Showing papers on "Nuclear matter published in 1996"

Hyperon rich matter in neutron stars

Abstract: We study the equation of state of hyperon-rich matter in neutron stars using an extended relativistic mean-field model. We take special care of the recently proposed nonlinear behavior of the vector field providing a much better agreement with Dirac-Br\"uckner calculations. The hyperon-hyperon interaction is also implemented by introducing additional meson exchanges. With these new terms we avoid the instability found at high densities while keeping the excellent description for finite nuclear systems. We also demonstrate within the mean-field approach that the presence of hyperons inside neutron stars on one hand and the hyperon-hyperon interactions on the other hand make the onset of kaon condensation less favorable. \textcopyright{} 1996 The American Physical Society.

Relativistic Mean-Field Theory and the High-Density Nuclear Equation of State

Abstract: The properties of high-density nuclear and neutron matter are studied using a relativistic mean-field approximation to the nuclear matter energy functional. Based on ideas of effective field theory, nonlinear interactions between the fields are introduced to parametrize the density dependence of the energy functional. Various types of nonlinearities involving scalar-isoscalar ($\sigma$), vector-isoscalar ($\omega$), and vector-isovector ($\rho$) fields are studied. After calibrating the model parameters at equilibrium nuclear matter density, the model and parameter dependence of the resulting equation of state is examined in the neutron-rich and high-density regime. It is possible to build different models that reproduce the same observed properties at normal nuclear densities, but which yield maximum neutron star masses that differ by more than one solar mass. Implications for the existence of kaon condensates or quark cores in neutron stars are discussed.

Realistic Model of the Nucleon Spectral Function in Few- and Many- Nucleon Systems

Abstract: By analyzing the high-momentum features of the nucleon momentum distribution in light and complex nuclei, it is argued that the basic two-nucleon configurations generating the structure of the nucleon spectral function at high values of the nucleon momentum and removal energy can be properly described by a factorized ansatz for the nuclear wave function, which leads to a nucleon spectral function in the form of a convolution integral involving the momentum distributions describing the relative and center-of-mass motion of a correlated nucleon-nucleon pair embedded in the medium. The spectral functions of $^{3}\mathrm{He}$ and infinite nuclear matter resulting from the convolution formula and from many-body calculations are compared, and a very good agreement in a wide range of values of nucleon momentum and removal energy is found. Applications of the model to the analysis of inclusive and exclusive processes are presented, illustrating those features of the cross section which are sensitive to that part of the spectral function which is governed by short-range and tensor nucleon-nucleon correlations. \textcopyright{} 1996 The American Physical Society.

Relativistic Mean-Field Theory and the High-Density Nuclear Equation of State.

Abstract: The properties of high-density nuclear and neutron matter are studied using a relativistic mean-field approximation to the nuclear matter energy functional. Based on ideas of effective field theory, nonlinear interactions between the fields are introduced to parametrize the density dependence of the energy functional. Various types of nonlinearities involving scalar-isoscalar (σ), vector-isoscalar (ω) and vector-isovector (ϱ) fields are studied. After calibrating the model parameters at equilibrium nuclear matter density, the model and parameter dependence of the resulting equation of state is examined in the neutron-rich and high-density regime. It is possible to build different models that reproduce the same observed properties at normal nuclear densities, but which yield maximum neutron star masses that differ by more than one solar mass. Implications for the existence of kaon condensates or quark cores in neutron stars are discussed.

The Time-Delay Signature of Quark-Gluon-Plasma Formation in Relativistic Nuclear Collisions

Abstract: The hydrodynamic expansion of quark-gluon plasmas with spherical and longitudinally boost-invariant geometries is studied as a function of the initial energy density. The sensitivity of the collective flow pattern to uncertainties in the nuclear matter equation of state is explored. We concentrate on the effect of a possible finite width, $\Delta T \sim 0.1 T_c$, of the transition region between quark-gluon plasma and hadronic phase. Although slow deflagration solutions that act to stall the expansion do not exist for $\Delta T > 0.08 T_c$, we find, nevertheless, that the equation of state remains sufficiently soft in the transition region to delay the propagation of ordinary rarefaction waves for a considerable time. We compute the dependence of the pion-interferometry correlation function on $\Delta T$, since this is the most promising observable for time-delayed expansion. The signature of time delay, proposed by Pratt and Bertsch, is an enhancement of the ratio of the inverse width of the pion correlation function in out-direction to that in side-direction. One of our main results is that this generic signature of quark-gluon plasma formation is rather robust to the uncertainties in the width of the transition region. Furthermore, for longitudinal boost-invariant geometries, the signal is likely to be maximized around RHIC energies, $\sqrt{s} \sim 200$ AGeV.

