Model Of Nuclear Matter Research Articles

ρ-mass modification in3He: Signal of restoration of chiral symmetry or test for nuclear matter models?

Two recent experiments have demonstrated that the effective $\rho$-mass in nuclear medium, as extracted from the $^3He(\gamma, \pi^+ \pi^-)$ reaction, is substantially reduced. This has been advocated as an indication of partial restoration of chiral symmetry in nuclear matter. We show that even in the absence of chiral symmetry, effective mean field nuclear matter models can explain these findings quantitatively.

Coulomb instability of hot nuclei in a chiral symmetry model

A chiral symmetry model for nuclear matter is extended to finite temperature and then used to study the Coulomb instability of hot nuclei. The role of the $\ensuremath{\rho}$ meson degree of freedom is discussed. The calculated limiting temperature is compared with the results obtained from other theories and experiments.

THE MEAN π-FIELD SHOULD BE CALCULATED IN A RELATIVISTIC MEAN FIELD THEORY FOR NUCLEAR MATTER

We extend our work on the finite value of mean π-field in the relativistic mean field theory to the derivative coupling models for nuclear matter proposed by Zimanyi and Moszkowski. We show that a finite mean π-field may lower the energy per nucleon at a not too high density for all of those models. This qualitative result seems true in general and is model independent, although the quantitative details may be different for different models.

Ground-state structure and thermodynamics of Yukawa Bose fluids in dimensionality D = 3 and D = 2

We evaluate the structure and the thermodynamic properties (internal energy, pressure and compressibility) of zero-temperature fluids of Bose particles interacting via the Yukawa potential in dimensionality D = 3 and D = 2. These systems provide simplified models for nuclear matter in D = 3 and for assemblies of flux lines in high- superconductors in D = 2. Our calculations are based on the dielectric formalism, with short-range correlations being treated in the self-consistent scheme of Singwi et al. In both dimensionalities our results for the ground-state energy demonstrate a crucial role of short-range correlations and are in good agreement with those of variational and diffusion Monte Carlo studies over extended ranges of values for the system parameters (reduced DeBoer length and reduced particle density). Reasonable agreement is also found for the pressure with the available diffusion Monte Carlo data in D = 2. The extent of the deviations from the compressibility sum rule in the theory is assessed for both dimensionalities. On all the above grounds it appears that the present approach is quite accurate for the high-density fluid and provides a useful starting point for fully quantitative studies of the low-density fluid.

Quasifree [formula omitted][formula omitted] reaction in nuclear emulsion

A study of 1.66 GeV c K − beam induced ( K −,K + ) reactions in the nuclear emulsion target is presented and 796 ( K −, K + ) reaction vertices ( p K + ⩾ 1 GeV/c ) containing 18 single hyperfragments are identified. There is no clear(spatially separated) double hyperfragment nor twin single hyperfragment in the identified 796 ( K −,K + ) reactions. These events are classified and their characteristics are described in detail. In the region p K + ⩾ 1 GeV/c where the quasifree K −(p) → K +Ξ − is the dominant process, the probability of Ξ − emission from the reaction vertex is determined to be 73.9 −4.6 +4.5%. From this value, the mean free path of Ξ − in a nucleus is estimated to be 4.7 −1.0 +1.5 fm for 〈p Ξ − 〉 ∼ 0.6 GeV/c by assuming a simple model of uniform nuclear matter.

Compressibility of nuclear matter and nucleon-nucleus optical potential in an extended quark-meson coupling model

The quark-meson coupling model for nuclear matter has been extended in order to take into account the quark structure of the mesons. The mesons, like the nucleons, are described by the MIT bag model, while the meson mass has been constrained to scale like the meson mass. In addition, we have also taken into account the in-medium change of the bag constants which have been made density dependent. We have constrained the real part of the Schrödinger-equivalent nucleon-nucleus optical potential at saturation density to an experimental value. We have found that a nuclear compressibility in the range allowed by breathing-mode analyses can be obtained.

