Neutron stars with isovector scalar correlations

Neutron stars with the isovector scalar δ-field are studied in the framework of the relativistic mean-field (RMF) approach in a pure-nucleon-plus-lepton scheme. The δ-field leads to a larger repulsion in dense neutron-rich matter and to a definite splitting of proton and neutron effective masses. Both features are influencing the stability conditions of the neutron stars. Two parametrizations for the effective nonlinear Lagrangian density are used to calculate the nuclear equation of state (EOS) and the neutron star properties, and compared to correlated Dirac-Brueckner results. We conclude that in order to reproduce reasonable nuclear structure and neutron star properties within a RMF approach, a density dependence of the coupling constants is required.     

Physics at the Thomas Jefferson National Accelerator Facility

We present here a brief summary of the presentation given at the Quark-Gluon-Plasma Thermalization Workshop in Vienna, Austria in August 2005, directly following the International Quark Matter Conference in Hungary.     

A realistic model of superfluidity in the neutron star inner crust

A semi-microscopic self-consistent quantum approach developed recently to describe the inner-crust structure of neutron stars within the Wigner-Seitz (WS) method with the explicit inclusion of neutron and proton pairing correlations is further developed. In this approach, the generalized energy functional is used which contains the anomalous term describing the pairing. It is constructed by matching the realistic phenomenological functional by Fayans et al. for describing the nuclear-type cluster in the center of the WS cell with the one calculated microscopically for neutron matter. Previously, the anomalous part of the latter was calculated within the BCS approximation. In this work corrections to the BCS theory which are known from the many-body theory of pairing in neutron matter are included into the energy functional in an approximate way. These modifications have a sizable influence on the equilibrium configuration of the inner crust, i.e. on the proton charge Z and the radius R _c of the WS cell. The effects are quite significant in the region where the neutron pairing gap is larger.     

Formation of charmonium states in heavy-ion collisions and thermalization of charm

We examine the possibility to utilize in-medium charmonium formation in heavy-ion interactions at collider energy as a probe of the properties of the medium. This is possible because the formation process involves recombination of charm quarks which imprints a signal on the resulting normalized transverse momentum distribution containing information about the momentum distribution of the quarks. We have contrasted the transverse momentum spectra of J/ψ, characterized by 〈p _T ²〉, which result from the formation process in which the charm quark distributions are taken at opposite limits with regard to thermalization in the medium. The first uses charm quark distributions unchanged from their initial production in a pQCD process, appropriate if their interaction with the medium is negligible. The second uses charm quark distributions which are in complete thermal equilibrium with the transversely expanding medium, appropriate if a very strong interaction between charm quarks and medium exists. We find that the resulting 〈p _T ²〉 of the formed J/ψ should allow one to differentiate between these extremes, and that this differentiation is not sensitive to variations in the detailed dynamics of in-medium formation. We include a comparison of predictions of this model with preliminary PHENIX measurements, which indicates compatibility with a substantial fraction of in-medium formation.     

Signals of a Weibel instability in the melting color glass condensate

Based on our work hep-ph/0510121, we discuss further the numerical study of classical SU(2) 3+1-D Yang-Mills equations for matter produced in a high-energy heavy-ion collision. The growth of the amplitude of fluctuations as exp(Γ ) (where g ²μ is a scale arising from the saturation of gluons in the nuclear wave function) is shown to be robust over a wide range of initial amplitudes that violate boost invariance. We argue that this growth is due to a non-Abelian Weibel instability, the scale of which is set by a dynamically generated plasmon mass. We discuss the relation of Γ to the prediction from kinetic theory.     

Nuclear matter properties and relativistic mean-field theory

Nuclear matter properties are calculated in the relativistic mean-field theory by using a number of different parameter sets. The result shows that the volume energy a₁ and the symmetry energy J are around the acceptable values 16MeV and 30MeV, respectively; the incompressibility K₀ is unacceptably high in the linear model, but assumes reasonable value if nonlinear terms are included; the density symmetry L is around 100MeV for most parameter sets, and the symmetry incompressibility K _s has positive sign which is opposite to expectations based on the nonrelativistic model. In almost all parameter sets there exists a critical point (,), where the minimum and the maximum of the equation of state are coincident and the incompressibility equals zero, falling into ranges 0.014fm^-3 < < 0.039fm^-3 and 0.74 < ≤0.95; for a few parameter sets there is no critical point and the pure neutron matter is predicted to be bound. The maximum mass M _NS of neutron stars is predicted in the range 2.45M ?M _NS? 3.26M , the corresponding neutron star radius R _NS is in the range 12.2km ?R _NS? 15.1km. Received: 5 May 2000 / Accepted: 28 November 2000     

