Showing papers on "Nuclear matter published in 1999"

Phases of dense matter in neutron stars

Abstract: After a brief history of neutron stars and supernovae recent developments are discussed. Based on modern nucleon-nucleon potentials more reliable equations of state for dense nuclear matter have been constructed. Furthermore, phase transitions such as pion, kaon and hyperon condensation, superfluidity and quark matter can occur in cores of neutron stars. Specifically, the nuclear to quark matter phase transition and its mixed phases with intriguing structures is treated. Rotating neutron stars with and without phase transitions are discussed and compared to observed masses, radii and glitches. The observations of possible heavy $\sim 2M_\odot$ neutron stars in X-ray binaries and QPO's require relatively stiff equation of states and restricts strong phase transitions to occur at very high nuclear densities only.

Asymmetric nuclear matter from an extended Brueckner-Hartree-Fock approach

Abstract: The properties of isospin-asymmetric nuclear matter have been investigated in the framework of the extended Brueckner-Hartree-Fock approximation at zero temperature. Self-consistent calculations using the Argonne ${V}_{14}$ interaction are reported for several values of the asymmetry parameter $\ensuremath{\beta}=(N\ensuremath{-}Z)/A,$ ranging from symmetric nuclear matter to pure neutron matter. The binding energy per nucleon fulfills the ${\ensuremath{\beta}}^{2}$ law in the whole asymmetry range. The symmetry energy is calculated for different densities and discussed in comparison with other predictions. At the saturation point it is in fairly good agreement with the empirical value. The present approximation, based on the Landau definition of quasiparticle energy, is investigated in terms of the Hugenholtz--Van Hove theorem, which is proved to be fulfilled with a good accuracy at various asymmetries. The isospin dependence of the single-particle properties is discussed, including mean field, effective mass, and mean free path of neutrons and protons. The isospin effects in nuclear physics and nuclear astrophysics are briefly discussed.

Neutron matter model

Abstract: The Bertsch, nonparametric model of neutron matter is analyzed and strong indications are found that, in the infinite system limit, the ground state is a Fermi liquid with an effective mass, except for a set of measure zero.

First order kaon condensate

Abstract: First order Bose condensation in asymmetric nuclear matter and in neutron stars is studied, with particular reference to kaon condensation. We demonstrate explicitly why the Maxwell construction fails to assure equilibrium in multicomponent substances. Gibbs conditions and conservation laws require that for phase equilibrium, the charge density must have opposite sign in the two phases of isospin asymmetric nuclear matter. The mixed phase will therefore form a Coulomb lattice with the rare phase occupying lattice sites in the dominant phase. Moreover, the kaon condensed phase differs from the normal phase, not by the mere presence of kaons in the first, but also by a difference in the nucleon effective masses. The mixed phase region, which occupies a large radial extent amounting to some kilometers in our model neutron stars, is thus highly heterogeneous. It should be particularly interesting in connection with the pulsar glitch phenomenon as well as transport properties.

Effects of strong and electromagnetic correlations on neutrino interactions in dense matter

Abstract: An extensive study of the effects of correlations on both charged and neutral current weak interaction rates in dense matter is performed. Both strong and electromagnetic correlations are considered. The propagation of particle-hole interactions in the medium plays an important role in determining the neutrino mean free paths. The effects due to Pauli blocking and density, spin, and isospin correlations in the medium significantly reduce the neutrino cross sections. As a result of the lack of experimental information at high density, these correlations are necessarily model dependent. For example, spin correlations in nonrelativistic models are found to lead to larger suppressions of neutrino cross sections compared to those of relativistic models. This is due to the tendency of the nonrelativistic models to develop spin instabilities. Notwithstanding the above caveats, and the differences between nonrelativistic and relativistic approaches such as the spin- and isospin-dependent interactions and the nucleon effective masses, suppressions of order 2{endash}3, relative to the case in which correlations are ignored, are obtained. Neutrino interactions in dense matter are especially important for supernova and early neutron star evolution calculations. The effects of correlations for protoneutron star evolution are calculated. Large effects on the internal thermodynamic properties of protoneutron stars,more » such as the temperature, are found. These translate into significant early enhancements in the emitted neutrino energies and fluxes, especially after a few seconds. At late times, beyond about 10 s, the emitted neutrino fluxes decrease more rapidly compared to simulations without the effects of correlations, due to the more rapid onset of neutrino transparency in the protoneutron star. {copyright} {ital 1999} {ital The American Physical Society}« less

