3,994 research outputs found

### On the splitting of nucleon effective masses at high isospin density: reaction observables

We review the present status of the nucleon effective mass splitting $puzzle$
in asymmetric matter, with controversial predictions within both
non-relativistic $and$ relativistic approaches to the effective in medium
interactions. Based on microscopic transport simulations we suggest some rather
sensitive observables in collisions of asymmetric (unstable) ions at
intermediate ($RIA$) energies: i) Energy systematics of Lane Potentials; ii)
Isospin content of fast emitted nucleons; iii) Differential Collective Flows.
Similar measurements for light isobars (like $^3H-^3He$) could be also
important.Comment: 13 pages, 10 figures; NSCL/RIA Workshop on "Reaction Mechanisms for
Rare Isotope Beams", March 2005, AIP Proc. Latex Styl

### Isospin in Reaction Dynamics. The Case of Dissipative Collisions at Fermi Energies

A key question in the physics of unstable nuclei is the knowledge of the
$EOS$ for asymmetric nuclear matter ($ANM$) away from normal conditions. We
recall that the symmetry energy at low densities has important effects on the
neutron skin structure, while the knowledge in high densities region is crucial
for supernovae dynamics and neutron star properties. The $only$ way to probe
such region of the isovector $EOS$ in terrestrial laboratories is through very
dissipative collisions of asymmetric (up to exotic) heavy ions from low to
relativistic energies. A general introduction to the topic is firstly
presented. We pass then to a detailed discussion on the $neck-fragmentation$
process as the main dissipative mechanism at the Fermi energies and to the
related isospin dynamics. From Stochastic Mean Field simulations the isospin
effects on all the phases of the reaction dynamics are thoroughly analysed,
from the fast nucleon emission to the mid-rapidity fragment formation up to the
dynamical fission of the $spectator$ residues. Simulations have been performed
with an increasing stiffness of the symmetry term of the $EOS$.
Some differences have been noticed, especially for the fragment charge
asymmetry. New isospin effects have been revealed from the correlation of
fragment asymmetry with dynamical quantities at the freeze-out time. A series
of isospin sensitive observables to be further measured are finally listed.Comment: 16 pages, 6 figures, Contribution to the 5th Italy-Japan Symposium,
Recent Achievements and Perspectives in Nuclear Physics, Naples Nov.3-7 2004,
World Sci. in press. Latex in WorldSci/proc/styl

### Searching for statistical equilibrium in a dynamical multifragmentation path

A method for identifying statistical equilibrium stages in dynamical
multifragmentation paths as provided by transport models, already successfully
tested for for the reaction ^{129}Xe+^{119}Sn at 32 MeV/u is applied here to a
higher energy reaction, ^{129}Xe+^{119}Sn at 50 MeV/u. The method evaluates
equilibrium from the point of view of the microcanonical multifragmentation
model (MMM) and reactions are simulated by means of the stochastic mean field
model (SMF). A unique solution, corresponding to the maximum population of the
system phase space, was identified suggesting that a huge part of the available
phase space is occupied even in the case of the 50 MeV/u reaction, in presence
of a considerable amount of radial collective flow. The specific equilibration
time and volume are identified and differences between the two systems are
discussed.Comment: 7 pages, 10 figures, accepted for publication in Physical Review

### Impact of temperature dependence of the energy loss on jet quenching observables

The quenching of jets (particles with $p_T>>T, \Lambda_{QCD}$) in
ultra-relativistic heavy-ion collisions has been one of the main prediction and
discovery at RHIC. We have studied, by a simple jet quenching modeling, the
correlation between different observables like the nuclear modification factor
\Rapt, the elliptic flow $v_2$ and the ratio of quark to gluon suppression
$R_{AA}(quark)/R_{AA}(gluon)$. We show that the relation among these
observables is strongly affected by the temperature dependence of the energy
loss. In particular the large $v_2$ and and the nearly equal \Rapt of quarks
and gluons can be accounted for only if the energy loss occurs mainly around
the temperature $T_c$ and the flavour conversion is significant.Finally we
point out that the efficency in the conversion of the space eccentricity into
the momentum one ($v_2$) results to be quite smaller respect to the one coming
from elastic scatterings in a fluid with a viscosity to entropy density ratio
$4\pi\eta/s=1$.Comment: 7 pages, 8 figures, Workshop WISH 201

### Fast nucleon emission as a probe of the isospin momentum dependence

In this article we investigate the structure of the non-local part of the
symmetry term, that leads to a splitting of the effective masses of protons and
neutrons in asymmetric matter. Based on microscopic transport simulations we
suggest some rather sensitive observables in collisions of neutron-rich
(unstable) ions at intermediate ($RIA$) energies. In particular we focus the
attention on pre-equilibrium nucleon emissions. We discuss interesting
correlations between the N/Z content of the fast emitted particles and their
rapidity or transverse momentum, that show a nice dependence on the
prescription used for the effective mass splitting.Comment: 5 pages, 6 figures, revtex

### 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 $v_2(p_T)$ with eccentricity $\epsilon_x$ and system
size and its sensitivity to finite shear viscosity. We present calculations for
shear viscosity to entropy density ratio $\eta/s$ in the range from $1/4\pi$ up
to $1/\pi$, finding that the $v_2$ saturation value varies by about a factor 2.
Scaling of $v_2(p_T)/\epsilon_x$ is seen also for finite $\eta/s$ which
indicates that it does not prove a perfect hydrodynamical behavior, but is
compatible with a plasma at finite $\eta/s$. Introducing a suitable freeze-out
condition, we see a significant reduction of $v_2(p_T)$ especially at
intermediate $p_T$ and for more peripheral collisions. This causes a breaking
of the scaling for both $v_2(p_T)$ and the $p_T-$averaged $v_2$, 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 $\eta/s$
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 $\eta/s$ from intermediate $p_T$ 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 $v_2=$ and the
$v_4=$, on both the finite shear viscosity $\eta$ 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 $\eta/s$ from a QGP
phase to a hadronic one. We address the relation between the scaling of
$v_2(p_T)$ with the eccentricity $\epsilon_x$ and with the integrated elliptic
flow. We show that the breaking of the $v_2(p_T)/\epsilon_x$ scaling is not
coming mainly from the finite $\eta/s$ but from the f.o. dynamics and that the
$v_2(p_T)$ is weakly dependent on the f.o. scheme. On the other hand the
$v_4(p_T)$ is found to be much more dependent on both the $\eta/s$ 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 $v_2$ and $v_4$
(with the smooth f.o.) consistently indicate a plasma with $4\pi \eta/s \sim
1-2$.Comment: 7 pages. Proceedings of the International School of Nuclear Physics
in Erice, Sicily, to appear in Progress in Particle and Nuclear Physic

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