Showing papers on "Nuclear matter published in 2001"

Neutron Star Structure and the Equation of State

Abstract: The structure of neutron stars is considered from theoretical and observational perspectives We demonstrate an important aspect of neutron star structure: the neutron star radius is primarily determined by the behavior of the pressure of matter in the vicinity of nuclear matter equilibrium density In the event that extreme softening does not occur at these densities, the radius is virtually independent of the mass and is determined by the magnitude of the pressure For equations of state with extreme softening or those that are self-bound, the radius is more sensitive to the mass Our results show that in the absence of extreme softening, a measurement of the radius of a neutron star more accurate than about 1 km will usefully constrain the equation of state We also show that the pressure near nuclear matter density is primarily a function of the density dependence of the nuclear symmetry energy, while the nuclear incompressibility and skewness parameters play secondary roles In addition, we show that the moment of inertia and the binding energy of neutron stars, for a large class of equations of state, are nearly universal functions of the star's compactness These features can be understood by considering two analytic, yet realistic, solutions of Einstein's equations, by, respectively, Buchdahl and Tolman We deduce useful approximations for the fraction of the moment of inertia residing in the crust, which is a function of the stellar compactness and, in addition, the pressure at the core-crust interface

Fermi systems with long scattering lengths

Abstract: Ground-state energies and superfluid gaps are calculated for degenerate Fermi systems interacting via long attractive scattering lengths such as cold atomic gases, neutron, and nuclear matter. In the intermediate region of densities, where the interparticle spacing $(\ensuremath{\sim}{1/k}_{F})$ is longer than the range of the interaction but shorter than the scattering length, the superfluid gaps and the energy per particle are found to be proportional to the Fermi energy and thus differ from the dilute and high-density limits. The attractive potential increase linearly with the spin-isospin or hyperspin statistical factor such that, e.g., symmetric nuclear matter undergoes spinodal decomposition and collapses whereas neutron matter and Fermionic atomic gases with two hyperspin states are mechanically stable in the intermediate density region. The regions of spinodal instabilities in the resulting phase diagram are reduced and do not prevent a superfluid transition.

Minimal color-flavor-locked-nuclear interface

Abstract: At nuclear matter density, electrically neutral strongly interacting matter in weak equilibrium is made of neutrons, protons, and electrons. At sufficiently high density, such matter is made of up, down, and strange quarks in the color-flavor-locked (CFL) phase, with no electrons. As a function of increasing density (or, perhaps, increasing depth in a compact star) other phases may intervene between these two phases, which are guaranteed to be present. The simplest possibility, however, is a single first order phase transition between CFL and nuclear matter. Such a transition, in space, could take place either through a mixed phase region or at a single sharp interface with electron-free CFL and electron-rich nuclear matter in stable contact. Here we construct a model for such an interface. It is characterized by a region of separated charge, similar to an inversion layer at a metal-insulator boundary. On the CFL side, the charged boundary layer is dominated by a condensate of negative kaons. We then consider the energetics of the mixed phase alternative. We find that the mixed phase will occur only if the nuclear-CFL surface tension is significantly smaller than dimensional analysis would indicate.

Density dependent hadron field theory for asymmetric nuclear matter and exotic nuclei

Abstract: Published in: Phys. Rev. C 64 (2001) , pp.034314 citations recorded in [Science Citation Index] Abstract: The density dependent relativistic hadron field (DDRH) theory is applied to strongly asymmetric nuclear matter and finite nuclei far off stability. A new set of in-medium meson-nucleon vertices is derived from Dirac-Brueckner Hartree-Fock (DBHF) calculations in asymmetric matter, now accounting also for the density dependence of isovector coupling constants. The scalar-isovector $delta$ meson is included. Nuclear matter calculations show that it is necessary to introduce a momentum correction in the extraction of coupling constants from the DBHF self-energies in order to reproduce the DBHF equation of state by DDRH mean-field calculations. The properties of DDRH vertices derived from the Groningen and the Bonn A nucleon-nucleon (NN) potentials are compared in nuclear matter calculations and for finite nuclei. Relativistic Hartree results for binding energies, charge radii, separation energies and shell gaps for the Ni and Sn isotopic chains are presented. Using the momentum corrected vertices an overall agreement to data on a level of a few percent is obtained. In the accessible range of asymmetries the $delta$ meson contributions to the self-energies are found to be of minor importance but asymmetry dependent fluctuations may occur.

