Showing papers on "Nuclear matter published in 2003"

Energy loss in perturbative QCD

Abstract: ▪ Abstract We review the propagation of energetic partons in hot or cold QCD matter, as known from recent work. We summarize advances in the understanding of both collisional and radiative energy loss. Our emphasis is on radiative energy loss, which has very interesting properties that may help to detect the quark-gluon plasma produced in heavy-ion collisions. We describe two different theoretical approaches, which lead to the same radiated gluon energy spectrum. The case of a longitudinally expanding QCD plasma is investigated. The energy lost by a jet with given opening angle is calculated with the aim of making predictions for the suppression (quenching) of hard jet production. Phenomenological implications for the difference between hot and cold matter are discussed. Numerical estimates of the loss suggest that it may be significantly greater in hot matter than in cold. This makes the magnitude of the radiative energy loss a remarkable signal for quark-gluon plasma formation.

Pairing in Nuclear Systems: From Neutron Stars to Finite Nuclei

Abstract: We discuss several pairing-related phenomena in nuclear systems, ranging from superfluidity in neutron stars to the gradual breaking of pairs in finite nuclei. We focus on the links between many-body pairing as it evolves from the underlying nucleon-nucleon interaction and the eventual experimental and theoretical manifestations of superfluidity in infinite nuclear matter and of pairing in finite nuclei. We analyse the nature of pair correlations in nuclei and their potential impact on nuclear structure experiments. We also describe recent experimental evidence that points to a relation between pairing and phase transitions (or transformations) in finite nuclear systems. Finally, we discuss recent investigations of ground-state properties of random two-body interactions where pairing plays little role although the interactions yield interesting nuclear properties such as 0+ ground states in even-even nuclei.

Jet Quenching and Radiative Energy Loss in Dense Nuclear Matter

Abstract: We review recent finite opacity approaches (GLV, WW, WOGZ) to the computation of the induced gluon radiative energy loss and their application to the tomographic studies of the density evolution in ultra-relativistic nuclear collisions.

Nuclear matter and neutron-star properties calculated with the Skyrme interaction

Abstract: The effective Skyrme interaction has been used extensively in mean-field models for several decades and many different parametrizations of the interaction have been proposed. All of these give similar agreement with the experimental observables of nuclear ground states as well as with the properties of infinite symmetric nuclear matter at the saturation density n0. However, when applied over a wider range of densities (up to ∼3n0) they predict widely varying behavior for the observables of both symmetric and asymmetric nuclear matter. A particularly relevant example of naturally occurring asymmetric nuclear matter is the material of which neutron stars are composed. At around nuclear matter density, this can be well represented as a mixture of neutrons, protons, electrons, and muons (n+p+e+μ matter) in β-equilibrium, and these densities turn out to be the key ones for determining the properties of neutron-star models with masses near to the widely used “canonical” value of 1.4M⊙. By constructing equations of state for neutron-star matter using the different Skyrme parametrizations, calculating corresponding neutron-star models and then comparing these with observational data, an additional constraint can be obtained for the values of the Skyrme parameters. Such a constraint is particularly relevant because the parametrizations are initially determined by fitting to the properties of doubly closed-shell nuclei and it is an open question how suitable they then are for nuclei with high values of isospin, such as those at the neutron drip-line and beyond. The neutron-star environment provides an invaluable testing ground for this. We have carried out an investigation of 87 different Skyrme parametrizations in order to examine how successful they are in predicting the expected properties of infinite nuclear matter and generating plausible neutron-star models. This is the first systematic study of the predictions of the various Skyrme parametrizations for the density dependence of the characteristic observables of nuclear matter; the density dependence of the symmetry energy for β-equilibrium matter turns out to be a crucial property for indicating which Skyrme parameter sets will apply equally well for finite nuclei and for neutron-star matter. Only 27 of the 87 parametrizations investigated pass the test of giving satisfactory neutron-star models and we present a list of these.