The effects of the tensor coupling term in the Zimanyi-Moszkowski model for unpolarized nuclear matter

The effects of the tensor coupling term on nuclear matter in the Zimanyi-Moszkowki (ZM) model are investigated. It is shown that the tensor coupling term in the ZM model leaves the thermodynamical properties of nuclear matter almost unchanged. The corrections of tensor coupling to the critical point of the liquid-gas phase transition are given.

Critical analysis of quark-meson coupling models for nuclear matter and finite nuclei

Three versions of the quark-meson coupling (QMC) model are applied to describe properties of nuclear matter and finite nuclei. The models differ in the treatment of the bag constant and in terms of non-linear scalar self-interactions. In two versions of the model the bag constant is held fixed at its free-space value whereas in the third model it depends on the density of the nuclear environment. As a consequence opposite predictions for the medium modifications of the internal nucleon structure arise. After calibrating the model parameters at equilibrium nuclear matter density, binding energies, charge radii, single-particle spectra and density distributions of spherical nuclei are analyzed and compared with QHD calculations. For the models which predict a decreasing size of the nucleon in the nuclear environment, unrealistic features of the nuclear shapes arise.

Study of nuclei in the drip-line regions

The ground state masses of 7188 nuclei covering the drip-line regions predicted in the infinite nuclear matter model are analysed. On the basis of systematics of two neutron separation energies, several new islands of stability in the exotic regions are predicted. The shell quenching for the N = 82 shell is clearly seen, in agreement with the expectation from the astrophysical r-process studies of abundances of heavy elements. This points to a solution of the puzzle existing in r-process nucleosynthesis. The shell quenching in the N = 126 shell is also predicted.

High-density nuclear matter with nonlocal confining solitons

An infinite system of nonlocal, individually confining solitons is considered as a model of high-density nuclear matter. The soliton-lattice problem is discussed in the Wigner-Seitz approximation. The cell size is varied to study the density dependence of physical quantities of interest. A transition to a system where quarks can migrate between solitons is found. We argue that this signals quark deconfinement. The model is applied to the calculation of selected in-medium properties.

Multichannel approach to clustering matter

An approach is developed, combining the ideas of quantum statistical mechanics and multichannel theory of scattering, for treating statistical systems whose constituents can possess different bound states realized as compact clusters. The main principles for constructing multichannel cluster Hamiltonians are formulated: principle of statistical correctness, principle of cluster coexistence, and principle of potential scaling. The importance of the principle of statistical correctness is emphasized by showing that when it does not hold the behaviour of thermodynamic functions becomes essentially distorted. And moreover, unphysical instabilities can appear. The ideas are carefully illustrated by a statistical model of hot nuclear matter.

Model for analytical calculation of nuclear photoabsorption at intermediate energies

The universal curve σ A of nuclear photoabsorption is investigated within a Fermi gas model of nuclear matter. An energy range from pion threshold up to 400 MeV is considered. The interactions between nucleon, pion, Δ-isobar and photon are considered in the non-relativistic approximation with corrections of the order 1 M taken into account with respect to proton mass. Analytical expressions are obtained, in which the influence of nuclear correlations and two-nucleon contributions is studied explicitly. The contributions of real and virtual pions are found to be sufficient to obtain agreement with experimental data in this energy range. An extension of the model calculation to nucleon knock-out reactions is discussed.

Multi-lambda matter in a quark - meson coupling model

The quark - meson coupling model for nuclear matter is extended to include the s quark and to describe strange hadronic matter. In a mean-field approximation, it is used to investigate the saturation properties of multi-lambda hypernuclear matter. The results are compared with those given by the Zimanyi - Moszkowski model.

Spin polarised nuclear matter and its application to neutron stars

An equation of state (EOS) of nuclear matter with explicit inclusion of a spin-isospin dependent force is constructed from a finite range, momentum and density dependent effective interaction. This EOS is found to be in good agreement with those obtained from more sophisticated models for unpolarised nuclear matter. Introducing spin degrees of freedom, it is found that it is possible for neutron matter to undergo a ferromagnetic transition at densities realisable in the core of neutron stars. The maximum mass and the surface magnetic field of the neutron star can be fairly explained in this model. Since finding quark matter rather than hadronic matter at the core of neutron stars is a possibility, the proposed EOS is also applied to the study of hybrid stars. It is found using the bag model picture that one can in principle describe both the mass as well as the surface magnetic field of hybrid stars satisfactorily.

