297 research outputs found
Quark core impact on hybrid star cooling
In this paper we investigate the thermal evolution of hybrid stars, objects
composed of a quark matter core, enveloped by ordinary hadronic matter. Our
purpose is to investigate how important are the microscopic properties of the
quark core to the thermal evolution of the star. In order to do that we use a
simple MIT bag model for the quark core, and a relativistic mean field model
for the hadronic envelope. By choosing different values for the microscopic
parameters (bag constant, strange quark mass, strong coupling constant) we
obtain hybrid stars with different quark core properties. We also consider the
possibility of color superconductivity in the quark core. With this simple
approach, we have found a set of microscopic parameters that lead to a good
agreement with observed cooling neutron stars. Our results can be used to
obtain clues regarding the properties of the quark core in hybrid stars, and
can be used to refine more sophisticated models for the equation of state of
quark matter.Comment: 8 pages, 10 figures. Accepted for publication in Physical Review
Hybrid Stars in an SU(3) Parity Doublet Model
We apply an extended version of the SU(3) parity model, containing quark
degrees of freedom, to study neutron stars. The model successfully reproduces
the main thermodynamic features of QCD which allows us to describe the
composition of dense matter. Chiral symmetry restoration is realized inside the
star and the chiral partners of the baryons appear, their masses becoming
degenerate. Furthermore, quark degrees of freedom appear in a transition to a
deconfined state. Performing an investigation of the macroscopic properties of
neutron stars, we show that observational constraints, like mass and thermal
evolution, are satisfied and new predictions can be made
Terahertz and infrared spectroscopic evidence of phonon-paramagnon coupling in hexagonal piezomagnetic YMnO3
Terahertz and far-infrared electric and magnetic responses of hexagonal
piezomagnetic YMnO3 single crystals are investigated. Antiferromagnetic
resonance is observed in the spectra of magnetic permeability mu_a [H(omega)
oriented within the hexagonal plane] below the Neel temperature T_N. This
excitation softens from 41 to 32 cm-1 on heating and finally disappears above
T_N. An additional weak and heavily-damped excitation is seen in the spectra of
complex dielectric permittivity epsilon_c within the same frequency range. This
excitation contributes to the dielectric spectra in both antiferromagnetic and
paramagnetic phases. Its oscillator strength significantly increases on heating
towards room temperature thus providing evidence of piezomagnetic or
higher-order couplings to polar phonons. Other heavily-damped dielectric
excitations are detected near 100 cm-1 in the paramagnetic phase in both
epsilon_c and epsilon_a spectra and they exhibit similar temperature behavior.
These excitations appearing in the frequency range of magnon branches well
below polar phonons could remind electromagnons; however, their temperature
dependence is quite different. We have used density functional theory for
calculating phonon dispersion branches in the whole Brillouin zone. A detailed
analysis of these results and of previously published magnon dispersion
branches brought us to the conclusion that the observed absorption bands stem
from phonon-phonon and phonon- paramagnon differential absorption processes.
The latter is enabled by a strong short-range in-plane spin correlations in the
paramagnetic phase.Comment: subm. to PR
On the Density Dependent Nuclear Matter Compressibility
In the present work we apply a quantum hadrodynamic effective model in the
mean-field approximation to the description of neutron stars. We consider an
adjustable derivative-coupling model and study the parameter influence on the
dynamics of the system by analyzing the full range of values they can take. We
establish a set of parameters which define a specific model that is able to
describe phenomenological properties such as the effective nucleon mass at
saturation as well as global static properties of neutron stars (mass and
radius). If one uses observational data to fix the maximum mass for neutron
stars by a specific model, we are able to predict the compression modulus of
nuclear matter K = 257,2MeV
Effective Model Approach to the Dense State of QCD Matter
The first-principle approach to the dense state of QCD matter, i.e. the
lattice-QCD simulation at finite baryon density, is not under theoretical
control for the moment. The effective model study based on QCD symmetries is a
practical alternative. However the model parameters that are fixed by hadronic
properties in the vacuum may have unknown dependence on the baryon chemical
potential. We propose a new prescription to constrain the effective model
parameters by the matching condition with the thermal Statistical Model. In the
transitional region where thermal quantities blow up in the Statistical Model,
deconfined quarks and gluons should smoothly take over the relevant degrees of
freedom from hadrons and resonances. We use the Polyakov-loop coupled
Nambu--Jona-Lasinio (PNJL) model as an effective description in the quark side
and show how the matching condition is satisfied by a simple ansatz on the
Polyakov loop potential. Our results favor a phase diagram with the chiral
phase transition located at slightly higher temperature than deconfinement
which stays close to the chemical freeze-out points.Comment: 8 pages, 4 figures; Talk at International Workshop on High Density
Nuclear Matter, Cape Town, South Africa, April 6-9, 201
Compact Stars - How Exotic Can They Be?
Strong interaction physics under extreme conditions of high temperature
and/or density is of central interest in modern nuclear physics for
experimentalists and theorists alike. In order to investigate such systems,
model approaches that include hadrons and quarks in a unified approach, will be
discussed. Special attention will be given to high-density matter as it occurs
in neutron stars. Given the current observational limits for neutron star
masses, the properties of hyperonic and hybrid stars will be determined. In
this context especially the question of the extent, to which exotic particles
like hyperons and quarks affect star masses, will be discussed.Comment: Contributon to conference "Nuclear Physics: Present and Future", held
in Boppard (Germany), May 201
An effective chiral Hadron-Quark Equation of State
We construct an effective model for the QCD equation of state, taking into
account chiral symmetry restoration as well as the deconfinement phase
transition. The correct asymptotic degrees of freedom at the high and low
temperature limits are included (quarks hadrons). The model
shows a rapid crossover for both order parameters, as is expected from lattice
calculations. We then compare the thermodynamic properties of the model at
which turn out to be in qualitative agreement with lattice data,
while apparent quantitative differences can be attributed to hadronic
contributions and excluded volume corrections. Furthermore we discuss the
effects of a repulsive vector type quark interaction at finite baryon number
densities on the resulting phase diagram of the model. Our current model is
able to reproduce a first-order liquid gas phase transition as expected, but
does not show any signs of a first order deconfinement or chiral phase
transition. Both transitions rather appear as a very wide crossover in which
heavily medium modified hadron coexist with free quarks.Comment: 19 pages, 13 figures Version accepted by J. Phys.
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