Momentum dependence of symmetry potential in asymmetric nuclear matter for transport model calculations

Abstract: For transport model simulations of collisions between two nuclei which have $N/Z$ significantly different from unity one needs a one-body potential which is both isospin and momentum dependent. This work provides sets of such potentials.

Compact stars with color superconducting quark matter

Abstract: We study compact stars that contain quark matter. We look at the effect of color superconductivity in the quark matter on the nuclear-quark matter transition density, mass-radius relationship, and the density discontinuity at the boundary between nuclear and quark matter. We find that color superconducting quark matter will occur in compact stars at values of the bag constant where ordinary quark matter would not be allowed. We are able to construct ``hybrid'' stars with a color superconducting quark matter interior and nuclear matter surface, with masses in the range $1.3\char21{}{1.6M}_{\ensuremath{\bigodot}}$ and radii 8\char21{}11 km. Our results are consistent with recent mass-radius limits based on absorption lines from EXO0748-676.

A microscopic estimate of the nuclear matter compressibility and symmetry energy in relativistic mean-field models

Abstract: The relativistic mean-field plus random phase and quasiparticle random phase approximation calculations, based on effective Lagrangians with density-dependent meson-nucleon vertex functions, are employed in a microscopic analysis of the nuclear matter compressibility and symmetry energy. We compute the isoscalar monopole response of 90Zr, 116Sn, 144Sm, the isoscalar monopole and isovector dipoles response of 208Pb, and also the differences between the neutron and proton radii for 208Pb and several Sn isotopes. The comparison of the calculated excitation energies with the experimental data on the giant monopole resonances restricts the nuclear matter compression modulus of structure models based on the relativistic mean-field approximation to Knm 250-270 MeV. The isovector giant dipole resonance in 208Pb and the available data on differences between the neutron and proton radii limit the range of the nuclear matter symmetry energy at saturation (volume asymmetry) of these effective interactions to 32 MeV < a4 < 36 MeV.

Quantum Monte Carlo calculations of neutron matter

Abstract: Uniform neutron matter is approximated by a cubic box containing a finite number of neutrons, with periodic boundary conditions. We report variational and Green's function Monte Carlo calculations of the ground state of fourteen neutrons in a periodic box using the Argonne $\vep $ two-nucleon interaction at densities up to one and half times the nuclear matter density. The effects of the finite box size are estimated using variational wave functions together with cluster expansion and chain summation techniques. They are small at subnuclear densities. We discuss the expansion of the energy of low-density neutron gas in powers of its Fermi momentum. This expansion is strongly modified by the large nn scattering length, and does not begin with the Fermi-gas kinetic energy as assumed in both Skyrme and relativistic mean field theories. The leading term of neutron gas energy is ~ half the Fermi-gas kinetic energy. The quantum Monte Carlo results are also used to calibrate the accuracy of variational calculations employing Fermi hypernetted and single operator chain summation methods to study nucleon matter over a larger density range, with more realistic Hamiltonians including three-nucleon interactions.

Nuclear symmetry energy and the neutron skin in neutron-rich nuclei

Abstract: The symmetry energy for nuclear matter and its relation to the neutron. skin in finite nuclei is discussed. The symmetry energy as a function of density obtained in a self-consistent Green function approach is presented and compared to the results of other recent theoretical approaches. A partial explanation of the linear relation between the symmetry energy and the neutron skin is proposed. The potential of several experimental methods to extract the neutron skin is examined.

