Showing papers on "Nuclear matter published in 2002"

Determination of the equation of state of dense matter.

Abstract: Nuclear collisions can compress nuclear matter to densities achieved within neutron stars and within core-collapse supernovae. These dense states of matter exist momentarily before expanding. We analyzed the flow of matter to extract pressures in excess of 10 34 pascals, the highest recorded under laboratory-controlled conditions. Using these analyses, we rule out strongly repulsive nuclear equations of state from relativistic mean field theory and weakly repulsive equations of state with phase transitions at densities less than three times that of stable nuclei, but not equations of state softened at higher densities because of a transformation to quark matter.

Relativistic Hartree-Bogoliubov model with density-dependent meson-nucleon couplings

Abstract: The relativistic Hartree-Bogoliubov (RHB) model is extended to include density-dependent meson-nucleon couplings. The effective Lagrangian is characterized by a phenomenological density dependence of the $\ensuremath{\sigma},$ $\ensuremath{\omega},$ and $\ensuremath{\rho}$ meson-nucleon vertex functions, adjusted to properties of nuclear matter and finite nuclei. Pairing correlations are described by the pairing part of the finite range Gogny interaction. The new density-dependent effective interaction DD-ME1 is tested in the analysis of the equations of state for symmetric and asymmetric nuclear matter, and of ground-state properties of the Sn and Pb isotopic chains. Results of self-consistent RHB calculations are compared with experimental data, and with results previously obtained in the RHB model with nonlinear self-interactions, as well as in the density-dependent relativistic hadron field (DDRH) model. Parity-violating elastic electron scattering on Pb and Sn nuclei is calculated using a relativistic optical model with inclusion of Coulomb distortion effects, and the resulting asymmetry parameters are related to the neutron ground-state density distributions.

Probing the high density behavior of the nuclear symmetry energy with high energy heavy-ion collisions.

Abstract: High energy heavy-ion collisions are proposed as a novel means to constrain stringently the high density (HD) behavior of nuclear symmetry energy. Within an isospin-dependent hadronic transport model, it is shown for the first time that the isospin asymmetry of the HD nuclear matter formed in high energy heavy-ion collisions is uniquely determined by the HD behavior of the nuclear symmetry energy. Experimental signatures in two sensitive probes, i.e., ${\ensuremath{\pi}}^{\ensuremath{-}}$ to ${\ensuremath{\pi}}^{+}$ ratio and neutron-proton differential collective flow, are also investigated.

Asymmetric nuclear matter: The role of the isovector scalar channel

Abstract: We try to single out some qualitative effects of coupling to a \ensuremath{\delta}-isovector-scalar meson, introduced in a minimal way in a phenomenological hadronic field theory. Results for the equation of state (EOS) and the phase diagram of asymmetric nuclear matter (ANM) are discussed. We stress the consistency of the \ensuremath{\delta}-coupling introduction in a relativistic approach. Contributions to the slope and curvature of the symmetry energy and to the neutron-proton effective mass splitting appear particularly interesting. A more repulsive EOS for neutron matter at high baryon densities is expected. Effects on the critical properties of warm ANM, mixing mechanical and chemical instabilities and isospin distillation, are also presented. The \ensuremath{\delta} influence is mostly on the isovectorlike collective response. The results are largely analytical, and this makes the physical meaning quite transparent. Implications for nuclear structure properties of drip-line nuclei and for reaction dynamics with radioactive beams are finally pointed out.

Nuclear ground state observables and QCD scaling in a refined relativistic point coupling model

Abstract: We present results obtained in the calculation of nuclear ground-state properties in relativistic Hartree approximation using a Lagrangian whose QCD-scaled coupling constants are all natural (dimensionless and of order one). Our model consists of four-, six-, and eight-fermion point couplings (contact interactions) together with derivative terms representing, respectively, two-, three-, and four-body forces and the finite ranges of the corresponding mesonic interactions. The coupling constants have been determined in a self-consistent procedure that solves the model equations for representative nuclei simultaneously in a generalized nonlinear least-squares adjustment algorithm. The extracted coupling constants allow us to predict ground-state properties of a much larger set of even-even nuclei to good accuracy. The fact that the extracted coupling constants are all natural leads to the conclusion that QCD scaling and chiral symmetry apply to finite nuclei.

