154 research outputs found
Cold uniform matter and neutron stars in the quark-mesons-coupling model
A new density dependent effective baryon-baryon interaction has been recently
derived from the quark-meson-coupling (QMC) model, offering impressive results
in application to finite nuclei and dense baryon matter. This self-consistent,
relativistic quark-level approach is used to construct the Equation of State
(EoS) and to calculate key properties of high density matter and cold, slowly
rotating neutron stars. The results include predictions for the maximum mass of
neutron star models, together with the corresponding radius and central
density, as well the properties of neutron stars with mass of order 1.4
. The cooling mechanism allowed by the QMC EoS is explored and the
parameters relevant to slow rotation, namely the moment of inertia and the
period of rotation investigated. The results of the calculation, which are
found to be in good agreement with available observational data, are compared
with the predictions of more traditional EoS. The QMC EoS provides cold neutron
star models with maximum mass 1.9--2.1 M, with central density less
than 6 times nuclear saturation density () and
offers a consistent description of the stellar mass up to this density limit.
In contrast with other models, QMC predicts no hyperon contribution at
densities lower than , for matter in -equilibrium. At higher
densities, and hyperons are present
Spontaneous symmetry breaking and response functions
We study the quantum phase transition occurring in an infinite homogeneous
system of spin 1/2 fermions in a non-relativistic context. As an example we
consider neutrons interacting through a simple spin-spin Heisenberg force. The
two critical values of the coupling strength -- signaling the onset into the
system of a finite magnetization and of the total magnetization, respectively
-- are found and their dependence upon the range of the interaction is
explored. The spin response function of the system in the region where the
spin-rotational symmetry is spontaneously broken is also studied. For a
ferromagnetic interaction the spin response along the direction of the
spontaneous magnetization occurs in the particle-hole continuum and displays,
for not too large momentum transfers, two distinct peaks. The response along
the direction orthogonal to the spontaneous magnetization displays instead,
beyond a softened and depleted particle-hole continuum, a collective mode to be
identified with a Goldstone boson of type II. Notably, the random phase
approximation on a Hartree-Fock basis accounts for it, in particular for its
quadratic -- close to the origin -- dispersion relation. It is shown that the
Goldstone boson contributes to the saturation of the energy-weighted sum rule
for ~25% when the system becomes fully magnetized (that is in correspondence of
the upper critical value of the interaction strength) and continues to grow as
the interaction strength increases.Comment: 36 pages, 17 figure
On the coefficients of the liquid drop model mass formulae and nuclear radii
The coefficients of different mass formulae derived from the liquid drop
model and including or not the curvature energy, the diffuseness correction to
the Coulomb energy, the charge exchange correction term, different forms of the
Wigner term and different powers of the relative neutron excess
have been determined by a least square fitting procedure to 2027 experimental
atomic masses. The Coulomb diffuseness correction term or the charge
exchange correction term plays the main role to improve the
accuracy of the mass formula. The Wigner term and the curvature energy can also
be used separately for the same purpose. The introduction of an
dependence in the surface and volume energies improves slightly the efficiency
of the expansion and is more effective than an dependence. Different
expressions reproducing the experimental nuclear charge radius are provided.
The different fits lead to a surface energy coefficient of around 17-18 MeV and
a relative equivalent rms charge radius r of 1.22-1.23 fm.Comment: 19 pages, 1 figure, accepted for publication in Nuclear Physics
Microscopic calculation of the equation of state of nuclear matter and neutron star structure
We present results for neutron star models constructed with a new equation of
state for nuclear matter at zero temperature. The ground state is computed
using the Auxiliary Field Diffusion Monte Carlo (AFDMC) technique, with
nucleons interacting via a semi-phenomenological Hamiltonian including a
realistic two-body interaction. The effect of many-body forces is included by
means of additional density-dependent terms in the Hamiltonian. In this letter
we compare the properties of the resulting neutron-star models with those
obtained using other nuclear Hamiltonians, focusing on the relations between
mass and radius, and between the gravitational mass and the baryon number.Comment: modified version with a slightly different Hamiltonian and
parametrization of the EO
Hyperons and massive neutron stars: the role of hyperon potentials
The constituents of cold dense matter are still far from being understood.