Lane-consistent, semimicroscopic nucleon-nucleus optical model

Abstract: A semimicroscopic, Lane-consistent optical model is established up to 200 MeV for nucleons incident on spherical and near-spherical nuclei with masses $40l~Al~209.$ This model, based on the earlier approach of Jeukenne, Lejeune, and Mahaux in nuclear matter, is an extension of our previous work [E. Bauge, J. P. Delaroche, and M. Girod, Phys. Rev. C 58, 1118 (1998)]. The modulus of the isovector potential is extracted and compared with measurements and fully microscopic predictions. Good overall descriptions of nucleon scattering, of transitions to isobaric analog states, and of reaction observables are obtained down to 1 keV. Those results are discussed in detail.

Superfluidity in Neutron Star Matter

Abstract: The research on the superfluidity of neutron matter can be traced back to Migdal’s observation that neutron stars are good candidates for being macroscopic superfluid systems [1] And, in fact, during more than two decades of neutron-star physics the presence of neutron and proton superfluid phases has been invoked to explain the dynamical and thermal evolution of a neutron star The most striking evidence is given by post-glitch timing observations [2],[3], but also the cooling history is strongly influenced by the possible presence of super- fluid phases [4],[5] On the theoretical side, the onset of superfluidity in neutron matter or in the more general context of nuclear matter was investigated soon after the formulation of the Bardeen, Cooper, and Schrieffer (BCS) theory of superconductivity [6] and the pairing theory in atomic nuclei [7],[8]

Isospin effects in Nuclear Fragmentation

Abstract: We investigate properties of the symmetry term in the equation-of-state (EOS) of nuclear matter (NM) from the analysis of simulations of fragmentation events in intermediate energy heavy ion collisions. For charge asymmetric systems a qualitative new feature in the liquid-gas phase transition is predicted: the onset of chemical instabilities with a mixture of isoscalar and isovector components. This leads to a separation into a higher density (``liquid'') symmetric and a low density (``gas'') neutron-rich phase, the so-called neutron distillation effect. We analyse the simulations with respect to the time evolution of the isospin dynamics, as well as with respect to the distribution and asymmetry of the final primary fragments. Qualitatively different effects arise in central collisions, with bulk fragmentation, and peripheral collisions with neck-fragmentation. The neck fragments produced in this type of process appear systematically more neutron-rich from a dynamical nucleon migration effect which is very sensitive to the symmetry term in regions just below normal density. In general the isospin dynamics plays an important role in all the steps of the reaction, from prompt nucleon emission to the sequential decay of the primary fragments. A fully microscopic description of the reaction dynamics including stochastic elements to treat fluctuations realistically is absolutely necessary in order to extract precise information on the fragmentation and the nuclear equation of state. We have performed simulations for fragment production events in $n$-rich ($^{124}Sn$) and $n$-poor ($^{112}Sn$) symmetric colliding systems.

Application of the density dependent hadron field theory to neutron star matter

Abstract: Published in: Phys. Rev., C 64 (2001) 025804 citations recorded in [Science Citation Index] Abstract: The density dependent hadron field (DDRH) theory, previously applied to isospin nuclei and hypernuclei is used to describe $beta$-stable matter and neutron stars under consideration of the complete baryon octet. The meson-hyperon vertices are derived from Dirac-Brueckner calculations of nuclear matter and extended to hyperons. We examine properties of density dependent interactions derived from the Bonn A and from the Groningen NN potential as well as phenomenological interactions. The consistent treatment of the density dependence introduces rearrangement terms in the expression for the baryon chemical potential. This leads to a more complex condition for the $beta$-equilibrium compared to standard relativistic mean field (RMF) approaches. We find a strong dependence of the equation of state and the particle distribution on the choice of the vertex density dependence. Results for neutron star masses and radii are presented. We find a good agreement with other models for the maximum mass. Radii are smaller compared to RMF models and indicate a closer agreement with results of non-relativistic Brueckner calculations.

Transition from BCS pairing to Bose-Einstein condensation in low-density asymmetric nuclear matter

Abstract: We study the isospin-singlet neutron-proton pairing in bulk nuclear matter as a function of density and isospin asymmetry within the BCS formalism. In the high-density, weak-coupling regime the neutron-proton paired state is strongly suppressed by a minor neutron excess. As the system is diluted, the BCS state with large, overlapping Cooper pairs evolves smoothly into a Bose-Einstein condensate of tightly bound neutron-proton pairs (deuterons). In the resulting low-density system a neutron excess is ineffective in quenching the pair correlations because of the large spatial separation of the deuterons and neutrons. As a result, the Bose-Einstein condensation of deuterons is weakly affected by an additional gas of free neutrons even at very large asymmetries.