Bremsstrahlung of a quark propagating through a nucleus

Abstract: The density of gluons produced in the central rapidity region of a heavy-ion collision is poorly known. We investigate the influence of the effects of quantum coherence on the transverse momentum distribution of photons and gluons radiated by a quark propagating through nuclear matter. We describe the case where the radiation time substantially exceeds the nuclear radius (the relevant case for RHIC and LHC energies), which is different from what is known as the Landau-Pomeranchuk-Migdal effect corresponding to an infinite medium. We find suppression of the radiation spectrum at small transverse photon(gluon) momentum ${k}_{T},$ but enhancement for ${k}_{T}g1\mathrm{GeV}.$ Any nuclear effects vanish for ${k}_{T}g~10\mathrm{GeV}.$ Our results also allow us to calculate the ${k}_{T}$-dependent nuclear effects in prompt photon, light, and heavy (Drell-Yan) dilepton and hadron production.

Nuclear liquid-gas phase transition

Abstract: The microscopic theory of the nuclear matter equation of state at finite temperature is developed within the Bloch--De Dominicis diagrammatic expansion. The liquid gas phase transition of symmetric nuclear matter is identified, with a critical temperature ${T}_{c}\ensuremath{\approx}20 \mathrm{MeV},$ using the Argonne ${v}_{14}$ as the bare $\mathrm{NN}$ interaction and a phenomenological three-body force adjusted to give the correct saturation point. Pure neutron and asymmetric matter, relevant to supernovae explosions, are also studied. It is found that the liquid-gas phase transition disappears at asymmetries $ag0.9.$ At the bounce-off of the supernova collapse, temperatures of several tens of MeV are reached and we find that the compressibility steeply increases at such temperatures. Finally, we find that the equation of state gives a ``limiting temperature'' of finite nuclei consistent with the experimental observation in compound nucleus reactions. A careful analysis of the diagrammatic expansion reveals that the dominant terms are the ones that correspond to the zero-temperature Bethe-Brueckner-Goldstone diagrams, where the temperature is introduced in the occupation numbers only, represented by Fermi distributions, thus justifying this commonly used procedure of naively introducing the temperature effect.

Magnetic oscillations in dense cold quark matter with four-fermion interactions

Abstract: The phase structures of Nambu–Jona-Lasinio models with one or two flavours have been investigated at non-zero values of µ and H, where H is an external magnetic field and µ is the chemical potential. In the phase portraits of both models there arise infinitely many massless chirally symmetric phases, as well as massive ones with spontaneously broken chiral invariance, reflecting the existence of infinitely many Landau levels. Phase transitions of first and second orders and a lot of tricritical points have been shown to exist in phase diagrams. In the massless case, such a phase structure leads unavoidably to the standard van Alphen–de Haas magnetic oscillations of some thermodynamical quantities, including magnetization, pressure and particle density. In the massive case we have found an oscillating behaviour not only for thermodynamical quantities, but also for a dynamical quantity as the quark mass. Besides, in this case we have non-standard, i.e. non-periodic, magnetic oscillations, since the frequency of oscillations is an H-dependent quantity.

Effective density-dependent pairing forces in the T=1 and T=0 channels

Abstract: Effective density-dependent pairing forces of zero range are adjusted on gap values in $T=0,1$ channels calculated with the Paris force in symmetric nuclear matter General discussions on the pairing force are presented In conjunction with the effective k mass the nuclear pairing force seems to need very little renormalization in the $T=1$ channel The situation in the $T=0$ channel is also discussed

Phase Transitions in Neutron Stars and Maximum Masses.