However, neutron star observations such as the recently observed pulsar PSR
J1614-2230 with a mass of 1.97+/-0.04 M_solar help to considerably constrain
the hadronic equation of state (EoS). We systematically investigate the
influence of the hyperon potentials on the stiffness of the EoS. We find that
they have but little influence on the maximum mass compared to the inclusion of
an additional vector meson mediating repulsive interaction amongst hyperons.
The new mass limit can only be reached with this additional meson regardless of
the hyperon potentials. Further, we investigate the impact of the nuclear
compression modulus and the effective mass of the nucleon at saturation density
on the high density regime of the EoS. We show that the maximum mass of purely
nucleonic stars is very sensitive to the effective nucleon mass but only very
little to the compression modulus.Comment: 24 pages, 8 figure
Photoproduction of hypernuclei within the quark-meson coupling model
We study the photoproduction of the ^12{_Lambda}B hypernucleus within a fully
covariant effective Lagrangian based model, employing Lambda bound state
spinors derived from the latest quark-meson coupling model. The kaon production
vertex is described via creation, propagation and decay of N*(1650), N*(1710),
and N*(1720) intermediate baryonic resonant states in the initial collision of
the photon with a target proton in the incident channel. The parameters of the
resonance vertices are fixed by describing the total and differential cross
section data on the elementary gamma (p, K+) Lambda reaction in the energy
regime relevant to the hypernuclear production. It is found that the
hypernuclear production cross sections calculated with the quark model based
hyperon bound state spinors differ significantly from those obtained with the
phenomenological Dirac single particle wave functions.Comment: 16 pages, 5 figures, version to appear in Phys. Lett.
Production of cascade hypernuclei via the (K-,K+) reaction within a quark-meson coupling model
We study the production of bound cascade hypernuclei via the (K-,K+) reaction
on 12C and 28Si targets within a covariant effective Lagrangian model,
employing the cascade bound state spinors derived from the latest quark-meson
coupling model as well as Dirac single particle wave functions. The K+-cascade
production vertex is described by excitation, propagation and decay of Lambda
and Sigma resonance states in the initial collision of a K- meson with a target
proton in the incident channel. The parameters of the resonance vertices are
fixed by describing the available data on total and differential cross sections
for the cascade production in elementary (K-,K+) reaction. We find that both
the elementary and hypernuclear production cross sections are dominated by the
contributions from the Lambda(1520) intermediate resonant state. The 0 degree
differential cross sections for the formation of simple s-state cascade
particle-hole states peak at a beam momentum around 1.0 GeV/c, with a value in
excess of 1 mub.Comment: 17 pages, 8 figures, version accepted for publication in Nucl. Phys.
Phase transition to the state with nonzero average helicity in dense neutron matter
The possibility of the appearance of the states with a nonzero average
helicity in neutron matter is studied in the model with the Skyrme effective
interaction. By providing the analysis of the self-consistent equations at zero
temperature, it is shown that neutron matter with the Skyrme BSk18 effective
force undergoes at high densities a phase transition to the state in which the
degeneracy with respect to helicity of neutrons is spontaneously removed.Comment: 4 pages, 3 figures; v2: journal versio
Structure and Coulomb dissociation of 23O within the quark-meson coupling model
We study the ground-state structure of nuclei in the vicinity of the
one-neutron dripline within the latest version of the quark-meson coupling
(QMC) model with a particular emphasis on 23O. For this nucleus the model
predicts a [22O(0+) + n (2s_{1/2})] configuration for its ground state, with a
one neutron separation energy in close agreement with the corresponding
experimental value. The wave function describing the valence neutron-core
relative motion was then used to calculate the Coulomb dissociation of 23O on a
lead target at a beam energy of 422 MeV/nucleon. The experimental neutron-core
relative energy spectrum and the total one-neutron removal cross sections are
well described by the calculations. The widths of the longitudinal momentum
distributions of the 22O fragment are found to be broad, which do not support
the formation of a neutron halo in this nucleus.Comment: Revised and extended version, accepted for publication in Nucl. Phys.
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