Hypernuclear structure with the new Nijmegen potentials

Abstract: We perform continuous-choice Brueckner-Hartree-Fock calculations of hypernuclear matter, using the recent Nijmegen potentials for hyperon-nucleon and hyperon-hyperon interactions. Single-particle observables of the various hyperons in bulk matter, as well as properties of single- and double-lambda hypernuclei, employing an extended Skyrme-Hartree-Fock scheme, are presented. We find that the potentials tend to overbind the single hypernuclei and strongly underbind the double hypernuclei.

Effects of new nonlinear couplings in relativistic effective field theory

Abstract: We extend the relativistic mean field theory model of Sugahara and Toki by adding new couplings suggested by modern effective field theories. An improved set of parameters is developed with the goal to test the ability of the models based on effective field theory to describe the properties of finite nuclei and, at the same time, to be consistent with the trends of Dirac-Brueckner-Hartree-Fock calculations at densities away from the saturation region. We compare our calculations with other relativistic nuclear force parameters for various nuclear phenomena.

Off-shell behavior of the in-medium nucleon-nucleon cross-section

Abstract: The properties of nucleon-nucleon scattering inside dense nuclear matter are investigated. We use the relativistic Brueckner-Hartree-Fock model to determine on-shell and half off-shell in-medium transition amplitudes and cross sections. At finite densities the on-shell cross sections are generally suppressed. This reduction is, however, less pronounced than found in previous works. In case the outgoing momenta are allowed to be off energy shell the amplitudes show a strong variation with momentum. This description allows one to determine in-medium cross sections beyond the quasiparticle approximation, accounting thereby for the finite width which nucleons acquire in the dense nuclear medium. For reasonable choices of the in-medium nuclear spectral width, i.e., $\ensuremath{\Gamma}l~40\mathrm{MeV},$ the resulting total cross sections are, however, reduced by not more than about 25% compared to the on-shell values. Off-shell effects are generally more pronounced at large nuclear matter densities.

Self consistent propagation of hyperons and antikaons in nuclear matter based on relativistic chiral SU(3) dynamics

Abstract: We evaluate the antikaon spectral density in isospin symmetric nuclear matter. The in-medium antikaon-nucleon scattering process and the antikaon propagation is treated in a self consistent and relativistic manner where a maximally scheme-independent formulation is derived by performing a partial density resummation in terms of the free-space antikaon-nucleon scattering amplitudes. The latter amplitudes are taken from a covariant and chiral coupled-channel SU(3) approach which includes s-, p- and d-waves systematically. Particular care is taken on the proper evaluation of the in-medium mixing of the partial waves. Our analysis establishes a rich structure of the antikaon spectral function with considerable strength at small energies. At nuclear saturation density we predict attractive mass shifts for the $\Lambda(1405)$, $\Sigma (1385)$ and $\Lambda(1520)$ of about 60 MeV, 60 MeV and 100 MeV respectively. The hyperon states are found to exhibit at the same time an increased decay width of about 120 MeV for the s-wave $\Lambda(1405)$, 70 MeV for the p-wave $\Sigma (1385)$ and 90 MeV for the d-wave $\Lambda(1520)$ resonance.

