Azimuthal anisotropy: transition from hydrodynamic flow to jet suppression*

The $2^{\rm{nd}}$ and $4^{\rm{th}}$ azimuthal anisotropy coefficients $v_{2,4}(N_{\rm{part}}, p_T)$ are scaled with the initial eccentricity $\varepsilon_{2,4}(N_{\rm{part}})$ of the collision zone and studied as a function of the number of participants $N_{\rm{part}}$ and the transverse momenta $p_T$. Scaling violations are observed for $p_T \alt 3$ GeV/c, consistent with a quadratic increase of viscous corrections with $p_T$. The predicted viscous corrections to flow and the thermal distribution function at freeze-out constrain estimates of the specific viscosity ($4\pi\frac{\eta}{s}$) to $1.1 \pm 0.1$ or $2.1 \pm 0.2 $ and the freeze-out temperature ($T_{\!f}$) to $162 \pm 11$ MeV or $173 \pm 11$ MeV for two different models for the initial collision geometry. For $p_T \agt 3$ GeV/c, the apparent viscous corrections exhibit a rapid decrease with $p_T$, suggesting a breakdown of the hydrodynamic ansatz and the onset of a change from flow-driven to suppression-driven anisotropy.