588 research outputs found
Estimation of the shear viscosity at finite net-baryon density from A+A collision data at GeV
Hybrid approaches based on relativistic hydrodynamics and transport theory
have been successfully applied for many years for the dynamical description of
heavy ion collisions at ultrarelativistic energies. In this work a new viscous
hybrid model employing the hadron transport approach UrQMD for the early and
late non-equilibrium stages of the reaction, and 3+1 dimensional viscous
hydrodynamics for the hot and dense quark-gluon plasma stage is introduced.
This approach includes the equation of motion for finite baryon number, and
employs an equation of state with finite net-baryon density to allow for
calculations in a large range of beam energies. The parameter space of the
model is explored, and constrained by comparison with the experimental data for
bulk observables from SPS and the phase I beam energy scan at RHIC. The favored
parameter values depend on energy, but allow to extract the effective value of
the shear viscosity coefficient over entropy density ratio in the
fluid phase for the whole energy region under investigation. The estimated
value of increases with decreasing collision energy, which may
indicate that of the quark-gluon plasma depends on baryochemical
potential .Comment: minor changes in the text, results for constant eta*T/w added.
Version accepted for publication in Phys. Rev.
Effect of temperature-dependent eta/s on flow anisotropies
We investigate the effects of a temperature-dependent shear viscosity over
entropy density ratio eta/s on the flow anisotropy coefficients v_2 and v_4 in
ultrarelativistic heavy-ion collisions at RHIC and LHC. We find that v_4 is
more sensitive to the viscosity at low temperatures than v_2. At RHIC v_2 is
mostly affected by the viscosity around the phase transition, but the larger
the collision energy, the more the quark-gluon plasma viscosity affects v_2.Comment: 6 pages, 11 figures, parallel talk in Strangeness in Quark Matter
conference, Cracow, Sept 201
Systematic Investigation of Negative Cooper-Frye Contributions in Heavy Ion Collisions Using Coarse-grained Molecular Dynamics
In most heavy ion collision simulations involving relativistic hydrodynamics,
the Cooper-Frye formula is applied to transform the hydrodynamical fields to
particles. In this article the so-called negative contributions in the
Cooper-Frye formula are studied using a coarse-grained transport approach. The
magnitude of negative contributions is investigated as a function of hadron
mass, collision energy in the range of -- GeV, collision
centrality and the energy density transition criterion defining the
hypersurface. The microscopic results are compared to negative contributions
expected from hydrodynamical treatment assuming local thermal equilibrium.
The main conclusion is that the number of actual microscopic particles flying
inward is smaller than the negative contribution one would expect in an
equilibrated scenario. The largest impact of negative contributions is found to
be on the pion rapidity distribution at midrapidity in central collisions. For
this case negative contributions in equilibrium constitute 8--13\% of positive
contributions depending on collision energy, but only 0.5--4\% in cascade
calculation. The dependence on the collision energy itself is found to be
non-monotonous with a maximum at 10-20 GeV.Comment: 10 pages, 9 figure
Dynamical freeze-out condition in ultrarelativistic heavy ion collisions
We determine the decoupling surfaces for the hydrodynamic description of
heavy ion collisions at RHIC and LHC by comparing the local hydrodynamic
expansion rate with the microscopic pion-pion scattering rate. The pion
spectra for nuclear collisions at RHIC and LHC are computed by applying the
Cooper-Frye procedure on the dynamical-decoupling surfaces, and compared with
those obtained from the constant-temperature freeze-out surfaces. Comparison
with RHIC data shows that the system indeed decouples when the expansion rate
becomes comparable with the pion scattering rate. The dynamical decoupling
based on the rates comparison also suggests that the effective decoupling
temperature in central heavy ion collisions remains practically unchanged from
RHIC to LHC.Comment: 7 pages, 9 figure
Elliptic flow in nuclear collisions at the Large Hadron Collider
We use perfect-fluid hydrodynamical model to predict the elliptic flow
coefficients in Pb + Pb collisions at the Large Hadron Collider (LHC). The
initial state for the hydrodynamical calculation for central collisions
is obtained from the perturbative QCD + saturation (EKRT) model. The centrality
dependence of the initial state is modeled by the optical Glauber model. We
show that the baseline results obtained from the framework are in good
agreement with the data from the Relativistic Heavy Ion Collider (RHIC), and
show predictions for the spectra and elliptic flow of pions in Pb + Pb
collisions at the LHC. Also mass and multiplicity effects are discussed.Comment: 11 pages, 10 figure
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