Universal Fluctuations in Heavy-Ion Collisions in the Fermi Energy Domain

Abstract: We discuss the scaling laws of both the charged fragments multiplicity n fluctuations and the charge of the largest fragment Zmax fluctuations for Xe+Sn collisions in the range of bombarding energies between 25 A·MeV and 50 A·MeV . We show at Elab � 32MeV/A the transition in the fluctuation regime of Zmax which is compatible with the transition from the ordered to disordered phase of excited nuclear matter. The size (charge) of the largest fragment is closely related to the order parameter characterizing this process. Theoretical description of the fragment production in heavy-ion (HI) collisions depends on whether the equilibrium has been reached before the system starts fragmenting. Possibility of the critical behavior associated with the transition from the particle evaporation regime at low excitation energies to the explosion of the hot source at about 5 - 10 MeV/nucleon cannot be excluded. Unfortunately, this exciting possibility is difficult to study because all standard models and methods of characterizing different phases and transitions of the nuclear matter in HI collisions assume an equilibrium mechanism of the fragment production. In this work, we shall apply new methods of the theory of universal fluctuations of observables in finite systems [1] to examine what can be said in a model independent way about the fragmentation mechanism and the phase changement in HI collisions in the Fermi energy domain. Our analysis, which is independent of the assumption of the equilibrium in the fragments production process, uses the data of the INDRA multidetector system for Xe + Sn collisions at 25 MeV ≤ Elab/A ≤ 50 MeV [2–5]. Several features of finite systems are important if one wants to study either the criticality or the distance to the critical point [1]. These are : (i) The �-scaling of the normalized probability distribution P [m] of the variable m for different ’system sizes’ :

Correlations and the Dirac structure of the nucleon self-energy

Abstract: The Dirac structure of the nucleon self-energy in symmetric nuclear matter as well as neutron matter is derived from a realistic meson exchange model for the nucleon-nucleon (NN) interaction. It is demonstrated that the effects of correlations on the effective NN interaction in the nuclear medium can be parameterized by means of an effective meson exchange. This analysis leads to a very intuitive interpretation of correlation effects and also provides an efficient parametrization of an effective interaction to be used in relativistic structure calculations for finite nuclei.

Event-by-event analysis of proton-induced nuclear multifragmentation: determination of the phase transition universality class in a system with extreme finite-size constraints.

Abstract: A percolation model of nuclear fragmentation is used to interpret 10.2 GeV/c p + 1 9 7 Au multifragmentation data. Emphasis is put on finding signatures of a continuous nuclear matter phase transition in finite nuclear systems. Based on model calculations, corrections accounting for physical constraints of the fragment detection and sequential decay processes are derived. Strong circumstantial evidence for a continuous phase transition is found, and the values of two critical exponents, σ = 0.5 ′ 0.1 and Τ = 2.35 ′ 0.05, are extracted from the data. A critical temperature of T c = 8.3 ′ 0.2 MeV is found.

Proton differential elliptic flow and the isospin dependence of the nuclear equation of state

Abstract: Within an isospin-dependent transport model for nuclear reactions involving neutron-rich nuclei, we study the first-order direct transverse flow of protons and their second-order differential elliptic flow as a function of transverse momentum. It is found that the differential elliptic flow of midrapidity protons, especially at high transverse momenta, is much more sensitive to the isospin dependence of the nuclear equation of state than the direct flow. Origins of these different sensitivities and their implications to the experimental determination of the isospin dependence of the nuclear equation of state by using neutron-rich heavy-ion collisions at intermediate energies are discussed. The isospin dependence of the nuclear equation of state ~EOS! is one of the most important but very poorly known properties of neutron-rich matter @1#. Its determination in laboratory-controlled experiments has profound implications for the study of the structure and evolution of many astrophysical objects @2#. Nuclear reactions induced by stable neutron-rich nuclei and/or radioactive beams provide a means to extract useful information about the isospin dependence of the nuclear EOS and to explore novel phenomena in nuclear matter at extreme isospin asymmetries. A number of dedicated experiments to study the isospin dependence of the nuclear EOS have been performed/planned at several available radioactive beam facilities and the future Rare Isotope Accelerator @3#. For these experiments to be fruitful it is important to first understand well the role of the isospin degree of freedom in nuclear reaction dynamics. Moreover, theoretical predictions on the sensitivity of experimental observables to the isospin dependence of the nuclear EOS are useful for explaining available data and planning new experiments. Recently, several useful observables have been identified in neutron-rich heavy-ion collisions at intermediate energies. These include the isospin fractionation @ 4‐1 0#, neutron to proton ratio of heavy residues @11# or projectilelike fragments @7,12#, and the neutron-proton differential flow @13#. Most of these observables make use of the relative multiplicities and kinetic energies of mirror nuclei. Information about the isospin dependence of the nuclear EOS can also be obtained from studying free nucleons as neutrons and protons have the opposite symmetry potentials in nuclear media. In a recent work, one of the present authors has shown that the neutron-proton differential flow is a rather useful probe of the isospin dependence of the nuclear EOS. The method utilizes constructively both the isospin fractionation and the nuclear collective flow as well as their sensitivities to the nuclear EOS. It, however, requires measuring the neutron and proton collective flows simultaneously. Experimentally, it is easier to identify charged particles and measure their momenta accurately, thus observables using charged particles only are more useful. In this work, within an isospin-dependent transport model for nuclear reactions involving neutron-rich nuclei, we explore the sensitivity of proton collective flow to the isospin dependence of the nuclear EOS in heavy-ion collisions at intermediate energies. We examine both the first-order transverse flow and the second-order differential elliptic flow as functions of transverse momenta. It is found that the differential elliptic flow of midrapidity protons, especially at high transverse momenta, is very sensitive to the isospin dependence of the nuclear EOS. This sensitivity is much stronger than that found in the first-order transverse collective flow around the projectile and target rapidity. At present, nuclear many-body theories predict vastly different isospin dependence of the nuclear EOS depending on both the calculation techniques and the bare two-body and/or three-body interactions employed, see, e.g., Refs. @14 ‐17#. Various theoretical studies ~e.g., Refs. @18,19#! have shown that the energy per nucleon e(r,d) in nuclear matter of density r and isospin-asymmetry parameter d defined as d[~r n2r p!/~r n1r p! ~1!

