Estimation of the shear viscosity at finite net-baryon density fromA+Acollision data atsNN=7.7–200GeV

Abstract

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 η/s in the fluid phase for the whole energy region under investigation. The estimated value of η/s increases with decreasing collision energy, which may indicate that η/s of the quark-gluon plasma depends on baryochemical potential µB.

Results from the RHIC Beam Energy Scan

“…In contrast to v n , the excitation function of the viscous parameter β " shows a non-monotonic behaviour over the same beam energy range. A similar non-monotonic trend for η/s has been observed in the hybrid viscous hydrodynamical calculations [28], tuned to describe the STAR BES-I v 2 data.…”

Azimuthal anisotropies and initial-state fluctuations from SPS to LHC energies

“…In figure 1, the comparison between the hydrosolver+UrQMD [21] [23]. Within the analyzed range, the CERES v 3 does not show centrality dependence [22].…”

Triangular flow of negative pions emitted in PbAu collisions at top SPS energy

“…In Ref. [19], the authors of the hybrid model investigated parameter dependence of the model results in the case of 0-5% centrality AuAu collisions at √ s NN = 19.6 GeV. It is shown that the model results change less than 10% when the parameters of the model are varied by 10%.…”

“…Within it, kinetic and chemical freeze-outs are described dynamically by the UrQMD hadron cascade without a unique freeze-out temperature. The transition from fluid to cascade part, so called the 'particlization' [19], is set to take place on a constant energy density surface with = 0.5 GeV/fm 3 . But, as the net baryon density is not uniform on such a surface, this density does not correspond to a single temperature.…”

“…The ideal fluid dynamics is used to describe the evolution of the hot and dense quark-gluon plasma, while hadron transport to describe the late sparse hadron gas. The model calculations [19] shown in this analysis are performed using the vHLLE viscous hydrosolver [26] combined with UrQMD hadron cascade [27]. Within it, kinetic and chemical freeze-outs are described dynamically by the UrQMD hadron cascade without a unique freeze-out temperature.…”