More Effective Field Theory for Nonrelativistic Scattering

Abstract: An effective field theory treatment of nucleon-nucleon scattering at low energy shows much promise and could prove a useful tool in the study of nuclear matter at both ordinary and extreme densities. The analysis is complicated by the existence a large length scale --- the scattering length --- which arises due to couplings in the short distance theory being near critical values. I show how this can be dealt with by introducing an explicit s-channel state in the effective field theory. The procedure is worked out analytically in a toy example. I then demonstrate that a simple effective field theory excellently reproduces the 1S_0 np phase shift up to the pion production threshold.

Role of color neutrality in nuclear physics: Modifications of nucleonic wave functions

Abstract: The influence of the nuclear medium upon the internal structure of a composite nucleon is examined. The interaction with the medium is assumed to depend on the relative distances between the quarks in the nucleon consistent with the notion of color neutrality, and to be proportional to the nucleon density. In the resulting description the nucleon in matter is a superposition of the ground state (free nucleon) and radial excitations. The effects of the nuclear medium on the electromagnetic and weak nucleon form factors and the nucleon structure function are computed using a light-front constituent quark model. Further experimental consequences are examined by considering the electromagnetic nuclear response functions. The effects of color neutrality supply small but significant corrections to predictions of observables. {copyright} {ital 1996 The American Physical Society.}

Early fragment formation in heavy ion collisions

Abstract: The fragmentation pattern of central multifragmentation events observed in the collision of heavy systems can be recognized at a time when the system is still dense and the particles are still interacting with each other. This is the result obtained by applying simulated annealing algorithms to molecular dynamics simulations. We employ this algorithm to central and peripheral reactions of heavy nuclei simulated by the quantum molecular dynamics model (QMD). We see that in central collisions the fragments can already be identified when the density is still close to normal nuclear matter density and hence the fragment nucleons never pass through a density sufficiently low to allow for a liquid gas phase transition. In peripheral reactions, however, we observe that shortly after the nuclei have passed each other a division of the spectator matter into several medium-size clusters would yield the highest binding energy. However, the spectator matter does not break into these clusters but approaches thermal equilibrium.

Effective kaon mass in dense baryonic matter: role of correlations

Abstract: We evaluate the effective kaon mass in dense nuclear matter. Pauli blocking and nucleon-nucleon short-range correlations are incorporated. The effects of short-range correlations are shown to be moderate and figure importantly only at densities larger than 2 times normal nuclear density. We discuss the relations between the present results and the results obtained in next-to-next-to-leading order chiral perturbation theory (${{\cal O} (Q^3)}$, where $Q$ is the characteristic small energy-momentum scale probed). We also discuss mean-field aspects, with some remarks on the relation between the short-range correlations and the four-Fermi contact terms in the chiral effective Lagrangian.

Variation of hadron masses in finite nuclei

Abstract: The quark-meson coupling model, based on a mean-field description of nonoverlapping nucleon bags bound by the self-consistent exchange of $\ensuremath{\sigma}$, $\ensuremath{\omega}$, and $\ensuremath{\rho}$ mesons, is extended to investigate the change of hadron properties in finite nuclei. Relativistic Hartree equations for spherical nuclei have been derived from a relativistic quark model of the structure of bound nucleons and mesons. Using this unified, self-consistent description of both infinite nuclear matter and finite nuclei, we investigate the properties of some closed-shell nuclei and study the changes in the hadron masses of the nonstrange vector mesons, the hyperons, and the nucleon in those nuclei. We find a new, simple scaling relation for the changes of the hadron masses, which can be described in terms of the number of nonstrange quarks in the hadron and the value of the scalar mean field in a nucleus.

Self-consistent description of finite nuclei based on a relativistic quark model

Abstract: Relativistic Hartree equations for spherical nuclei have been derived from a relativistic quark model of the structure of bound nucleons which interact through the (self-consistent) exchange of scalar ($\sigma$) and vector ($\omega$ and $\rho$) mesons. The coupling constants and the mass of the $\sigma$-meson are determined from the properties of symmetric nuclear matter and the rms charge radius in $^{40}$Ca. Calculated properties of static, closed-shell nuclei from $^{16}$O to $^{208}$Pb are compared with experimental data and with results of Quantum Hadrodynamics (QHD). The dependence of the results on the nucleon size and the quark mass is investigated. Several possible extensions of the model are also discussed.

Light Vector Mesons in Nuclear Matter

Abstract: We summarize the current theoretical and experimental status of the spectral changes of vector mesons (p, w, ¢) in nuclear medium. Various approaches, including QeD sum rules, effective theory of hadrons and bag models show the decrease of vector meson masses in nuclear matter. The possibility to detect the mass shift through lepton pairs in r-A, p-A and A-A reactions is also discussed.