819 research outputs found
Directed flow, a signal for the phase transition in Relativistic Nuclear Collisions?
The sign change of the slope of the directed flow of baryons has been
predicted as a signal for a first order phase transition within fluid dynamical
calculations. Recently, the directed flow of identified particles has been
measured by the STAR collaboration in the beam energy scan (BES) program. In
this article, we examine the collision energy dependence of directed flow
in fluid dynamical model descriptions of heavy ion collisions for
GeV. The first step is to reproduce the existing
predictions within pure fluid dynamical calculations. As a second step we
investigate the influence of the order of the phase transition on the
anisotropic flow within a state-of-the-art hybrid approach that describes other
global observables reasonably well. We find that, in the hybrid approach, there
seems to be no sensitivity of the directed flow on the equation of state and in
particular on the existence of a first order phase transition. In addition, we
explore more subtle sensitivities like e.g. the Cooper-Frye transition
criterion and discuss how momentum conservation and the definition of the event
plane affects the results. At this point, none of our calculations matches
qualitatively the behavior of the STAR data, the values of the slopes are
always larger than in the data.Comment: 7 pages, 7 figure
Chiral Hadronic Mean Field Model including Quark Degrees of Freedom
In an approach inspired by Polyakov loop extended NJL models, we present a
nonlinear hadronic SU(3) sigma-omega mean field model augmented by quark
degrees of freedom. By introducing the effective Polyakov loop related scalar
field \Phi and an associated effective potential, the model includes all known
hadronic degrees of freedom at low temperatures and densities as well as a
quark phase at high temperatures and densities. Hadrons in the model exhibit a
finite volume in order to suppress baryons at high T and \mu. This ensures that
the right asymptotic degrees of freedom are attained for the description of
strongly interacting matter and allows to study the QCD phase diagram in a wide
range of temperatures and chemical potentials. Therefore, with this model it is
possible to study the phase transition of chiral restoration and deconfinement.
In this paper, the impact of quarks on the resulting phase diagram is shown.
The results from the chiral model are compared to recent data from lattice QCD.Comment: 25 pages, 10 figure
Entropy Production in Collisions of Relativistic Heavy Ions -- a signal for Quark-Gluon Plasma phase transition?
Entropy production in the compression stage of heavy ion collisions is
discussed within three distinct macroscopic models (i.e. generalized RHTA,
geometrical overlap model and three-fluid hydrodynamics). We find that within
these models \sim 80% or more of the experimentally observed final-state
entropy is created in the early stage. It is thus likely followed by a nearly
isentropic expansion. We employ an equation of state with a first-order phase
transition. For low net baryon density, the entropy density exhibits a jump at
the phase boundary. However, the excitation function of the specific entropy
per net baryon, S/A, does not reflect this jump. This is due to the fact that
for final states (of the compression) in the mixed phase, the baryon density
\rho_B increases with \sqrt{s}, but not the temperature T. Calculations within
the three-fluid model show that a large fraction of the entropy is produced by
nuclear shockwaves in the projectile and target. With increasing beam energy,
this fraction of S/A decreases. At \sqrt{s}=20 AGeV it is on the order of the
entropy of the newly produced particles around midrapidity. Hadron ratios are
calculated for the entropy values produced initially at beam energies from 2 to
200 AGeV.Comment: 17 pages, 8 figures, uses epsfig.sty; Submitted to Nucl.Phys.
Energy Spectra of Anti-nucleons in Finite Nuclei
The quantum vacuum in a many-body system of finite nuclei has been
investigated within the relativistic Hartree approach which describes the bound
states of nucleons and anti-nucleons consistently. The contributions of the
Dirac sea to the source terms of the meson-field equations are taken into
account up to the one-nucleon loop and one-meson loop. The tensor couplings for
the - and -meson are included in the model. The overall nucleon
spectra of shell-model states are in agreement with the data. The calculated
anti-nucleon spectra in the vacuum differ about 20 -- 30 MeV with and without
the tensor-coupling effects.Comment: 4 pages, to appear in the Proceedings of MENU 2004 (Beijing, Aug. 29
-- Sept. 4, 2004
Some Comments on Relativistic Hydrodynamics and Fuzzy Bags
Three subjects are considered here: the relativistic hydrodynamics equations
for a boost-invariant expanding fluid; the fuzzy bag model for the pressure
which recently appeared in QCD phenomenology; and the early space-time
evolution of the QCD matter, drawn from model studies, which can also be
expected to arise in realistic fluid dynamics relevant to heavy ion collisions
at LHC.Comment: 10 pages, 5 figures; v3: additional discussion of lattice data, minor
clarifications, references adde
Hypernuclei, dibaryon and antinuclei production in high energy heavy ion collisions: Thermal production vs. Coalescence
We study the production of (hyper-)nuclei and di-baryons in most central
heavy Ion collisions at energies of GeV. In particular we are
interested in clusters produced from the hot and dense fireball. The formation
rate of strange and non-strange clusters is estimated by assuming thermal
production from the intermediate phase of the UrQMD-hydro hybrid model and
alternatively by the coalescence mechanism from a hadronic cascade model. Both
model types are compared in detail. For most energies we find that both
approaches agree in their predictions for the yields of the clusters. Only for
very low beam energies, and for di-baryons including 's, we observe
considerable differences. We also study the production of anti-matter clusters
up to top RHIC energies and show that the observation of anti- and even
anti- is feasible. We have found a considerable qualitative
difference in the energy dependence of the strangeness population factor
when comparing the thermal production with the coalescence results.Comment: 9 pages, 8 figures and 2 tables, version accepted by PL
Open-charm enhancement at FAIR?
We have calculated the D-meson spectral density at finite temperature within
a self-consistent coupled-channel approach that generates dynamically the
(2593) resonance. We find a small mass shift for the D-meson in
this hot and dense medium while the spectral density develops a sizeable width.
The reduced attraction felt by the D-meson in hot and dense matter together
with the large width observed have important consequences for the D-meson
production in the future CBM experiment at FAIR.Comment: 4 pages, 2 figures, to appear in the proceedings of 9th International
Conference on Strangeness in Quark Matter (SQM2006), Los Angeles, USA, March
26-31, 200
- …