92 research outputs found
Higher-order effects on the incompressibility of isospin asymmetric nuclear matter
Analytical expressions for the saturation density as well as the binding
energy and incompressibility at the saturation density of asymmetric nuclear
matter are given exactly up to 4th-order in the isospin asymmetry delta =(rho_n
- rho_p)/rho using 11 characteristic parameters defined at the normal nuclear
density rho_0. Using an isospin- and momentum-dependent modified Gogny (MDI)
interaction and the SHF approach with 63 popular Skyrme interactions, we have
systematically studied the isospin dependence of the saturation properties of
asymmetric nuclear matter, particularly the incompressibility at the
saturation density. Our results show that the magnitude of the high-order
parameter is generally small compared to that of the K_{\sat,2}
parameter. The latter essentially characterizes the isospin dependence of the
incompressibility at the saturation density and can be expressed as
, Furthermore, we have constructed a
phenomenological modified Skyrme-like (MSL) model which can reasonably describe
the general properties of symmetric nuclear matter and the symmetry energy
predicted by both the MDI model and the SHF approach. The results indicate that
the high-order contribution to generally cannot be
neglected. In addition, it is found that there exists a nicely linear
correlation between and as well as between and
. These correlations together with the empirical constraints on ,
, and the nucleon effective mass lead to an estimate of
MeV.Comment: 61 pages, 12 figures, 6 Tables. Title changed a little and results of
several Skyrme interactions updated. Accepted version to appear in PR
Medium effects on charged pion ratio in heavy ion collisions
We have recently studied in the delta-resonance--nucleon-hole model the
dependence of the pion spectral function in hot dense asymmetric nuclear matter
on the charge of the pion due to the pion p-wave interaction in nuclear medium.
In a thermal model, this isospin-dependent effect enhances the ratio of
negatively charged to positively charged pions in neutron-rich nuclear matter,
and the effect is comparable to that due to the uncertainties in the
theoretically predicted stiffness of nuclear symmetry energy at high densities.
This effect is, however, reversed if we also take into account the s-wave
interaction of the pion in nuclear medium as given by chiral perturbation
theory, resulting instead in a slightly reduced ratio of negatively charged to
positively charged pions. Relevance of our results to the determination of the
nuclear symmetry energy from the ratio of negatively to positively charged
pions produced in heavy ion collisions is discussed.Comment: 11 pages, 4 figures, contribution to The International Workshop on
Nuclear Dynamics in Heavy-Ion Reactions and the Symmetry Energy (IWND2009),
Shanghai, China, 22-25 August, 200
Density slope of the nuclear symmetry energy from the neutron skin thickness of heavy nuclei
Expressing explicitly the parameters of the standard Skyrme interaction in
terms of the macroscopic properties of asymmetric nuclear matter, we show in
the Skyrme-Hartree-Fock approach that unambiguous correlations exist between
observables of finite nuclei and nuclear matter properties. We find that
existing data on neutron skin thickness of Sn isotopes give an
important constraint on the symmetry energy and its
density slope at saturation density . Combining these
constraints with those from recent analyses of isospin diffusion and double
neutron/proton ratio in heavy-ion collisions at intermediate energies leads to
a more stringent limit on approximately independent of . The implication of these new constraints on the of
Pb as well as the core-crust transition density and pressure in neutron
stars is discussed.Comment: 18 pages, 9 figures, 1 table. Significantly expanded to include a
number of details and discussions. Title shortened. Accepted version to
appear in PR
Transition density and pressure in hot neutron stars
Using the momentum-dependent MDI effective interaction for nucleons, we have
studied the transition density and pressure at the boundary between the inner
crust and liquid core of hot neutron stars. We find that their values are
larger in neutrino-trapped neutron stars than in neutrino-free neutron stars.
Furthermore, both are found to decrease with increasing temperature of a
neutron star as well as increasing slope parameter of the nuclear symmetry
energy, except that the transition pressure in neutrino-trapped neutron stars
for the case of small symmetry energy slope parameter first increases and then
decreases with increasing temperature. We have also studied the effect of the
nuclear symmetry energy on the critical temperature above which the inner crust
in a hot neutron star disappears and found that with increasing value of the
symmetry energy slope parameter, the critical temperature decreases slightly in
neutrino-trapped neutron stars but first decreases and then increases in
neutrino-free neutron stars.Comment: 7 pages, 6 figures, version to appear in Phys. Rev.
