Azimuthal anisotropy: Transition from hydrodynamic flow to jet suppression

Measured second and fourth azimuthal anisotropy coefficients v _2,4(N_part,p_T) are scaled with the initial eccentricity ε_2,4(N_part) of the collision zone and studied as a function of the number of participants N_part and the transverse momenta p_T. Scaling violations are observed for p _T3 GeV/c, consistent with a pT2 dependence of viscous corrections and a linear increase of the relaxation time with p_T. These empirical viscous corrections to flow and the thermal distribution function at freeze-out constrain estimates of the specific viscosity and the freeze-out temperature for two different models for the initial collision geometry. The apparent viscous corrections exhibit a sharp maximum for p_T3 GeV/c, suggesting a breakdown of the hydrodynamic ansatz and the onset of a change from flow-driven to suppression-driven anisotropy.