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H. -J. Schulze

Publications and source records attributed to H. -J. Schulze.

72 records · Page 4Linked to original sources
arXiv2004

Hybrid stars with the color dielectric and the MIT bag models

We study the hadron-quark phase transition in the interior of neutron stars (NS). For the hadronic sector, we use a microscopic equation of state (EOS) involving nucleons and hyperons derived within the Brueckner-Bethe-Goldstone many-body theory, with realistic two-body and three-body forces. For the description of quark matter, we employ both the MIT bag model with a density dependent bag constant, and the color dielectric model. We calculate the structure of NS interiors with the EOS comprising both phases, and we find that the NS maximum masses are never larger than 1.7 solar masses, no matter the model chosen for describing the pure quark phase.

nucl-th↗
arXiv2004

1S0 Proton and Neutron Superfluidity in beta-stable Neutron Star Matter

We investigate the effect of a microscopic three-body force on the proton and neutron superfluidity in the $^1S_0$ channel in $β$-stable neutron star matter. It is found that the three-body force has only a small effect on the neutron $^1S_0$ pairing gap, but it suppresses strongly the proton $^1S_0$ superfluidity in $β$-stable neutron star matter.

nucl-th↗
arXiv2004

Pairing in two-dimensional boson-fermion mixtures

The possibilities of pairing in two-dimensional boson-fermion mixtures are carefully analyzed. It is shown that the boson-induced attraction between two identical fermions dominates the p-wave pairing at low density. For a given fermion density, the pairing gap becomes maximal at a certain optimal boson concentration. The conditions for observing pairing in current experiments are discussed

cond-mat.supr-con↗
arXiv2003

Neutron Star Structure with Hyperons and Quarks

We discuss the high-density nuclear equation of state within the Brueckner-Hartree-Fock approach. Particular attention is paid to the effects of nucleonic three-body forces, the presence of hyperons, and the joining with an eventual quark matter phase. The resulting properties of neutron stars, in particular the mass-radius relation, are determined. It turns out that stars heavier than 1.3 solar masses contain necessarily quark matter.

astro-ph↗
arXiv2003

On the maximum rotational frequency of neutron and hybrid stars

We construct self-consistent equilibrium sequences of general relativistic, rotating neutron star models. Special emphasis in put on the determination of the maximum rotation frequency of such objects. Recently proposed models for the equation of state of neutron star matter are employed, which are derived by describing the hadronic phase within the many-body Brueckner--Bethe--Goldstone formalism, and the quark matter phase within the MIT bag model using a density dependent bag constant. We find that the rotational frequencies of neutron stars with deconfined quark phases in their cores rival those of absolutely stable, self-bound strange quark matter stars. This finding is of central importance for the interpretation of extremely rapidly rotating pulsars, which are the targets of present pulsar surveys.

astro-ph↗
arXiv2003

Neutron stars and the transition to color-superconducting quark matter

We explore the relevance of color superconductivity inside a possible quark matter core for the bulk properties of neutron stars. For the quark phase we use an Nambu--Jona-Lasinio (NJL) type model, extended to include diquark condensates. For the hadronic phase, a microscopic many-body model is adopted, with and without strangeness content. In our calculations, a sharp boundary is assumed between the hadronic and the quark phases. For NJL model parameters fitted to vacuum properties we find that no star with a pure quark core does exist. Nevertheless the presence of color superconducting phases can lower the neutron star maximum mass substantially. In some cases, the transition to quark matter occurs only if color superconductivity is present. Once the quark phase is introduced, the value of the maximum mass stays in any case below the value of two solar masses.

nucl-th↗
arXiv2003

Pion broadening and low-mass dilepton production

We determine mass and transverse momentum spectra of dileptons produced in Pb+Au (158 GeV/u) collisions within a pion annihilation model. A fit to the data requires simultaneous mass reduction and broadening of the in-medium rho propagator. The introduction of a finite pion width, as required within self-consistent approaches to the interacting pion gas, further improves the agreement with the data.

hep-ph↗
arXiv2002

Short-range Effects on Nuclear Pairing

We determine the influence of the three-body force and the medium modification of meson masses on pairing in nuclear and neutron matter. A reduction of the pairing gap is found and increasing with density.

nucl-th↗
arXiv2002

The hadron-quark phase transition in dense matter and neutron stars

We study the hadron-quark phase transition in the interior of neutron stars (NS's). We calculate the equation of state (EOS) of hadronic matter using the Brueckner-Bethe-Goldstone formalism with realistic two-body and three-body forces, as well as a relativistic mean field model. For quark matter we employ the MIT bag model constraining the bag constant by using the indications coming from the recent experimental results obtained at the CERN SPS on the formation of a quark-gluon plasma. We find necessary to introduce a density dependent bag parameter, and the corresponding consistent thermodynamical formalism. We calculate the structure of NS interiors with the EOS comprising both phases, and we find that the NS maximum masses fall in a relatively narrow interval, $1.4 M_\odot \leq M_{\rm max} \leq 1.7 M_\odot$. The precise value of the maximum mass turns out to be only weakly correlated with the value of the energy density at the assumed transition point in nearly symmetric nuclear matter.