Isospin- and momentum-dependent effective interactions for the baryon octet and the properties of hybrid stars
The isospin- and momentum-dependent MDI interaction, which has been
extensively used in intermediate-energy heavy-ion reactions to study the
properties of asymmetric nuclear matter, is extended to include the
nucleon-hyperon and hyperon-hyperon interactions by assuming same density,
momentum and isospin dependence as for the nucleon-nucleon interaction. The
parameters in these interactions are determined from the empirical hyperon
single-particle potentials in symmetric nuclear matter at saturation density.
The extended MDI interaction is then used to study in the mean-field
approximation the equation of state of hypernuclear matter and also the
properties of hybrid stars by including the phase transition from the
hypernuclear matter to the quark matter at high densities. In particular, the
effects of attractive and repulsive N interactions and different values
of symmetry energies on the hybrid star properties are investigated.Comment: 13 pages, 12 figures, version to appear in Phys. Rev.
Probing isospin- and momentum-dependent nuclear effective interactions in neutron-rich matter
The single-particle potentials for nucleons and hyperons in neutron-rich
matter generally depends on the density and isospin asymmetry of the medium as
well as the momentum and isospin of the particle. It further depends on the
temperature of the matter if the latter is in thermal equilibrium. We review
here the extension of a Gogny-type isospin- and momentum-dependent interaction
in several aspects made in recent years and their applications in studying
intermediate-energy heavy ion collisions, thermal properties of asymmetric
nuclear matter and properties of neutron stars. The importance of the isospin-
and momentum-dependence of the single-particle potential, especially the
momentum dependence of the isovector potential, is clearly revealed throughout
these studies.Comment: 27 pages, 19 figures, 1 table, accepted version to appear in EPJA
special volume on Nuclear Symmetry Energ
Shear viscosity of neutron-rich nucleonic matter near its liquid-gas phase transition
Within a relaxation time approach using free nucleon-nucleon cross sections
modified by the in-medium nucleon masses that are determined from an isospin-
and momentum-dependent effective nucleon-nucleon interaction, we investigate
the specific shear viscosity () of neutron-rich nucleonic matter near
its liquid-gas phase transition. It is found that as the nucleonic matter is
heated at fixed pressure or compressed at fixed temperature, its specific shear
viscosity shows a valley shape in the temperature or density dependence, with
the minimum located at the boundary of the phase transition. Moreover, the
value of drops suddenly at the first-order liquid-gas phase transition
temperature, reaching as low as times the KSS bound of .
However, it varies smoothly for the second-order liquid-gas phase transition.
Effects of the isospin degree of freedom and the nuclear symmetry energy on the
value of are also discussed.Comment: 6 pages, 5 figure
Energy dependence of pion in-medium effects on \pi^-/\pi^+ ratio in heavy-ion collisions
Within the framework of a thermal model with its parameters fitted to the
results from an isospin-dependent Boltzmann-Uehling-Uhlenbeck (IBUU) transport
model, we study the pion in-medium effect on the charged-pion ratio in
heavy-ion collisions at various energies. We find that due to the cancellation
between the effects from pion-nucleon s-wave and p-wave interactions in nuclear
medium, the \pi^-/\pi^+ ratio generally decreases after including the pion
in-medium effect. The effect is larger at lower collision energies as a result
of narrower pion spectral functions at lower temperatures.Comment: 4 pages, 4 figures, 1 table, minor modifications, version to appear
in Physical Review
Transition Density and Pressure at the Inner Edge of Neutron Star Crusts
Using the nuclear symmetry energy that has been recently constrained by the
isospin diffusion data in intermediate-energy heavy ion collisions, we have
studied the transition density and pressure at the inner edge of neutron star
crusts, and they are found to be 0.040 fm
fm and 0.01 MeV/fm MeV/fm,
respectively, in both the dynamical and thermodynamical approaches. We have
also found that the widely used parabolic approximation to the equation of
state of asymmetric nuclear matter gives significantly higher values of
core-crust transition density and pressure, especially for stiff symmetry
energies. With these newly determined transition density and pressure, we have
obtained an improved relation between the mass and radius of neutron stars.Comment: 7 pages, 3 figures, proceeding of "The International Workshop on
Nuclear Dynamics in Heavy-Ion Reactions and the Symmetry Energy (IWND2009)
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