general collision dynamics between heavy ions at the AGS energies. We then show that in central collisions there exists a large volume of sufficiently long-lived superdense hadronic matter whose local baryon and energy densities exceed the critical densities for the hadronic matter to quark-gluon plasma transition. The size and lifetime of this matter are found to depend strongly on the equation of state. We also investigate the degree and time scale of thermalization as well as the radial flow during the expansion of the superdense hadronic matter. The flow velocity profile and the temperature of the hadronic matter at freeze-out are extracted. The transverse momentum and rapidity distributions of protons, pions, and kaons calculated with and without the mean field are compared with each other and also with the preliminary data from the E866/E802 Collaboration to search for experimental observables that are sensitive to the equation of state. It is found that these inclusive, single particle observables depend weakly on the equation of state. The difference between results obtained with and without the nuclear mean field is only about 20%. The baryon transverse collective flow in the reaction plane is also analyzed. It is shown that both the flow parameter and the strength of the ``bounce-off'' effect are very sensitive to the equation of state. In particular, a soft equation of state with a compressibility of 200 MeV results in an increase of the flow parameter by a factor of 2.5 compared to the cascade case without the mean field. This large effect makes it possible to distinguish the predictions from different theoretical models and to detect the signatures of the quark-gluon plasma which is expected to significantly soften the equation of state.

Theoretical nuclear and subnuclear physics

Abstract: Nuclear Forces: A Review Nuclear Matter The Shell Model The Many-Particle Shell Model Why Field Theory QCD and a Phase Transition Pions Dynamic Resonances Path Integrals Strong-Coupling Limit Include Fermions Quarks in the Standard Model Semileptonic Weak Processes and other topics.

Theoretical Nuclear And Subnuclear Physics

Abstract: Nuclear Forces: A Review Nuclear Matter The Shell Model The Many-Particle Shell Model Why Field Theory QCD and a Phase Transition Pions Dynamic Resonances Path Integrals Strong-Coupling Limit Include Fermions Quarks in the Standard Model Semileptonic Weak Processes and other topics.

Density dependent hadron field theory

Abstract: A fully covariant approach to a density dependent hadron field theory is presented. The relation between in-medium NN interactions and field-theoretical meson-nucleon vertices is discussed. The medium dependence of nuclear interactions is described by a functional dependence of the meson-nucleon vertices on the baryon field operators. As a consequence, the Euler-Lagrange equations lead to baryon rearrangement self-energies which are not obtained when only a parametric dependence of the vertices on the density is assumed. It is shown that the approach is energy-momentum conserving and thermodynamically consistent. Solutions of the field equations are studied in the mean-field approximation. Descriptions of the medium dependence in terms of the baryon scalar and vector density are investigated. Applications to infinite nuclear matter and finite nuclei are discussed. Density dependent coupling constants obtained from Dirac-Brueckner calculations with the Bonn NN potentials are used. Results from Hartree calculations for energy spectra, binding energies, and charge density distributions of $^{16}\mathrm{O}$, $^{40,48}\mathrm{Ca}$, and $^{208}\mathrm{Pb}$ are presented. Comparisons to data strongly support the importance of rearrangement in a relativistic density dependent field theory. Most striking is the simultaneous improvement of charge radii, charge densities, and binding energies. The results indicate the appearance of a new ``Coester line'' in the nuclear matter equation of state.

Strangeness in hadronic stellar matter

Abstract: We examine the presence of strangeness-bearing components, hyperons and kaons, in dense neutron star matter. Calculations are performed using relativistic mean field models, in which both the baryon-baryon and kaon-baryon interactions are mediated by meson exchange. Results of kaon condensation are found to be qualitatively similar to previous work with chiral models, if compatibility of the kaon optical potentials is required. The presence of strangeness, be it in the form of hyperons or kaons, implies a reduction in the maximum mass and a relatively large number of protons, sufficient to allow rapid cooling to take place. The need to improve upon the poorly known couplings of the strange particles, which determine the composition and structure of neutron stars, is stressed. We also discuss generic problems with effective masses in mean field theories.

Isospin dependence of the spin-orbit force and effective nuclear potentials.

Abstract: The isospin dependence of the spin-orbit potential is investigated for an effective Skyrme-like energy functional suitable for density dependent Hartree-Fock calculations. The magnitude of the isospin dependence is obtained from a fit to experimental data on finite spherical nuclei. It is found to be close to that of relativistic Hartree models. Consequently, the anomalous kink in the isotope shifts of Pb nuclei is well reproduced.

Vacuum Contributions in a Chiral Effective Lagrangian for Nuclei

Abstract: A relativistic hadronic model for nuclear matter and finite nuclei, which incorporates nonlinear chiral symmetry and broken scale invariance, is presented and applied at the one-baryon-loop level to finite nuclei. The model contains an effective light scalar field that is responsible for the midrange nucleon-nucleon attraction and which has anomalous scaling behavior. One-loop vacuum contributions in this background scalar field at finite density are constrained by low-energy theorems that reflect the broken scale invariance of quantum chromodynamics. A mean-field energy functional for nuclear matter and nuclei is derived that contains small powers of the fields and their derivatives, and the validity of this truncation is discussed. Good fits to the bulk properties of finite nuclei and single-particle spectra are obtained.