Microscopic calculations in asymmetric nuclear matter

Abstract: A microscopic calculation of the equation of state for asymmetric nuclear matter is presented We employ realistic nucleon-nucleon forces and operate within the Dirac-Brueckner-Hartree-Fock approach to nuclear matter The focal point of this paper is a (momentum-space) G matrix that properly accounts for the asymmetry between protons and neutrons This will merge naturally into the development of an effective interaction suitable for applications to asymmetric nuclei, which will be the object of extensive study in the future

Phi meson mass and decay width in nuclear matter

Abstract: The $\phi$ meson spectrum, which in vacuum is dominated by its coupling to the $\bar{K} K$ system, is modified in nuclear matter. Following a model based on chiral SU(3) dynamics we calculate the $\phi$ meson selfenergy in nuclear matter considering the $K$ and $\bar{K}$ in-medium properties. For the latter we use the results of previous calculations which account for $S-$ and $P-$wave kaon-nucleon interactions based on the lowest order meson-baryon chiral effective Lagrangian, and this leads to a dressing of the kaon propagators in the medium. In addition, a set of vertex corrections is evaluated to fulfill gauge invariance, which involves contact couplings of the $\phi$ meson to $S-$wave and $P-$wave kaon-baryon vertices. Within this scheme the mass shift and decay width of the $\phi$ meson in nuclear matter are studied.

Initial State Parton Broadening and Energy Loss Probed in d+Au at RHIC

Abstract: The impact parameter and rapidity dependence of the Cronin effect for massless pions in $d+Au$ reactions at $\sqrt{s}_{NN}=200$ GeV at RHIC is computed in the framework of pQCD multiple elastic scattering on a nuclear target. We introduce a formalism to incorporate initial state energy loss in perturbative calculations and take into account the elastic energy loss in addition to the transverse momentum broadening of partons.We argue that the centrality dependence of the Cronin effect can distinguish between different hadron production scenarios at RHIC. Its magnitude and rapidity dependence are shown to carry important experimental information about the properties of cold nuclear matter up to the moderate- and large-$x$ antishadowing/EMC regions.

Structure of cold nuclear matter at subnuclear densities by Quantum Molecular Dynamics

Abstract: Structure of cold nuclear matter at subnuclear densities for the proton fraction x = 0.5, 0.3 and 0.1 is investigated by quantum molecular dynamics (QMD) simulations. We demonstrate that the phases with slablike and rodlike nuclei, etc. can be formed dynamically from hot uniform nuclear matter without any assumptions on nuclear shape, and also systematically analyze the structure of cold matter using two-point correlation functions and Minkowski functionals. In our simulations, we also observe intermediate phases, which has complicated nuclear shapes. It is found out that these phases can be characterized as those with negative Euler characteristic. Our result implies the existence of these kinds of phases in addition to the simple “pasta” phases in neutron star crusts and supernova inner cores. In addition, we investigate the properties of the effective QMD interaction used in the present work to examine the validity of our results. The resultant energy per nucleon ǫn of the pure neutron matter, the proton chemical � (0) p in pure neutron matter and the nuclear surface tension Esurf are generally reasonable in comparison with other nuclear interactions.

Nuclear isospin diffusivity

Abstract: The isospin diffusion and other irreversible phenomena are discussed for a two-component nuclear Fermi system. The set of Boltzmann transport equations, such as that employed for reactions, is linearized, for weak deviations of a system from uniformity, in order to arrive at nonreversible fluxes linear in the nonuniformities. Besides the diffusion driven by a concentration gradient, also the diffusion driven by temperature and pressure gradients is considered. Diffusivity, conductivity, heat-conduction, and shear-viscosity coefficients are formally expressed in terms of the responses of distribution functions to the nonuniformities. The linearized Boltzmann-equation set is solved, under the approximation of constant form factors in the distribution-function responses, to find concrete expressions for the transport coefficients in terms of weighted collision integrals. The coefficients are calculated numerically for nuclear matter, using experimental nucleon-nucleon cross sections. The isospin diffusivity is inversely proportional to the neutron-proton cross section and is also sensitive to the symmetry energy. At low temperatures in symmetric matter, the diffusivity is directly proportional to the symmetry energy.

Saturation of nuclear matter and short-range correlations.