Neutron stars in a class of nonlinear relativistic models

Abstract: In this work we introduce a class of relativistic models for nuclear matter and neutron stars which exhibits a parametrization, through mathematical constants, of the nonlinear meson-baryon couplings. For appropriate choices of the parameters, it recovers current quantum hadrodynamics models found in the literature: the Walecka model and Zimanyi-Moszkowski models (ZM and ZM3). For other choices of parameters, the models give very interesting and new physical results. The phenomenology of neutron stars in ZM models is presented and compared to the phenomenology obtained in other versions of the Walecka model. We have found that the ZM3 model is too soft, and predicts a very small maximum neutron star mass, $\ensuremath{\sim}{0.72M}_{\ensuremath{\bigodot}}.$ A strong similarity between the results of ZM-like models and those with exponential couplings is noted. The sensibility of the results to the specific choice of the values for the binding energy and saturation density is pointed out. Finally, we discuss the very intense scalar condensates found in the interior of neutron stars, which may lead to negative effective masses.

Conditions for isoscaling in nuclear reactions

Abstract: With the availability of rare isotope beams as well as detection systems that can resolve the masses and charges of the detected particles, isotope yields become an important observable for studying nuclear collisions of heavy ions @1,2#. This additional freedom on isospin asymmetry allows one to study the properties of bulk nuclear matter that are affected by the nucleon composition of the nuclei such as the isospin dependence of the liquid gas phase transition of nuclear matter @3‐5# and the asymmetry term @ 6‐9 # in the nuclear equation of state. To minimize undesirable complications stemming from the sequential decays of primary unstable fragments, it has been proposed that isospin effects can best be studied by comparing the same observables in two similar reactions that differ mainly in isospin asymmetry @5,7,9#. If two reactions, 1 and 2, have the same temperature but different isospin asymmetry, for example, the ratios of a specific isotope yield with neutron and proton numberN and Z obtained from system 2 and system 1 have been observed to exhibit isoscaling, i.e., exponential dependence of the form @5,7# R21~ N,Z!5Y 2~ N,Z!/Y 1~ N,Z!5C exp~ Na1Zb!, ~1!

Effective YN and YY Interactions and Hyperon-Mixing in Neutron Star Matter Y ≡ Λ Case

Abstract: Effective ΛN and ΛΛ interactions in dense hyperonic nuclear matter are constructed on the basis of the G-matrix calculation with Nijmegen hard-core potentials. With these effective interactions, the mixing of Λ in neutron star matter and the equation of state are analyzed. The Λ-mixed phase is shown to appear in neutron star cores with a baryon number density ρ > ρt(Λ) � (3 − 5)ρ0, where ρt(Λ) is the threshold density for the Λmixing and ρ0 is the normal nuclear matter density. The density ρt(Λ) depends not only on the ΛN but also on the NN interactions. The three-body force introduced in the NN interaction to reproduce the proper nuclear saturation properties enhances the Λ-mixing and drastically softens the equation of state. The resulting equation of state is not consistent with the observed neutron star mass Mobs =1 .44M� . It is found that this crucial problem can be resolved by the introduction of a three-body repulsion also for the ΛN and ΛΛ interactions. The finite-temperature effect on the Λ-mixing is found to be large, especially at lower densities and is expected to affect the properties of neutron stars at birth.