nucl-th↗
arXiv2001

Maximum mass of neutron stars with a quark core

Massive neutron stars (NS) are expected to possess a quark core. While the hadronic side of the NS equation of state (EOS) can be considered well established, the quark side is quite uncertain. While calculating the EOS of hadronic matter we have used the Brueckner-Bethe-Goldstone formalism with realistic two-body and three-body forces, as well as a relativistic mean field model. For quark matter we employ the MIT bag model constraining the bag constant by exploiting the recent experimental results obtained at CERN on the formation of a quark-gluon plasma. We calculate the structure of NS interiors with the EOS comprising both phases, and we find that the NS maximum masses fall in a relatively narrow interval, $1.45 M_\odot \leq M_{\rm max} \leq 1.65 M_\odot$, near the lower limit of the observational range.

astro-ph↗
arXiv2001

Pairing in asymmetric two-component fermion matter

We analyze the possibilities of pairing between two different fermion species in asymmetric matter at low density. While the direct interaction allows pairing only for very small asymmetries, the pairing mediated by polarization effects is always possible, with a pronounced maximum at finite asymmetry. We present analytical results up to second order in the low-density parameter $k_F a$.

cond-mat.soft↗
arXiv2001

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

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.

nucl-th↗
arXiv2000

Strange hadronic stellar matter within the Brueckner-Bethe-Goldstone theory

In the framework of the non-relativistic Brueckner-Bethe-Goldstone theory, we derive a microscopic equation of state for asymmetric and $β$-stable matter containing $Σ^-$ and $Λ$ hyperons. We mainly study the effects of three-body forces (TBFs) among nucleons on the hyperon formation and the equation of state (EoS). We find that, when TBFs are included, the stellar core is almost equally populated by nucleons and hyperons. The resulting EoS, which turns out to be extremely soft, has been used in order to calculate the static structure of neutron stars. We obtain a value of the maximum mass of 1.26 solar masses (1 solar mass $M_o \simeq 1.99 \cdot 10^{33} g$). Stellar rotations increase this value by about 12%.

nucl-th↗
arXiv1999

Hyperon stars in the Brueckner-Bethe-Goldstone theory

In the framework of the Brueckner-Bethe-Goldstone theory, we determine a fully microscopic equation of state for asymmetric and $β$-stable nuclear matter containing $\sim$ and $\la$ hyperons. We use the Paris and the new Argonne $Av_{18}$ two-body nucleon interaction, whereas the nucleon-hyperon interaction is described by the Njimegen soft-core model. We stress the role played by the three-body nucleon interaction, which produces a strong repulsion at high densities. This enhances enormously the hyperon population, and produces a strong softening of the equation of state, which turns out almost independent on the nucleon-nucleon interaction. We use the new equation of state in order to calculate the structure of static neutron stars. We obtain a maximum mass configuration with $M_{\rm max}$ = 1.26 (1.22) when the Paris ($Av_{18}$) nucleon potential is adopted. Central densities are about 10 times normal nuclear matter density. Stellar rotations, treated within a perturbative approach, increase the value of the limiting mass by about 12%.

nucl-th↗
arXiv1998

3P_2-3F_2 pairing in neutron matter with modern nucleon-nucleon potentials

We present results for the $^3P_2 - ^3F_2$ pairing gap in neutron matter with several realistic nucleon-nucleon potentials, in particular with recent, phase-shift equivalent potentials. We find that their predictions for the gap cannot be trusted at densities above $ρ\approx 1.7ρ_0$, where $ρ_0$ is the saturation density for symmetric nuclear matter. In order to make predictions above that density, potential models which fit the nucleon-nucleon phase shifts up to about 1 GeV are required.

nucl-th↗
arXiv1997

Scalar correlations in a quark plasma and low mass dilepton production

We investigate possible consequences of resonant scalar interactions for dilepton production from a quark plasma at the chiral phase transition. It is found that this production mechanism is strongly suppressed compared to the Born process and has no significance for present experiments.

nucl-th↗
arXiv1996

Dilepton Enhancement by Thermal Pion Annihilation in the CERES Experiment

We compare the recent CERES data on dielectron production in 200 GeV/u S+Au collisions with the theoretical predictions due to pion annihilation in a thermal hadronization and a string fragmentation scenario. Both models yield similar results for the dilepton mass spectrum. A satisfactory description of the experimental spectrum requires an in-medium reduction of the rho-mass and a freeze-out temperature of about 150 MeV in the thermal model. We emphasize and discuss the influence of experimental acceptance and resolution corrections.

nucl-th↗