335 research outputs found
Scalings of Elliptic Flow for a Fluid at Finite Shear Viscosity
Within a parton cascade approach we investigate the scaling of the
differential elliptic flow with eccentricity and system
size and its sensitivity to finite shear viscosity. We present calculations for
shear viscosity to entropy density ratio in the range from up
to , finding that the saturation value varies by about a factor 2.
Scaling of is seen also for finite which
indicates that it does not prove a perfect hydrodynamical behavior, but is
compatible with a plasma at finite . Introducing a suitable freeze-out
condition, we see a significant reduction of especially at
intermediate and for more peripheral collisions. This causes a breaking
of the scaling for both and the averaged , while keeping
the scaling of v_2(p_T)/\la v_2\ra. This is in better agreement with the
experimental observations and shows as a first indication that the
should be significantly lower than the pQCD estimates. We finally point out the
necessity to include the hadronization via coalescence for a definite
evaluation of from intermediate data.Comment: 5 pages, 5 figures. Two points in fig.4 has been change
Anisotropies in momentum space at finite Shear Viscosity in ultrarelativistic heavy-ion collisions
Within a parton cascade we investigate the dependence of anisotropies in
momentum space, namely the elliptic flow and the
, on both the finite shear viscosity and the
freeze-out (f.o.) dynamics at the RHIC energy of 200 AGeV. In particular it is
discussed the impact of the f.o. dynamics looking at two different procedures:
switching-off the collisions when the energy density goes below a fixed value
or reducing the cross section according to the increase in from a QGP
phase to a hadronic one. We address the relation between the scaling of
with the eccentricity and with the integrated elliptic
flow. We show that the breaking of the scaling is not
coming mainly from the finite but from the f.o. dynamics and that the
is weakly dependent on the f.o. scheme. On the other hand the
is found to be much more dependent on both the and the f.o.
dynamics and hence is indicated to put better constraints on the properties of
the QGP. A first semi-quantitative analysis show that both and
(with the smooth f.o.) consistently indicate a plasma with .Comment: 7 pages. Proceedings of the International School of Nuclear Physics
in Erice, Sicily, to appear in Progress in Particle and Nuclear Physic
Does the NJL chiral phase transition affect the elliptic flow of a fluid at fixed ?
We have derived and solved numerically the Boltzmann-Vlasov transport
equations that includes both two-body collisions and the chiral phase
transition by mean of NJL-field dynamics. The scope is to understand if the
field dynamics supply new genuine effects on the build-up of the elliptic flow
, a measure of the asymmetry in the momentum space, and in particular if
it can affect the relation between and the shear viscosity to entropy
ratio . Solving the transport equation with a constant cross section
for the condition of collisions at AGeV it is shown
a sizable suppression of due to the attractive nature of the field
dynamics that generates the constituent mass. However the key result is that if
of the system is kept fixed by an appropriate local renormalization of
the cross section the does not depend on the details of the collisional
and/or field dynamics and in particular it is not affected significantly by the
chiral phase transition.Comment: 5 pages, 5 figure
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
Isospin Effects on Strangeness in Heavy-Ion Collisions
Kaon properties are studied within the framework of a fully covariant
transport approach. The kaon-nucleon potential is evaluated in two schemes, a
chiral perturbative approach and an effective One-Boson-Exchange model. Isospin
effects are explicitly accounted for in both models. The transport calculations
indicate a significant sensitivity of momentum distributions and total yields
of isospin states on the choice of the kaon-nucleon interaction.
Furthermore, isospin effects are rather moderate on absolute kaon yields, but
appear on strangeness ratios. This is an important issue in determining the
high density symmetry energy from studies of strangeness production in
heavy-ion collisions.Comment: 15 papes, 5 figures. Accepted for publication in Nuclear Physic
THE EFFECTS OF ION IRRADIATION ON THE EVOLUTION OF THE CARRIER OF THE 3.4 MICRON INTERSTELLAR ABSORPTION BAND
Carbon grains in the interstellar medium evolve through exposure to UV photons, heat, gas, and cosmic rays. Understanding their formation, evolution, and destruction is an essential component of evaluating the composition of the dust available for newly forming planetary systems. The 3.4 lm absorption band, attributed to the aliphatic C"H stretch vibration, is a useful probe of the degree to which energetic processing affects hydrogenated carbon grains. Here we report on the effects of ion bombardment of two different kinds of nano-size hydrogenated carbon grains with different hydrogen content. Grain samples, both with and without a mantle of H2O ice, were irradiated with 30 keV He + to simulate cosmic-ray processing in both diffuse and dense interstellar medium conditions. The ion fluences ranged between 1:5 � 10 13 and 7:9 � 10 15 ions cm � 2 . Infrared and Raman spectroscopy were used to study the effects of ion irradiation on grains. In both the dense and diffuse interstellar medium simulations, ion bombardment led to a reduction of the 3.4 lm band intensity. To discuss the effects of cosmic-ray irradiation of interstellar hydrogenated carbon materials we adopt the approximation of 1 MeV monoenergetic protons. An estimate of the C"H bond destruction cross section by 1 MeV protons was made based on experiments using 30 keV He + ions and model calculations. In combination with results from our previous studies, which focused on UV irradiation and thermal H atom bombardment, the present results indicate that the C"H bond destruction by fastcolliding charged particles is negligible with respect to that of UV photons in the diffuse ISM. However, in dense cloud regions, cosmic-ray bombardment is the most significant C"H bond destruction mechanism when the optical depth corresponds to values of the visual extinction larger than � 5 mag. The results presented here strengthen the new interpretation of the evolution of the interstellar aliphatic component (i.e., the C"H bonds in the CH2 and CH3 groups) as evidenced by the presence of the 3.4 lm absorption band in the diffuse medium and the absence of such a signature in the dense cloud environment. The evolutionary transformation of carbon grains, induced by H atoms, UV photons, and cosmic rays, indicates that C"H bonds are readily formed, in situ, in the diffuse interstellar medium and are destroyed in the dense cloud environment
Isospin Dynamics in Heavy Ion Collisions: EoS-sensitive Observables
Heavy Ion Collisions (HIC) represent a unique tool to probe the in-medium
nuclear interaction in regions away from saturation and at high nucleon
momenta. In this report we present a selection of reaction observables
particularly sensitive to the isovector part of the interaction, i.e. to the
symmetry term of the nuclear Equation of State (EoS) At low energies the
behavior of the symmetry energy around saturation influences dissipation and
fragment production mechanisms. Predictions are shown for deep-inelastic and
fragmentation collisions induced by neutron rich projectiles. Differential flow
measurements will also shed lights on the controversial neutron/proton
effective mass splitting in asymmetric matter. The high density symmetry term
can be derived from isospin effects on heavy ion reactions at relativistic
energies (few AGeV range), that can even allow a ``direct'' study of the
covariant structure of the isovector interaction in the hadron medium. Rather
sensitive observables are proposed from collective flows and from pion/kaon
production. The possibility of the transition to a mixed hadron-quark phase, at
high baryon and isospin density, is finally suggested. Some signatures could
come from an expected ``neutron trapping'' effect.Comment: 10 pages, 5 figures; espcrc1 style; IX Int.Conf. on Nucleus-Nucleus
Collisions, Rio de Janeiro Aug.2006; to appear in Nucl.Phys.
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