Abstract
We determine the probability distributions of the shear viscosity over the entropy density ratio $\ensuremath{\eta}/s$ in the quark-gluon plasma formed in Au + Au collisions at $\sqrt{{s}_{NN}}=19.6,\phantom{\rule{0.16em}{0ex}}39$, and $62.4\phantom{\rule{0.16em}{0ex}}\mathrm{GeV}$, using Bayesian inference and Gaussian process emulators for a model-to-data statistical analysis that probes the full input parameter space of a transport + viscous hydrodynamics hybrid model. We find the most likely value of $\ensuremath{\eta}/s$ to be larger at smaller $\sqrt{{s}_{NN}}$, although the uncertainties still allow for a constant value between 0.10 and 0.15 for the investigated collision energy range.
Highlights
After successfully verifying the existence of a new state of matter, the quark-gluon plasma (QGP), experiments at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC) are focused on quantifying the transport properties of the QGP and mapping out the QCD phase diagram
The agreement with the test point values is very good. This assures us that the principal components of the model variance have been correctly identified and the chosen kernel for Gaussian processes provides a good description of the model behavior with respect to the principal components
We find a moderate collision energy dependence for the most likely value of the effective shear viscosity over entropy density ratio in the QGP phase
Summary
After successfully verifying the existence of a new state of matter, the quark-gluon plasma (QGP), experiments at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC) are focused on quantifying the transport properties of the QGP and mapping out the QCD phase diagram. The beam energy scan has introduced new challenges to the theoretical modeling of relativistic heavy-ion collisions, as the initial nonequilibrium evolution of the system leading up to thermalization gains in importance and has a large effect on the outcome of the calculation. The equation of state needs to accommodate the nonzero baryon chemical potential μB.
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