Abstract: A fully self-consistent treatment of short-range correlations in nuclear matter is presented. Different implementations of the determination of the nucleon spectral functions for different interactions are shown to be consistent with each other. The resulting saturation densities are closer to the empirical result when compared with (continuous choice) Brueckner-Hartree-Fock values. Arguments for the dominance of short-range correlations in determining the nuclear matter saturation density are presented. A further survey of the role of long-range correlations suggests that the inclusion of pionic contributions to ring diagrams in nuclear matter leads to higher saturation densities than empirically observed. A possible resolution of the nuclear matter saturation problem is suggested.

Light clusters production as a probe to nuclear symmetry energy

Abstract: Using an isospin-dependent transport model for heavy-ion collisions induced by neutron-rich nuclei at intermediate energies, we study the production of light clusters such as deuteron, triton, and $^{3}$He via coalescence of nucleons. We find that both the yields and energy spectra of these light clusters are affected significantly by the density dependence of nuclear symmetry energy, with a stiffer symmetry energy giving a larger yield.

Warm stellar matter with deconfinement: Application to compact stars

Abstract: We investigate the properties of mixed stars formed by hadronic and quark matter in $\ensuremath{\beta}$ equilibrium described by appropriate equations of state (EOS) in the framework of relativistic mean-field theory We use the nonlinear Walecka model for the hadron matter and the MIT Bag and the Nambu-Jona-Lasinio (NJL) models for the quark matter The phase transition to a deconfined quark phase is investigated In particular, we study the dependence of the onset of a mixed phase and a pure quark phase on the hyperon couplings, quark model, and properties of the hadronic model We calculate the strangeness fraction with baryonic density for the different EOS With the NJL model the strangeness content in the mixed phase decreases The calculations were performed for $T=0$ and for finite temperatures in order to describe neutron and proto-neutron stars The star properties are discussed Both the Bag model and the NJL model predict a mixed phase in the interior of the star Maximum allowed masses for proto-neutron stars are larger for the NJL model $(\ensuremath{\sim}{19M}_{\ensuremath{\bigodot}})$ than that for the Bag model $(\ensuremath{\sim}{16M}_{\ensuremath{\bigodot}})$

Hartree-Fock approach to nuclear matter and finite nuclei with M3Y-type nucleon-nucleon interactions

Abstract: By introducing a density-dependent contact term, M3Y-type interactions applicable to the Hartree-Fock calculations are developed. In order to view basic characters of the interactions, we carry out calculations on the uniform nuclear matter as well as on several doubly magic nuclei. It is shown that a parameter set called M3Y-P2 describes various properties similarly well to the Skyrme SLy5 and/or the Gogny D1S interactions. A remarkable difference from the SLy5 and D1S interactions is found in the spin-isospin properties in the nuclear matter, to which the one-pion-exchange potential gives a significant contribution. Affecting the single-particle energies, this difference may play a certain role in the new magic numbers in unstable nuclei.

Self-consistent solution to the nuclear many-body problem at finite temperature

Abstract: The properties of symmetric nuclear matter are investigated within Green's functions approach. We have implemented an iterative procedure allowing for a self-consistent evaluation of the single-particle and two-particle propagators. The in-medium scattering equation is solved for a realistic (nonseparable) nucleon-nucleon interaction including both particle-particle and hole-hole propagation. The corresponding two-particle propagator is constructed explicitly from the single-particle spectral functions. Results are obtained for finite temperatures and an extrapolation to $T=0$ is presented.

