Understand
Flow coefficients $v_n$ for $n$ = 2, 3, 4, characterizing the anisotropic collective flow in Au+Au collisions at $\sqrt{s_{NN}} = 200$ GeV, are presented.
- They indicate the expected growth of viscous damping for sound propagation in the quark gluon plasma (QGP) produced in these collisions.
- Hydrodynamical model comparisons which include the effects of initial state geometry fluctuations, highlight the role of higher harmonics ($v_{n, n>2}$) as a constraint for disentangling the effects of viscosity and initial conditions, and suggest a small specific viscosity for the QGP.
- This viscosity is compatible with that obtained via a newly proposed technique \cite{Lacey:2011ug} which employs the relative magnitudes of $v_n$ to estimate the viscosity, and the "viscous horizon" or length-scale which characterizes the highest harmonic that survives viscous damping.