Hadron-quark phase transition in dense matter and neutron stars

Abstract: We study the hadron-quark phase transition in the interior of neutron stars (NS's). We calculate the equation of state (EOS) of hadronic matter using the Brueckner-Bethe-Goldstone formalism with realistic two-body and three-body forces, as well as a relativistic mean field model. For quark matter we employ the MIT bag model constraining the bag constant by using the indications coming from the recent experimental results obtained at the CERN SPS on the formation of a quark-gluon plasma. We find it necessary to introduce a density-dependent bag parameter and the corresponding consistent thermodynamical formalism. We calculate the structure of NS interiors with the EOS comprising both phases, and we find that the NS maximum masses fall in a relatively narrow interval, $1.4{M}_{\ensuremath{\bigodot}}l~{M}_{\mathrm{max}}l~{1.7M}_{\ensuremath{\bigodot}}.$ The precise value of the maximum mass turns out to be only weakly correlated with the value of the energy density at the assumed transition point in nearly symmetric nuclear matter.

Phase transition to hyperon matter in neutron stars

Abstract: Recent progress in the understanding of the high density phase of neutron stars advances the view that a substantial fraction of the matter consists of hyperons. The possible impacts of a highly attractive interaction between hyperons on the properties of compact stars are investigated. We find that a hadronic equation of state with hyperons allows for a first order phase transition to hyperonic matter. The corresponding hyperon stars can have rather small radii of R approximately equal 8 km.

Microscopic three-body force for asymmetric nuclear matter

Abstract: Brueckner calculations including a microscopic three-body force have been extended to isospin-asymmetric nuclear matter. The effects of the three-body force on the equation of state and on the single-particle properties of nuclear matter are discussed with a view to possible applications in nuclear physics and astrophysics. It is shown that, even in the presence of the three-body force, the empirical parabolic law of the energy per nucleon vs. isospin asymmetry β = (N - Z)/A is fulfilled in the whole asymmetry range 0≤β≤1 up to high densities. The three-body force provides a strong enhancement of the symmetry energy which increases with density in good agreement with the predictions of relativistic approaches. The Lane's assumption that proton and neutron mean fields linearly vary vs. the isospin parameter is violated at high density due to the three-body force, while the momentum dependence of the mean fields turns out to be only weakly affected. Consequently, a linear isospin split of the neutron and proton effective masses is found for both cases with and without the three-body force. The isospin effects on multifragmentation events and collective flows in heavy-ion collisions are briefly discussed along with the conditions for direct URCA processes to occur in the neutron star cooling.

Microscopic three-body force for asymmetric nuclear matter

Abstract: Brueckner calculations including a microscopic three-body force have been extended to isospin asymmetric nuclear matter. The effects of the three-body force on the equation of state and on the single-particle properties of nuclear matter are discussed with a view to possible applications in nuclear physics and astrophysics. It is shown that, even in the presence of the three-body force, the empirical parabolic law of the energy per nucleon vs isospin asymmetry $\beta=(N-Z)/A$ is fulfilled in the whole asymmetry range $0\le\beta\le 1$ up to high densities. The three-body force provides a strong enhancement of symmetry energy increasing with the density in good agreement with relativistic approaches. The Lane's assumption that proton and neutron mean fields linearly vary vs the isospin parameter is violated at high density in the presence of the three-body force. Instead the momentum dependence of the mean fields is rather insensitive to three body force which brings about a linear isospin deviation of the neutron and proton effective masses. The isospin effects on multifragmentation events and collective flows in heavy-ion collisions are briefly discussed along with the conditions for direct URCA processes to occur in the neutron-star cooling.