Deuteron Formation in Expanding Nuclear Matter from a Strong Coupling BCS Approach

Abstract: Central heavy ion collisions at E/A in the 50 to 200 MeV range can roughly be described by the initial build up of compressed and hot nuclear matter and by a sequential decompression. Some of these reactions have recently been reported [1] to end up with final fragments not heavier than α-particles, at most. This is the kind of scenario we shall address in this work. Our main aim is to develop the first steps of a transport theory which goes beyond BUU in a systematic fashion to include light cluster formation in a consistent way. By the latter we mean that cluster formation is followed throughout the entire reaction process without any ad hoc switching on and off of clustering or coalescence processes. In such a way for example memory effects in the formation process and Pauli principle will be fully accounted for. We here shall be mainly concerned with deuteron formation. However, in the end we shall shortly touch upon α-particle production as well. Our theoretical tools will be based on BCS and quasi-particle RPA theory.

Relativistic Hydrodynamics for Heavy--Ion Collisions -- II. Compression of Nuclear Matter and the Phase Transition to the Quark--Gluon Plasma

Abstract: We investigate the compression of nuclear matter in relativistic hydrodynamics. Nuclear matter is described by a $\sigma-\omega$--type model for the hadron matter phase and by the MIT bag model for the quark--gluon plasma, with a first order phase transition between both phases. In the presence of phase transitions, hydrodynamical solutions change qualitatively, for instance, one-dimensional stationary compression is no longer accomplished by a single shock but via a sequence of shock and compressional simple waves. We construct the analytical solution to the ``slab-on-slab'' collision problem over a range of incident velocities. The performance of numerical algorithms to solve relativistic hydrodynamics is then investigated for this particular test case. Consequences for the early compressional stage in heavy--ion collisions are pointed out.

Variations of hadron masses and matter properties in dense nuclear matter.

Abstract: Using a self-consistent quark model for nuclear matter we investigate variations of the masses of the nonstrange vector mesons, the hyperons, and the nucleon in dense nuclear matter (up to four times the normal nuclear density). We find that the changes in the hadron masses can be described in terms of the value of the scalar mean field in matter. The model is then used to calculate the density dependence of the quark condensate in-medium, which turns out to be well approximated by a linear function of the nuclear density. Some relations among the hadron properties and the in-medium quark condensate are discussed.

The phase transition to the quark-gluon plasma and its effect on hydrodynamic flow

Abstract: It is shown that in ideal relativistic hydrodynamics a phase transition from hadron to quark and gluon degrees of freedom in the nuclear matter equation of state leads to a minimum in the excitation function of the transverse collective flow.

Equation of state for cold nuclear matter from refractive 16O+16O elastic scattering.

Abstract: The nuclear density overlap, which occurs during refractive heavy-ion scattering, opens an alternative approach to study the equation of state (EOS) for cold nuclear matter. For this purpose elastic [sup 16]O+[sup 16]O scattering at incident enegies of 145, 250, 350, and 480 MeV has been measured very accurately, up to large angles. A systematic folding analysis of these data has been performed using an effective density dependent interaction based on the [ital G]-matrix elements of the Paris nucleon-nucleon potential. We find, with the observed refractive scattering patterns, that a soft EOS (with the nuclear incompressibility [ital K] around 200 MeV) is the most realistic one.

Kaon energies in dense matter.

Abstract: We discuss the role of kaon-nucleon and nucleon-nucleon correlations in kaon condensation in dense matter. Correlations raise the threshold density for kaon condensation, possibly to densities higher than those encountered in stable neutron stars.

Quarkonium Interactions in QCD

Abstract: Contents: 1. Introduction 2. Operator Product Expansion for Quarkonium Interactions 3. Scale Anomaly, Chiral Symmetry and Low-Energy Theorems 4. Quarkonium Interactions in Matter 5. Conclusions and Outlook

Correlations and pairing in nuclear matter within the Nozières-Schmitt-Rink approach

Abstract: The influence of correlations on the critical temperature and density for the onset of superfluidity in nuclear matter is investigated within the scheme of Nozieres and Schmitt-Rink [1]. For symmetric nuclear matter a smooth transition from Bose-Einstein condensation (BEC) of deuteronlike bound states at low densities and low temperatures to Bardeen-Cooper-Schrieffer (BCS) pairing at higher densities is described. Compared with the mean field approach a lowering of the critical temperature is obtained for symmetric nuclear matter as well as for pure neutron matter. The Mott transition in symmetric nuclear matter is discussed. Regions in the temperature-density plane are identified where correlated pairs give the main contribution to the composition of the system, so that approximations beyond the quasi-particle picture are requested.

Strange matter equation of state and the combustion of nuclear matter into strange matter in the quark mass-density-dependent model at T>0.

Abstract: We study the properties and stability of strange matter in the mass-density-dependent model at Tg0 We find the temperature dependence of the strange matter stability window and the critical temperature above which there is no stable strange matter It occurs at ${\mathit{T}}_{\mathit{c}}$=34 MeV Also, the resulting equation of state in this approach is presented We also study the combustion of nuclear matter into strange matter We employ for strange matter the equation of state of this paper and for nuclear matter a set of equations of state (free neutrons, Bethe-Johnson, Lattimer-Ravenhall, and Walecka) It is shown the the results are very similar to the ones found employing the MIT bag model However, the only equation of state which shows a noticeable dependence with temperature is the Walecka one Moreover, contrary to the former case, the Walecka equation of state is flammable in the present model It is found that, also at finite temperatures, the properties of strange quark matter in the quark mass-density-dependent model and in the MIT bag model are very similar