THE LOW TEMPERATURE BEHAVIOR OF SYMMETRIC NUCLEAR MATTER

The low temperature behavior of the nuclear binding energy of isospin symmetric nuclear matter recently derived by using a finite temperature extension to the semi-empirical mass formula appears in conflict with the Fermi gas result. We show that extensions to known mean field theoretical models of nuclear matter lead back to the Fermi gas result and hence are unable to resolve the conflict in the temperature dependence.

Mesonic contribution to the Compton scattering amplitude for heavy nuclei

The contribution of mesonic exchange currents to nuclear Compton scattering is investigated within the framework of a Fermi gas model of nuclear matter in the non-relativistic limit. The additional interaction between the nucleons is accounted for by including two- and three-body diagrams. As a test of this model, the enhancement constant κ is calculated. The full correlators for the central and tensor parts of the nucleon-nucleon interaction due to pion exchange are obtained and the energy dependence of the amplitude is investigated. The contribution of the Δ-excitation to the mesonic part of the Compton amplitude is calculated explicitly using an effective Hamiltonian in the static limit.

Modified quark-meson coupling model for nuclear matter.

The quark-meson coupling model for nuclear matter, which describes nuclear matter as nonoverlapping MIT bags bound by the self-consistent exchange of scalar and vector mesons, is modified by introducing medium modification of the bag constant. We model the density dependence of the bag constant in two different ways: One invokes a direct coupling of the bag constant to the scalar meson field, and the other relates the bag constant to the in-medium nucleon mass. Both models feature a decreasing bag constant with increasing density. We find that when the bag constant is significantly reduced in nuclear medium with respect to its free-space value, large canceling isoscalar Lorentz scalar and vector potentials for the nucleon in nuclear matter emerge naturally. Such potentials are comparable to those suggested by relativistic nuclear phenomenology and finite-density QCD sum rules. This suggests that the reduction of bag constant in nuclear medium may play an important role in low- and medium-energy nuclear physics. \textcopyright{} 1996 The American Physical Society.

Thermodynamical properties of relativistic nuclear matter in a quark - meson coupling model

The quark - meson coupling model for nuclear matter is extended to finite temperature. The resulting equation of state for nuclear matter is applied to discuss the liquid - gas phase transition and the Coulomb instability of hot nuclei. The calculated critical temperature and limiting temperature are compared with those given by the Walecka and other models.

ISOSPIN LATTICE GAS MODEL AND NUCLEAR-MATTER PHASE DIAGRAM AND PRESSURE-VOLUME ISOTHERMS

We study a cubic lattice gas model for nuclear matter where each lattice site can be either occupied, by one proton or one neutron, or unoccupied. A nearest-neighbor interaction of the form - ∑<ij>Jijτziτzj is assumed. Our model is an isospin-1 Ising model, with τz= (1, 0, –1) representing respectively (proton, vacancy, neutron). A kinetic-energy term has been included in our model. Under the Bragg-Williams mean-field approximation our model exhibits the existence of a dense phase (liquid-like) and a rare phase (gas-like). The nuclear-matter p−v isotherms given by our model are discussed.

Recovering relativistic nuclear phenomenology from the quark-meson coupling model

The quark-meson coupling (QMC) model for nuclear matter, which describes nuclear matter as non-overlapping MIT bags bound by the self-consistent exchange of scalar and vector mesons is modified by the introduction of a density dependent bag constant. The density dependence of the bag constant is related to that of the in-medium effective nucleon mass through a scaling ansatz suggested by partial chiral symmetry restoration in nuclear matter. This modification overcomes drawbacks of the QMC model and leads to the recovery of the essential features of relativistic nuclear phenomenology. This suggests that the modification of the bag constant in the nuclear medium may play an important role in low- and medium-energy nuclear physics.