Limiting temperatures and the equation of state of nuclear matter.

Abstract: From experimental observations of limiting temperatures in heavy ion collisions we derive the critical temperature of infinite nuclear matter T c = 16.6′0.86. Theoretical model correlations between T c , the compressibility modulus K, the effective mass m*, and the saturation density ρ s are then exploited to derive the quantity (K/m*) 1 / 2 ρ s -1/3. This quantity together with calculations employing Skyrme and Gogny interactions indicates a value of K in moderately excited nuclei that is in excellent agreement with the value determined from giant monopole resonance data.

Gluon production in the Color Glass Condensate model of collisions of ultrarelativistic finite nuclei

Abstract: We extend previous work on high energy nuclear collisions in the Color Glass Condensate model to study collisions of finite ultrarelativistic nuclei. The changes implemented include a) imposition of color neutrality at the nucleon level and b) realistic nuclear matter distributions of finite nuclei. The saturation scale characterizing the fields of color charge is explicitly position dependent, $\Lambda_s=\Lambda_s(x_T)$. We compute gluon distributions both before and after the collisions. The gluon distribution in the nuclear wavefunction before the collision is significantly suppressed below the saturation scale when compared to the simple McLerran-Venugopalan model prediction, while the behavior at large momentum $p_T\gg \Lambda_s$ remains unchanged. We study the centrality dependence of produced gluons and compare it to the centrality dependence of charged hadrons exhibited by the RHIC data. We demonstrate the geometrical scaling property of the initial gluon transverse momentum distributions for different centralities. Classical Yang-Mills results for $p_T \Lambda_s$-the resulting energy per particle is significantly lower than the purely classical estimates. Our results for nuclear collisions can be used as initial conditions for quantitative studies of the further evolution and possible equilibration of hot and dense gluonic matter produced in heavy ion collisions. Finally, we study $pA$ collisions within the classical framework. Our results agree well with previously derived analytical results in the appropriate kinematical regions.

Structure of the inner crust of neutron stars: Crystal lattice or disordered phase?

Abstract: We investigate the inner crust structure of neutron stars using the Skyrme-Hartree-Fock approach with the Coulomb interaction treated beyond the Wigner-Seitz approximation. Our results suggest that the shell effects associated with unbound neutrons play an important role and, in particular, lead to complicated phase transition pattern between various nuclear phases (as a function of the density). Namely, we show that the relative energies of different phases are rapidly oscillating functions of the neutron density. In the semiclassical approach this behavior is explained as an interference effect due to periodic orbits of similar lengths. We discuss also the dependence of the shell effects on pairing correlations.

Complete relativistic equation of state for neutron stars

Abstract: We construct the equation of state (EOS) in a wide density range for neutron stars using the relativistic mean field theory. The properties of neutron star matter with both uniform and nonuniform distributions are studied consistently. The inclusion of hyperons considerably softens the EOS at high densities. The Thomas-Fermi approximation is used to describe the nonuniform matter, which is composed of a lattice of heavy nuclei. The phase transition from uniform matter to nonuniform matter occurs around $0.06 {\mathrm{fm}}^{\ensuremath{-}3},$ and the free neutrons drip out of nuclei at about $2.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4} {\mathrm{fm}}^{\ensuremath{-}3}.$ We apply the resulting EOS to investigate the neutron star properties such as maximum mass and composition of neutron stars.

Equation of state and magnetic susceptibility of spin polarized isospin asymmetric nuclear matter

Abstract: Properties of spin-polarized isospin asymmetric nuclear matter are studied within the framework of the Brueckner-Hartree-Fock formalism. The single-particle potentials of neutrons and protons with spin up and down are determined for several values of the neutron and proton spin polarizations and the asymmetry parameter. It is found that the single-particle potentials exhibit an almost linear and symmetric variation as a function of these parameters. An analytic parametrization of the total energy per particle as a function of the asymmetry and spin polarizations is constructed. This parametrization is employed to compute the magnetic susceptibility of nuclear matter for several values of the asymmetry from neutron to symmetric matter. The results show no indication of a ferromagnetic transition at any density for any asymmetry of nuclear matter.

