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PHENIX Measurements of Higher-order Flow Harmonics in Au+Au collisions at Root_s = 200 GeV

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.