Abstract: Using the most recent realistic effective interactions for nuclear matter with a smooth extrapolation to high densities including causality, we constrain the equation of state and calculate maximum masses of rotating neutron stars. First- and second-order phase transitions to, e.g., quark matter at high densities are included. If neutron star masses of ~2.3 M☉ from quasi-periodic oscillations in low-mass X-ray binaries are confirmed, a soft equation of state as well as strong phase transitions can be excluded in neutron star cores.

Neutron star properties in the quark-meson coupling model

Abstract: The effects of internal quark structure of baryons on the composition and structure of neutron star matter with hyperons are investigated in the quark-meson coupling (QMC) model. The QMC model is based on a mean-field description of nonoverlapping spherical bags bound by self-consistent exchange of scalar and vector mesons. The predictions of this model are compared with a quantum hadrodynamic (QHD) model calibrated to reproduce identical nuclear matter saturation properties. By employing a density-dependent bag constant through direct coupling to the scalar field, the QMC model is found to exhibit identical properties as QHD near saturation density. Furthermore, this modified QMC model provides well-behaved and continuous solutions at high densities relevant to the core of neutron stars. Two additional strange mesons are introduced which couple only to the strange quark in the QMC model and to the hyperons in the QHD model. The constitution and structure of stars with hyperons in the QMC and QHD models reveal interesting differences. This suggests the importance of quark structure effects in the baryons at high densities.

Pairing in low-density Fermi gases

Abstract: In the theory of fermionic matter, the expansion about the low-density limit has been invaluable for understanding the structure of the theory and the role of the interaction. At low densities, the interaction needs only be characterized by its scattering length to get expansions for the energy density, excitation spectrum, etc. @1#. However, to our knowledge the pairing singularity has never been incorporated into this framework. We have for example only the qualitative statement in Ref. @1# that the pairing singularity is logarithmic and unimportant for integrated quantities. A more quantitative statement is needed to have complete understanding of low-density fermionic matter. Another motivation for our study is the general reexamination of nuclear physics with effective field theory which is now taking place @2‐9#. In the effective field theory approach, the interaction is systematically expanded in a power series in momentum with the object of getting relationships between observables such that the details of the shortdistance interaction need not be parameterized. We shall show here that the BCS theory of pairing is amenable to this approach, and the low-energy theory gives finite and analytic results. Within effective field theory many results can be obtained analytically opposed to the numerical treatment of potential models. In this sense our approach complements the large body of literature of pairing in nuclear and neutron matter that is based on potential models @10‐16#. We consider a Fermi gas with two-fold degeneracy interacting with a short-range attractive interaction. Examples are neutron matter or gaseous 3 He. The Hamiltonian is idealized to be of the form

Roles of Hyperons in Neutron Stars

Abstract: We examine the roles the presence of hyperons in the cores of neutron stars may play in determining global properties of these stars. The study is based on estimates that hyperons appear in neutron star matter at about twice the nuclear saturation density, and emphasis is placed on effects that can be attributed to the general multi-species composition of the matter, hence being only weakly dependent on the specific modeling of strong interactions. Our analysis indicates that hyperon formation not only softens the equation of state but also severely constrains its values at high densities. Correspondingly, the valid range for the maximum neutron star mass is limited to about 1.5-1.8 $M_\odot$, which is a much narrower range than available when hyperon formation is ignored. Effects concerning neutron star radii and rotational evolution are suggested, and we demonstrate that the effect of hyperons on the equation of state allows a reconciliation of observed pulsar glitches with a low neutron star maximum mass. We discuss the effects hyperons may have on neutron star cooling rates, including recent results which indicate that hyperons may also couple to a superfluid state in high density matter. We compare nuclear matter to matter with hyperons and show that once hyperons accumulate in neutron star matter they reduce the likelihood of a meson condensate, but increase the susceptibility to baryon deconfinement, which could result in a mixed baryon-quark matter phase.