Phase transition to color-flavor-locked matter and condensation of Goldstone bosons in neutron star matter

Deconfinement phase transition and condensation of Goldstone bosons in neutron star matter are investigated in a chiral hadronic model (also referred as to the FST model) for the hadronic phase (HP) and in the color-flavor-locked (CFL) quark model for the deconfined quark phase. It is shown that the hadronic-CFL mixed phase (MP) exists in the center of neutron stars with a small bag constant, while the CFL quark matter cannot appear in neutron stars when a large bag constant is taken. Color superconductivity softens the equation of state (EOS) and decreases the maximum mass of neutron stars compared with the unpaired quark matter. The K⁰ condensation in the CFL phase has no remarkable contribution to the EOS and properties of neutron star matter. The EOS and the properties of neutron star matter are sensitive to the bag constant B, the strange quark mass m_s and the color superconducting gap Δ. Increasing B and m_s or decreasing Δ can stiffen the EOS which results in the larger maximum masses of neutron stars.     

The beauty of the electromagnetic probe

Selected results from the first five years of RHIC data taking are reviewed with emphasis on the evidence for thermalization in central Au + Au collisions at = 200GeV.     

On the importance of low-energy beta beams for supernova neutrino physics

We argue that isotropization and, consequently, thermalization of the system of gluons and quarks produced in an ultrarelativistic heavy-ion collision does not follow from Feynman diagram analysis to any order in the coupling constant. We conclude that the apparent thermalization of quarks and gluons, leading to success of perfect fluid hydrodynamics in describing heavy-ion collisions at RHIC, can only be attributed to the non-perturbative QCD effects not captured by Feynman diagrams. We proceed by modeling these non-pertrubative thermalization effects using viscous hydrodynamics. We point out that matching Color Glass Condensate inital conditions with viscous hydrodynamics leads to a continuous evolution of all the components of the energy-momentum tensor and, unlike the case of ideal hydrodynamics, does not give rise to a discontinuity in the longitudinal pressure. An important consequence of such a matching is a relationship between the thermalization time and shear viscosity: we observe that small viscosity leads to short thermalization time.     

Fluctuations and correlations in the string clustering approach

The fluctuations due to the clustering of color sources can explain the behaviour of the scaled multiplicity variance and transverse momentum fluctuations with centrality. They also predict a nonmonotonic behaviour with centrality for the multiplicity associated to high-p_T events. The clustering of color sources gives rise to an increase in the long-range correlations with centrality as well as to a supression at high centrality with respect to superposition models.     

Shear viscosity and entropy of quark matter

We consider the shear viscosity of a system of quarks and its ratio to the entropy density above the critical temperature for deconfinement. Expressions for both quantities are derived in the quasi-particle approximation and calculations are carried out for different modeling of the quark self-energy, also allowing for a temperature dependence of the effective mass and width. Beyond the temperature dependence, the behaviour of the viscosity and the entropy density is discussed in terms of the strength of the coupling and of the main characteristics of the quark self-energy. A comparison with existing approaches is also discussed.     

Neutrino-nucleon scattering rate in proto-neutron star matter

We present a calculation of the neutrino-nucleon scattering cross-section which takes into account the nuclear correlations in the relativistic random phase approximation (RPA). Our approach is based on a quantum-hadrodynamics model with exchange of σ, ω, π, ρ and δ mesons. In view of applications to neutrino transport in the final stages of supernova explosion and proto-neutron star cooling, we study the evolution of the neutrino mean free path as a function of density, proton-neutron asymmetry and temperature. Special attention was paid to the issues of renormalization of the Dirac sea, residual interactions in the tensor channel, coupling to the delta-meson and meson mixing. In contrast with the results of other authors, we find that the neutral-current process is not sensitive to the strength g' of the residual contact interaction. As a consequence, it is found that RPA corrections with respect to the mean-field approximation amount to only 10% to 15% at high density. Received: 27 June 2001 / Accepted: 14 January 2002     

