Viscosity Information from Relativistic Nuclear Collisions: How Perfect is the Fluid Observed at RHIC? (0706.1522v2)

Abstract: Relativistic viscous hydrodynamic fits to RHIC data on the centrality dependence of multiplicity, transverse and elliptic flow for sqrt{s}=200 GeV Au+Au collisions are presented. For Glauber-type initial conditions, while data on integrated v_2 is consistent with a ratio of viscosity over entropy density up to eta/s=0.16, data on minimum bias v_2 seems to favor a much smaller viscosity over entropy ratio, below the bound from the AdS/CFT conjecture. Some caveats on this result are discussed.

• The paper employs viscous hydrodynamics to quantify the shear viscosity to entropy density (η/s) ratio of quark-gluon plasma.
• The paper compares simulation results with RHIC data, finding integrated elliptic flow consistent with η/s up to 0.16 and hints of lower values in minimum bias data.
• The paper highlights the need for advanced models and refined measurements to fully capture non-hydrodynamic effects in heavy-ion collisions.

Viscosity Information from Relativistic Nuclear Collisions: A Hydrodynamic Perspective

The study "Viscosity Information from Relativistic Nuclear Collisions: How Perfect is the Fluid Observed at RHIC?" by Paul Romatschke and Ulrike Romatschke investigates the properties of the quark-gluon plasma produced at the Relativistic Heavy-Ion Collider (RHIC) through the lens of relativistic viscous hydrodynamics. The primary focus is on determining the fluidity of this plasma, specifically addressing the ratio of shear viscosity to entropy density, denoted as $\eta/s$. This investigation is crucial as it challenges the minimal bound conjectured by the Anti-de-Sitter/Conformal Field Theory (AdS/CFT) correspondence.

Highlights and Methodology

1. Hydrodynamic Approach: The paper utilizes viscous hydrodynamics as opposed to ideal hydrodynamics, acknowledging that non-ideal effects such as shear viscosity are significant for a complete description of the quark-gluon plasma. The viscous hydrodynamic model incorporates shear viscosity while neglecting bulk viscosity and heat conduction.
2. Initial Conditions and Evolution: The initial conditions are modeled using Glauber-type frameworks, often employed to depict transverse energy density distribution using the nuclear thickness functions. Initial energy density is proportional to the number of binary collisions.
3. Numerical Simulations: Simulations were conducted across various shear viscosity to entropy density ratios, employing these conditions to predict particle multiplicity, transverse momentum distribution, and elliptic flow parameters. The code aligns with existing benchmarks in the field, offering confidence in its predictions.
4. Comparison with Experimental Data: The paper presents a comparative analysis with RHIC experimental data, focusing on centrality-dependent multiplicity and elliptic flow characteristics. Notably, this analysis determined that while integrated elliptic flow data is consistent with an $\eta/s$ up to 0.16, minimum bias flow data suggests a potentially smaller $\eta/s$, challenging the lower bound suggested by AdS/CFT.

Implications and Future Directions

Regarded in the context of RHIC physics, the findings provoke discussions around the underlying particle interactions and transport coefficients in extreme conditions. The potential indication of an $\eta/s$ ratio below the AdS/CFT limit is notable and implies a higher degree of fluidity than previously conjectured.

This study prompts further exploration into:

• Alternative Initial Condition Models:

Employing alternative models such as Color Glass Condensate (CGC) may provide different insights into the system's initial state, potentially affecting the interpretation of $\eta/s$.

• Advanced Hydrodynamic + Cascade Models:

A more comprehensive model integrating kinetic elements through hydro+cascade approaches could yield additional accuracy, offering insights into late-stage collision dynamics.

• Quantification of Non-Hydrodynamic Effects:

It is essential to explore other non-hydrodynamic factors or conditions, such as mean-field effects or instabilities in QCD plasma, that could influence viscosity.

In sum, this work by Romatschke et al. significantly contributes to our understanding of heavy-ion collisions and opens avenues for refining the theoretical and computational models, enhancing the alignment with experimental results. Continued refinement of transport coefficient measurements will be instrumental in further elucidating the properties of quark-gluon plasma in relativistic nuclear collision environments.