Neutrino-Nucleon Interactions in Magnetized Neutron-Star Matter: The Effects of Parity Violation

Abstract: We study neutrino-nucleon scattering and absorption in a dense, magnetized nuclear medium. These are the most important sources of neutrino opacity governing the cooling of a proto-neutron star in the first tens of seconds after its formation. Because the weak interaction is parity violating, the absorption and scattering cross sections depend asymmetrically on the directions of the neutrino momenta with respect to the magnetic field. We develop the moment formalism of neutrino transport in the presence of such asymmetric opacities and derive explicit expressions for the neutrino flux and other angular moments of the Boltzmann transport equation. For a given neutrino species, there is a drift flux of neutrinos along the magnetic field in addition to the usual diffusive flux. This drift flux depends on the deviation of the neutrino distribution function from thermal equilibrium. Hence, despite the fact that the neutrino cross sections are asymmetric throughout the star, the asymmetric neutrino flux can be generated only in the outer region of the proto-neutron star where the neutrino distribution deviates significantly from thermal equilibrium. The deviation from equilibrium is similarly altered by the asymmetric scattering and absorption, although its magnitude will still be quite small in the interior of themore » star. We clarify two reasons why previous studies have led to misleading results. First, inelasticity must be included in the phase space integrals in order to satisfy detail balance. Second, nucleon recoil must be included in order to find the leading order asymmetric cross sections correctly, even though it can be ignored to leading order to get the zero field opacities. In addition to the asymmetric absorption opacity arising from nucleon polarization, we also derive the contribution of the electron (or positron) ground state Landau level. For neutrinos of energy less than a few times the temperature, this is the dominant source of asymmetric opacity. Last, we discuss the implication of our result to the origin of pulsar kicks: in order to generate kick velocity of a few hundred kmhs{sup {minus}1} from asymmetric neutrino emission using the parity violation effect, the proto-neutron star must have a dipole magnetic field of at least 10{sup 15}{minus}10{sup 16} G. {copyright} {ital 1999} {ital The American Physical Society}« less

Electromagnetic form factors of the bound nucleon

Abstract: We calculate electromagnetic form factors of the proton bound in specified orbits for several closed-shell nuclei. The shell structure of the finite nuclei, together with the internal quark substructure of the nucleon, are self-consistently described by the quark-meson-coupling model. We find that the medium-modified electric and magnetic form factors of the bound nucleon deviate considerably from those of the free nucleon. Our results suggest that this medium correction on the nucleon's quark substructure may be detectable in forthcoming quasielastic electron-nucleus scattering.

Light quarks in the instanton vacuum at finite baryon density

Abstract: We consider the finite density, zero-temperature behavior of quark matter in the instanton picture. Since the instanton-induced interactions are attractive in both $\overline{q}q$ and $\mathrm{qq}$ channels, a competition ensues between phases of matter with condensation in either or both. It results in chiral symmetry restoration due to the onset of diquark condensation, a ``color superconductor,'' at finite density. Also possible is a state with both manners of condensation; however, such a phase is at best metastable for any chemical potential. The properties of quark matter in each phase are discussed, with emphasis on the microscopic effects of the effective mass and superconducting energy gap.

Quark description of hadronic phases

Abstract: We extend our proposal that major universality classes of hadronic matter can be understood, and in favorable cases calculated, directly in the microscopic quark variables, to allow for a splitting between strange and light quark masses. A surprisingly simple but apparently viable picture emerges, featuring essentially three phases, distinguished by whether strangeness is conserved (standard nuclear matter), conserved modulo 2 (hypernuclear matter), or locked to color (color flavor locking). These are separated by sharp phase transitions. There is also, potentially, a quark phase matching hadronic K condensation. The smallness of the secondary gap in two-flavor color superconductivity corresponds to the disparity between the primary dynamical energy scales of QCD and the much smaller energy scales of nuclear physics.

Kaonic nuclei excited by the (K-, N) reaction

Abstract: We show that kaonic nuclei can be produced by the ( ${K}^{\ensuremath{-}},p$) and ( ${K}^{\ensuremath{-}},n$) reactions. The reactions are shown to have cross sections experimentally measurable. The observation of kaonic nuclei gives a kaon-nucleus potential which answers the question as to the existence of kaon condensation in dense nuclear matter, especially in neutron stars.