Self-consistently thermodynamic treatment for strange quark matter in the effective mass bag model

In the framework of the effective mass bag model (EMBM) we have performed the thermodynamical treatment for strange quark matter (SQM) self-consistently, which overcomes the inconsistencies in the thermodynamical properties of the system. Because of the existence of the pressure extra term, the SQM equation of state (EOS) becomes stiffer comparing with the one for the original EMBM. It is interesting to find that in our treatment the SQM EOS is almost independent of the strong coupling constant g . In this case the SQM EOS seems to get back to the EOS for the original MIT bag model. However, this treatment still has influence on the EOS for hybrid star matter and the corresponding mass-radius relations. With the increase of the strong coupling constant g , the EOS for hybrid star matter gets obviously stiff. From our treatment we notice that the pressure extra term can make a hybrid star more compact than the one described in the original EMBM and this model is more suitable to describe the hybrid stars with small radii.     

Q¯ modes in the Quark-Gluon Plasma

We study the evolution of heavy quarkonium states with temperature in a Quark-Gluon Plasma (QGP) by evaluating an in-medium Qˉ T-matrix within a reduced Bethe-Salpeter equation in S- and P-wave channels. The interaction kernel is extracted from finite-temperature QCD lattice calculations of the singlet free energy of a Qˉ pair. Quarkonium bound states are found to gradually move across the Qˉ threshold after which they rapidly dissolve in the hot system. We calculate Euclidean-time correlation functions and compare to results from lattice QCD. We also study finite-width effects in the heavy-quark propagators.     

Few-nucleon systems (theory)

Three-gluon to three-gluon scatterings lead to rapid thermalization of gluon matter created in central Au-Au collisions at RHIC energies. Thermalization of quark matter is studied from three-quark to three-quark scatterings.     

Phase diagram of neutron star quark matter in nonlocal chiral models

We analyze the phase diagram of two-flavor quark matter under neutron star constraints for a nonlocal covariant quark model within the mean-field approximation. Applications to cold compact stars are discussed.     

Hawking-Unruh phenomenon in the parton language

Inelastic hadron interactions at high energies are accompanied by a pulse of a strong chromo-electric field. This field leads to the decay of QCD vacuum which proceeds through the emission of partons with a thermal spectrum. In a semi-classical treatment, the effective temperature of the spectrum is determined by the acceleration of partons in the classical chromo-electric field, in accord with the general arguments given by Hawking and Unruh.     

Screened potential and quarkonia properties at high temperatures

We perform a quark model calculation of the quarkonia b and c spectra using smooth and sudden string breaking potentials. The screening parameter is scale dependent and can be related to an effective running gluon mass that has a finite infrared fixed point. A temperature dependence for the screening mass is motivated by lattice QCD simulations at finite temperature. Qualitatively different results are obtained for quarkonia properties close to a critical value of the deconfining temperature when a smooth or a sudden string breaking potential is used. In particular, with a sudden string breaking potential quarkonia radii remain almost independent of the temperature up to the critical point, only well above the critical point the radii increase significantly. Such a behavior will impact the phenomenology of quarkonia interactions in medium, in particular for scattering dissociation processes.     

Two generalizations of the Boltzmann equation

We connect two different extensions of Boltzmann's kinetic theory by requiring the same stationary solution. Non-extensive statistics can be produced by either using corresponding collision rates nonlinear in the one-particle densities or equivalently by using nontrivial energy composition rules in the energy conservation constraint part. Direct transformation formulas between key functions of the two approaches are given.     

Coexistence of color superconductivity and chiral symmetry breaking within the NJL model

The phase diagram for quark matter is investigated within a simple Nambu-Jona-Lasinio model without vector correlations. It is found that the phase structure in the temperature-density plane depends sensitively on the parametrization of the model. We present two schemes of parametrization of the model where, within the first one, a first-order phase transition from a phase with broken chiral symmetry to a color superconducting phase for temperatures below the triple point at T _t = 55 MeV occurs, whereas for the second one a second-order phase transition for temperatures below T _t = 7 MeV is found. In the latter case, there is also a coexistence phase of broken chiral symmetry with color superconductivity, which is a new finding within this class of models. Possible consequences for the phenomenology of the QCD phase transition at high baryon densities are discussed. Received: 3 January 2003 / Accepted: 21 February 2003 / Published online: 24 April 2003