Collective modes of asymmetric nuclear matter in quantum hadrodynamics

Abstract: We discuss a fully relativistic Landau Fermi liquid theory based on the Quantum Hadro-Dynamics ($QHD$) effective field picture of Nuclear Matter ({\it NM}). From the linearized kinetic equations we get the dispersion relations of the propagating collective modes. We focus our attention on the dynamical effects of the interplay between scalar and vector channel contributions. A beautiful ``mirror'' structure in the form of the dynamical response in the isoscalar/isovector degree of freedom is revealed, with a complete parallelism in the role respectively played by the compressibility and the symmetry energy. All that strongly supports the introduction of an explicit coupling to the scalar-isovector channel of the nucleon-nucleon interaction. In particular we study the influence of this coupling (to a $\delta$-meson-like effective field) on the collective response of asymmetric nuclear matter ($ANM$). Interesting contributions are found on the propagation of isovector-like modes at normal density and on an expected smooth transition to isoscalar-like oscillations at high baryon density. Important ``chemical'' effects on the neutron-proton structure of the mode are shown. For dilute $ANM$ we have the isospin distillation mechanism of the unstable isoscalar-like oscillations, while at high baryon density we predict an almost pure neutron wave structure of the propagating sounds.

Hadrons in the nuclear medium

Abstract: Quantum chromodynamics (QCD), the microscopic theory of strong interactions, has not yet been applied to the calculation of nuclear wavefunctions. However, it certainly provokes a number of specific questions and suggests the existence of novel phenomena in nuclear physics which are not part of the traditional framework of the meson–nucleon description of nuclei. Many of these phenomena are related to high nuclear densities and the role of colour in nucleonic interactions. Quantum fluctuations in the spatial separation between nucleons may lead to local high-density configurations of cold nuclear matter in nuclei, up to four times larger than typical nuclear densities. We argue here that experiments utilizing the higher energies available upon completion of the Jefferson Laboratory energy upgrade will be able to probe the quark–gluon structure of such high-density configurations and therefore elucidate the fundamental nature of nuclear matter. We review three key experimental programmes: quasi-elastic electro-disintegration of light nuclei, deep inelastic scattering from nuclei at x > 1 and the measurement of tagged structure functions. These interrelated programmes are all aimed at the exploration of the quark structure of high-density nuclear configurations. The study of the QCD dynamics of elementary hard processes is another important research direction and nuclei provide a unique avenue to explore these dynamics. In particular, we argue that the use of nuclear targets and large values of momentum transfer at energies available with the Jefferson Laboratory upgrade would allow us to determine whether the physics of the nucleon form factors is dominated by spatially small configurations of three quarks. Similarly, one could determine if hard two-body processes such as exclusive vector meson electroproduction are dominated by production of mesons in small-size q configurations.

Saturation of Nuclear Matter and Radii of Unstable Nuclei

Abstract: We systematically examine relations among the parameters characterizing the phenomenological equation of state (EOS) of nearly symmetric, uniform nuclear matter near the saturation density by comparing macroscopic calculations of radii and masses of stable nuclei with experimental data. The EOS parameters of interest here are the symmetry energy coefficient S0, the symmetry energy density derivative coefficient L and the incompressibility K0 at normal nuclear density. We estimate a range of (K0, L) from empirically reasonable values of the slope of the saturation line (the line joining the saturation points of nuclear matter at finite neutron excess) and find a strong correlation between S0 and L. In light of the uncertainties on the values of K0 and L, we perform macroscopic calculations of the radii of unstable nuclei expected to be produced in future facilities. We find that the matter radii depend appreciably on L, while being almost independent of K0. This dependence implies that if the matter radii are measured with an accuracy of ±0.01 fm for a sufficiently large number of neutron-rich nuclides to allow one to smooth out the expected staggering of the radii due to shell and pairing effects, it might be possible to derive the value of L within ±20 MeV.

Effects of symmetry energy on two-nucleon correlation functions in heavy-ion collisions induced by neutron-rich nuclei.

Abstract: Using an isospin-dependent transport model, we study the effects of nuclear symmetry energy on two-nucleon correlation functions in heavy-ion collisions induced by neutron-rich nuclei We find that the density dependence of the nuclear symmetry energy affects significantly the nucleon emission times in these collisions, leading to larger values of two-nucleon correlation functions for a symmetry energy that has a stronger density dependence Two-nucleon correlation functions are thus useful tools for extracting information about the nuclear symmetry energy from heavy-ion collisions