200 A GeV Au+Au collisions serve a nearly perfect quark-gluon liquid (1011.2783v2)

Abstract: The specific shear viscosity (eta/s)_QGP of a Quark-Gluon-Plasma (QGP) at temperatures T_c < T < 2T_c is extracted from the centrality dependence of the eccentricity-scaled elliptic flow measured in ultra-relativistic heavy-ion collisions. Coupling viscous fluid dynamics for the QGP with a microscopic transport model for hadronic freeze-out we find that the eccentricity-scaled elliptic flow is a universal function of charged multiplicity per unit overlap area, (1/S)(dN_ch/dy), that depends only on the viscosity but not on the model used for computing the initial fireball eccentricity. Comparing with measurements we find 1 < (4pi)(eta/s)_QGP < 2.5 where the uncertainty range is dominated by model uncertainties for the eccentricity values used to normalize the measured elliptic flow.

• The paper introduces a novel method correlating eccentricity-scaled elliptic flow with multiplicity density to isolate the QGP’s shear viscosity.
• It employs a hybrid model combining viscous fluid dynamics with a hadronic cascade to manage uncertainties in initial conditions.
• The study’s results, with (η/s)_QGP estimated between 1 and 2.5 (in 4π units), reinforce the method’s universal applicability for high-energy collision experiments.

Analysis of Shear Viscosity in Quark-Gluon Plasma from 200 A GeV Au+Au Collisions

Abstract Review and Methodology

The research paper presents a comprehensive analysis of the specific shear viscosity $(\eta/s)_{QGP}$ of the Quark-Gluon Plasma (QGP) at temperatures spanning from slightly above the critical temperature $T_c$ to approximately twice this value. The focus is on measurements derived from 200 A GeV Au+Au collisions, employing a novel approach to relate the eccentricity-scaled elliptic flow $v_2/\varepsilon$ to multiplicity density $(1/S)(ch)$. This relationship provides key insights into understanding the transport properties of QGP under extreme conditions.

To achieve this, a hybrid model is deployed, integrating viscous fluid dynamics for the QGP with a hadronic cascade model for the post-hadronization stage. This methodological framework enables the isolation of the shear viscosity effect in the observed flow, while controlling for model dependency in initial eccentricity $\varepsilon$ calculations. The findings suggest that $v_2/\varepsilon$ is a robust functional form, invariant under different computational models of initial eccentricity.

Key Numerical Results

The analysis results in an estimated range for $(\eta/s)_{QGP}$ of $1<4\pi(\eta/s)_{QGP}<2.5$. This range, crucially, is bounded primarily by model uncertainties relating to initial conditions rather than the intrinsic behavior of the plasma. Importantly, the research asserts that the centrality dependence of $v_2/\varepsilon$ is largely model-independent, thereby reinforcing the universal applicability of the proposed method in different scenarios and system sizes.

Discussion of Model and Initial Conditions

A critical consideration is the model's treatment of the initial conditions. Two primary models are evaluated — MC-Glauber and MC-KLN — each providing disparate predictions for source eccentricity $\varepsilon$ and overlap area $S$. The importance of these parameters cannot be overstated; they are integral in normalizing $v_2$ and aligning experimental data with theoretical predictions. Consistency is maintained across models by ensuring accurate adjustment to experimental multiplicity data, which ultimately constrains the uncertainty range of $(\eta/s)_{QGP}$.

Implications and Future Directions

This paper delivers valuable insights into the fluid-like properties of QGP, emphasizing its minimal shear resistance under RHIC conditions. Given the technological and methodological advances exhibited, future work could explore broader temperature ranges, different collision energies, and further refinement of initial condition models to continue narrowing the error bounds. Such endeavors would enrich our understanding of QGP, contribute to precision measurements of transport coefficients, and offer significant theoretical developments in high-energy nuclear physics and QCD.

The conclusions drawn from this paper have ramifications both in experimental lattice QCD, hinting at an alignment with recent lattice results, and in informing the design of subsequent RHIC and LHC experiments. By clarifying the viscosity of the QGP, these findings aid in piecing together the complex thermodynamics of the early universe and the dynamics of strongly interacting matter.