Quantitative and Conceptual Considerations for Extracting the Knudsen Number in Heavy Ion Collisions

In this article, we examine the methodology for extracting the Knudsen number (K) and the ratio of shear viscosity to entropy density ({eta}/s) developed by Drescher et al. [1]. The final result for {eta}/s turns out to be quite sensitive to Glauber parameters, particularly the parameter x, which controls the balance between N{sub part} and N{sub coll}. We also explore how alternative formulations of the functional relation between the elliptic flow and Knudsen number (K={lambda}/R{sup -}) impacts the physics conclusions, based on Pade approximants. Finally, we extend the calculation to include a limiting minimum value on the mean free path proportional to the DeBroglie wavelength. These results emphasize the importance of clarifying the initial state used in different calculations as well as the ambiguities inherent in using a transport approach in a strongly coupled regime.