High-energy hadron-induced dilepton production from nucleons and nuclei

Abstract: ▪ Abstract We review the production of high-mass lepton pairs in fixed-target experiments, including both Drell-Yan (DY) and heavy quarkonium production [J/ψ, ψ′, ϒ(1S), ϒ(2S), and ϒ(3S)]. In recent years, DY data have become standard input to the determination of parton density distributions. DY data have recently yielded the first measurement of the x dependence of the , asymmetry of the proton. Similar to the observations in deeply inelastic scattering, precision measurements of the nuclear dependence of the proton-induced DY process exhibit shadowing at small target momentum fraction, x2. There is, however, no evidence of enhanced DY production from nuclear targets. Mean transverse momenta of DY pairs are observed to increase with target mass. These data, analyzed within a new theoretical framework, provide an estimation of the energy loss of fast quarks in nuclear matter. In contrast to the DY process, there are large nuclear effects in the production of all quarkonia. These effects show strong depen...

Isoscalar compression modes in relativistic random phase approximation

Abstract: A fully consistent relativistic RPA calculation is performed for the monopole and dipole compression modes in nuclei. The emphasis is put on the effects of Dirac sea states which are generally neglected in relativistic RPA calculations. It is found that these effects can be quite important for the isoscalar monopole mode. The main contributions from the pairs of Fermi to Dirac sea states are through the exchange of the scalar meson, while the vector mesons play a negligible role. Numerical results of relativistic RPA are checked with the constrained relativistic mean field model in the monopole case. A good agreement beteween monopole energies calculated in RRPA and in time-dependent relativistic mean field approach is achieved. For the monopole compression mode, a comparison of experimental and calculated energies gives a value of 250 $\sim$ 270 MeV for the nuclear matter incompressibility. A large discrepancy remains between theory and experiment in the case of the dipole compression mode.

The phase transition from nuclear matter to quark matter during proto‐neutron star evolution

Abstract: We explore the occurrence of a phase transition from nuclear matter to quark matter in proto-neutron stars. To this end, we employ recent results on such a phase transition in the presence of an electron--neutrino--degenerate gas, based on a mean field model nuclear equation of state together with a quark matter equation of state as described by the MIT `bag model'. Those results show that this neutrino gas does not favour the transition. By comparison with the proto-neutron star evolutionary calculations of Keil & Janka, we find that, if the bag constant B has a value B ≤ 126 MeV fm-3, the deconfinement transition indeed occurs. We also find that, if B ≥ 100 MeV fm-3, the phase transition is delayed by the presence of neutrinos by a few seconds after core bounce, thus providing a natural explanation for the second peak of neutrino emission detected in SN 1987A by the Kamiokande Group. The transition to quark matter and its subsequent decay should affect proto-neutron star evolution and supernova explosions in a non-trivial way.

Hadronic particle production in nucleus-nucleus collisions

Abstract: Data on hadronic particle production in symmetric nuclear collisions from SIS/BEVALAC to SPS energies are reviewed. The main emphasis is on the production of pions, kaons, and antibaryons. Global features are discussed in terms of rapidity and transverse momentum distributions and the total energy stored in produced particles. Pion and kaon production probabilities are studied as function of beam energy and their distribution in polar and azimuthal angle. Special emphasis is put on medium effects expected for kaons in dense nuclear matter at low energies. An enhanced strange particle yield is found at all energies; its explanation at SPS energies is still controversial. Experimental data on antibaryon and multistrange hyperon production is less complete and does not allow for similar systematic studies.

H-dibaryon

Abstract: The quark cluster model studies concerning the H-dibaryon are reviewed. The covered topics are the H-dibaryon itself, the interaction between a nucleon and an H-dibaryon, and the one between two H-dibaryons. A related study on the H-dibaryon in nuclear matter is also reviewed and its implication to double hypernuclei is discussed.