Antikaon nuclear potential in hot and dense matter

Abstract: The antikaon optical potential in hot and dense nuclear matter is studied within the framework of a coupled- channel self-consistent calculation taking, as bare meson-baryon interaction, the meson-exchange potential of the Julich group. Typical conditions found in heavy-ion collisions at GSI are explored. As in the case of zero temperature, the angular momentum components larger than L50 contribute significantly to the finite tem- perature antikaon optical potential at finite momentum. It is found that the particular treatment of the medium effects has a strong influence on the behavior of the antikaon potential with temperature. Our self-consistent model, in which antikaons and pions are dressed in the medium, gives a moderately temperature dependent antikaon potential which remains attractive at GSI temperatures, contrary to what one finds if only nuclear

Instabilities of infinite matter with effective Skyrme-type interactions

Abstract: The stability of the equation of state predicted by Skyrme-type interactions is examined. We consider simultaneously symmetric nuclear matter and pure neutron matter. The stability is defined by the inequalities that the Landau parameters must satisfy simultaneously. A systematic study is carried out to define interaction parameter domains where the inequalities are fulfilled. It is found that there is always a critical density ${\ensuremath{\rho}}_{\mathrm{cr}}$ beyond which the system becomes unstable. The results indicate in which parameter regions one can find effective forces to describe correctly finite nuclei and give at the same time a stable equation of state up to densities of 3--4 times the saturation density of symmetric nuclear matter.

eta in the nuclear medium within a chiral unitary approach

Abstract: The self-energy of an eta meson in the nuclear medium is calculated in a chiral unitary approach. A coupled channel Bethe-Salpeter equation is solved to obtain the effective eta-N interaction in the medium. The base model reproduces well the free space pi-N elastic and inelastic scattering at the eta-N threshold or N^*(1535) region. The Pauli blocking on the nucleons, binding potentials for the baryons and self-energies of the mesons are incorporated, including the eta self-energy in a self-consistent way. Our calculation predicts about -54 -i29 MeV for the optical potential at normal nuclear matter for an eta at threshold but also shows a strong energy dependence of the potential.

Tomography of cold and hot QCD matter : tools and diagnosis

Abstract: The probability distribution D() in the energy loss incurred by incoming and outgoing hard quarks in a QCD medium is computed numerically from the BDMPS gluon spectrum It is shown to follow an empirical log-normal behavior which allows us to give the quenching weight a simple analytic parameterization The dependence of our results under the infrared and ultraviolet sensitivity of the gluon spectrum is investigated as well Finally, as an illustration, we discuss and compare estimates for the quenching of hadron spectra in nuclear matter and in a quark-gluon plasma to HERA and RHIC preliminary data

Low momentum nucleon-nucleon potentials with half-on-shell T-matrix equivalence

Abstract: We study a method by which realistic nucleon-nucleon potentials V_NN can be reduced, in a physically equivalent way, to an effective low-momentum potential V-low-k confined within a cut-off momentum k-cut. Our effective potential is obtained using the folded-diagram method of Kuo, Lee and Ratcliff, and it is shown to preserve the half-on-shell T-matrix. Both the Andreozzi-Lee-Suzuki and the Andreozzi-Krenciglowa-Kuo iteration methods have been employed in carrying out the reduction. Calculations have been performed for the Bonn A and Paris NN potentials, using various choices for k-cut such as 2 fm-1. The deuteron binding energy, low-energy NN phase shifts, and the low-momentum half-on-shell T-matrix given by V_NN are all accurately reproduced by V-low-k. Possible applications of V-low-k directly to nuclear matter and nuclear structure calculations are discussed.