Effect of the δ\delta-meson on the instabilities of nuclear matter under strong magnetic fields

We study the influence of the isovector-scalar meson on the spinodal instabilities and the distillation effect in asymmetric non-homogenous nuclear matter under strong magnetic fields, of the order of $10^{18}-10^{19}$ G. Relativistic nuclear models both with constant couplings (NLW) and with density dependent parameters (DDRH) are considered. A strong magnetic field can have large effects on the instability regions giving rise to bands of instability and wider unstable regions. It is shown that for neutron rich matter the inclusion of the $\delta$ meson increases the size of the instability region for NLW models and decreases it for the DDRH models. The effect of the $\delta$ meson on the transition density to homogeneous $\beta$-equilibrium matter is discussed. The DDRH$\delta$ model predicts the smallest transition pressures, about half the values obtained for NL$\delta$.Comment: 6 pages, 5 figues, 3 tables, accepted for publication in Phys. Rev.

Color symmetrical superconductivity in a schematic nuclear quark model

In this note, a novel BCS-type formalism is constructed in the framework of a schematic QCD inspired quark model, having in mind the description of color symmetrical superconducting states. The physical properties of the BCS vacuum (average numbers of quarks of different colors) remain unchanged under an arbitrary color rotation. In the usual approach to color superconductivity, the pairing correlations affect only the quasi-particle states of two colors, the single particle states of the third color remaining unaffected by the pairing correlations. In the theory of color symmetrical superconductivity here proposed, the pairing correlations affect symmetrically the quasi-particle states of the three colors and vanishing net color-charge is automatically insured. It is found that the groundstate energy of the color symmetrical sector of the Bonn model is well approximated by the average energy of the color symmetrical superconducting state proposed here

Hyperons in neutron star matter within relativistic mean-field models

Since the discovery of neutron stars with masses around 2 solar masses the composition of matter in the central part of these massive stars has been intensively discussed. Within this paper we will (re)investigate the question of the appearance of hyperons. To that end we will perform an extensive parameter study within relativistic mean field models. We will show that it is possible to obtain high mass neutron stars (i) with a substantial amount of hyperons, (ii) radii of 12-13 km for the canonical mass of 1.4 solar masses, and (iii) a spinodal instability at the onset of hyperons. The results depend strongly on the interaction in the hyperon-hyperon channels, on which only very little information is available from terrestrial experiments up to now.Comment: 15 pages, 10 figure

Dynamical instabilities in density-dependent hadronic relativistic models

Unstable modes in asymmetric nuclear matter (ANM) at subsaturation densities are studied in the framework of relativistic mean-field density-dependent hadron models. The size of the instabilities that drive the system are calculated and a comparison with results obtained within the non-linear Walecka model is presented. The distillation and anti-distillation effects are discussed.Comment: 8 pages, 8 Postscript figures. Submitted for publication in Phys. Rev.

Aspects of short range correlations in a relativistic model

In the present work short range correlations are introduced for the first time in a relativistic approach to the equation of state of the infinite nuclear matter in the framework of the Hartree-Fock approximation using an effective Hamiltonian derived from the $\sigma-\omega$ Walecka model. The unitary correlation method is used to introduce short range correlations. The effect of the correlations in the ground state properties of the nuclear matter is discussed.Comment: 7 pages, 3 figure

Nuclear symmetry energy and core-crust transition in neutron stars: a critical study

The slope of the nuclear symmetry energy at saturation density $L$ is pointed out as a crucial quantity to determine the mass and width of neutron-star crusts. This letter clarifies the relation between $L$ and the core-crust transition. We confirm that the transition density is soundly correlated with $L$ despite differences between models, and we propose a clear understanding of this correlation based on a generalised liquid drop model. Using a large number of nuclear models, we evaluate the dispersion affecting the correlation between the transition pressure $P_t$ and $L$. From a detailed analysis it is shown that this correlation is weak due to a cancellation between different terms. The correlation between the isovector coefficients $K_{\rm sym}$ and $L$ plays a crucial role in this discussion

Mean-filed theories with mixed states and associated boson expansions

A variational derivation of the Liouville-von Neumann equation of quantum-statistical mechanics is presented, in order to formulate mean-field approximations appropriate to mixed states. The Hartree-Fock and the RPA at finite temperatures are particular cases of the general formalism. A thermal boson expansion is defined, which allows us to describe anharmonic motion around a thermal excited state. In a numerical application on the basis of the Lipkin model, temperature-dependent phase transitions are observed