6,744 research outputs found
Isovector Vibrations in Nuclear Matter at Finite Temperature
We consider the propagation and damping of isovector excitations in heated
nuclear matter within the Landau Fermi-liquid theory. Results obtained for
nuclear matter are applied to calculate the Giant Dipole Resonance (GDR) at
finite temperature in heavy spherical nuclei within Steinwedel and Jensen
model.
The centroid energy of the GDR slightly decreases with increasing temperature
and the width increases as for temperatures MeV in agreement with
recent experimental data for GDR in Pb and Sn.
The validity of the method for other Fermi fluids is finally suggested.Comment: gzipped LaTeX file with text: 19 pages, 26 blocks; 3 gzipped *.ps
files with figures: 50 block
Application of density dependent parametrization models to asymmetric nuclear matter
Density dependent parametrization models of the nucleon-meson effective
couplings, including the isovector scalar \delta-field, are applied to
asymmetric nuclear matter. The nuclear equation of state and the neutron star
properties are studied in an effective Lagrangian density approach, using the
relativistic mean field hadron theory. It is known that the introduction of a
\delta-meson in the constant coupling scheme leads to an increase of the
symmetry energy at high density and so to larger neutron star masses, in a pure
nucleon-lepton scheme. We use here a more microscopic density dependent model
of the nucleon-meson couplings to study the properties of neutron star matter
and to re-examine the \delta-field effects in asymmetric nuclear matter. Our
calculations show that, due to the increase of the effective \delta coupling at
high density, with density dependent couplings the neutron star masses in fact
can be even reduced.Comment: 5 pages, 4 figure
Nuclear collective dynamics within Vlasov approach
We discuss, in an investigation based on Vlasov equation, the properties of
the isovector modes in nuclear matter and atomic nuclei in relation with the
symmetry energy. We obtain numerically the dipole response and determine the
strength function for various systems, including a chain of Sn isotopes. We
consider for the symmetry energy three parametrizations with density providing
similar values at saturation but which manifest very different slopes around
this point. In this way we can explore how the slope affects the collective
response of finite nuclear systems. We focus first on the dipole polarizability
and show that while the model is able to describe the expected mass dependence,
A^{5/3}, it also demonstrates that this quantity is sensitive to the slope
parameter of the symmetry energy. Then, by considering the Sn isotopic chain,
we investigate the emergence of a collective mode, the Pygmy Dipole Resonance
(PDR), when the number of neutrons in excess increases. We show that the total
energy-weighted sum rule exhausted by this mode has a linear dependence with
the square of isospin I=(N-Z)/A, again sensitive to the slope of the symmetry
energy with density. Therefore the polarization effects in the isovector
density have to play an important role in the dynamics of PDR. These results
provide additional hints in the investigations aiming to extract the properties
of symmetry energy below saturation.Comment: 7 pages, 6 figure
Influence of vector interactions on the hadron-quark/gluon phase transition
The hadron-quark/gluon phase transition is studied in the two-phase model. As
a further study of our previous work, both the isoscalar and isovector vector
interactions are included in the Polyakov loop modified Nambu--Jona-Lasinio
model (PNJL) for the quark phase. The relevance of the exchange (Fock) terms is
stressed and suitably accounted for. The calculation shows that the isovector
vector interaction delays the phase transition to higher densities and the
range of the mixed phase correspondingly shrinks. Meanwhile the asymmetry
parameter of quark matter in the mixed phase decreases with the strengthening
of this interaction channel. This leads to some possible observation signals
being weakened, although still present. We show that these can be rather
general effects of a repulsion in the quark phase due to the symmetry energy.
This is also confirmed by a simpler calculation with the MIT--Bag model.
However, the asymmetry parameter of quark matter is slightly enhanced with the
inclusion of the isoscalar vector interaction, but the phase transition will be
moved to higher densities. The largest uncertainty on the phase transition lies
in the undetermined coupling constants of the vector interactions. In this
respect new data on the mixed phase obtained from Heavy Ion Collisions at
Intermediate Energies appear very important.Comment: submitted to Phys. Rev.
Symmetry Energy Effects on the Mixed Hadron-Quark Phase at High Baryon Density
The phase transition of hadronic to quark matter at high baryon and isospin
density is analyzed. Relativistic mean field models are used to describe
hadronic matter, and the MIT bag model is adopted for quark matter. The
boundaries of the mixed phase and the related critical points for symmetric and
asymmetric matter are obtained. Due to the different symmetry term in the two
phases, isospin effects appear to be rather significant. With increasing
isospin asymmetry the binodal transition line of the (T,\rho_B) diagram is
lowered to a region accessible through heavy ion collisions in the energy range
of the new planned facilities, e.g. the FAIR/NICA projects. Some observable
effects are suggested, in particular an "Isospin Distillation" mechanism with a
more isospin asymmetric quark phase, to be seen in charged meson yield ratios,
and an onset of quark number scaling of the meson/baryon elliptic flows. The
presented isospin effects on the mixed phase appear to be robust with respect
to even large variations of the poorly known symmetry term at high baryon
density in the hadron phase. The dependence of the results on a suitable
treatment of isospin contributions in effective QCD Lagrangian approaches, at
the level of explicit isovector parts and/or quark condensates, is finally
discussed.Comment: 14 two column pages, 14 figures, new results with other hadron EoS.
Accepted for publication in Phys.Rev.
Hadron-quark phase transition in asymmetric matter with dynamical quark masses
The two-Equation of State (EoS) model is used to describe the hadron-quark
phase transition in asymmetric matter formed at high density in heavy-ion
collisions. For the quark phase, the three-flavor Nambu--Jona-Lasinio (NJL)
effective theory is used to investigate the influence of dynamical quark mass
effects on the phase transition. At variance to the MIT-Bag results, with fixed
current quark masses, the main important effect of the chiral dynamics is the
appearance of an End-Point for the coexistence zone. We show that a first order
hadron-quark phase transition may take place in the region T=(50-80)MeV and
\rho_B=(2-4)\rho_0, which is possible to be probed in the new planned
facilities, such as FAIR at GSI-Darmstadt and NICA at JINR-Dubna. From isospin
properties of the mixed phase somepossible signals are suggested. The
importance of chiral symmetry and dynamical quark mass on the hadron-quark
phase transition is stressed. The difficulty of an exact location of
Critical-End-Point comes from its appearance in a region of competition between
chiral symmetry breaking and confinement, where our knowledge of effective QCD
theories is still rather uncertain.Comment: 13 pages, 16 figures (revtex
Isospin effects on sub-threshold kaon production at intermediate energies
We show that in collisions with neutron rich heavy ions at energies around
the production threshold K^0 and K^+ yields might probe the isospin dependent
part of the nuclearEquation of State (EoS) at high baryon densities. In
particular we suggest the K^0/K^+ ratio as a promising observable. Results
obtained in a fully covariant relativistic transport approach are presented for
central Au+Au collisions in the beam energy range 0.8-1.8~AGeV. The focus is
put on the EoS influence which goes beyond the "collision-cascade" picture. The
isovector part of the in-medium interaction affects the kaon multiplicities via
two mechanisms: i) a "symmetry potential" effect, i.e. a larger neutron
repulsion in n-rich systems (isospin fractionation); ii) a "threshold" effect,
due to the change in the self-energies of the particles involved in inelastic
processes. Genuine relativistic contributions are revealed, that could allow to
directly ``measure'' the Lorentz structure of the effective isovector
interaction.Comment: 5 pages, 2 figures, revtex
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