Is anisotropic flow really acoustic?

The flow harmonics for charged hadrons ($v_n$) and their ratios $(v_n\mathrm{/}{v}_2{)}_{n\ge 3}$, are studied for a broad range of transverse momenta ($p_T$) and centrality ($\text{cent}$) in Pb+Pb collisions at $\sqrt{s_{NN}}= 2.76$ TeV. They indicate characteristic scaling patterns for viscous damping consistent with the dispersion relation for sound propagation in the plasma produced in the collisions. These scaling properties are not only a unique signature for anisotropic expansion modulated by the specific shear viscosity ($\eta \mathrm{/}s$), they provide essential constraints for the relaxation time, a distinction between two of the leading models for initial eccentricity, as well as an extracted $<\eta \mathrm{/}s>$ value which is insensitive to the initial geometry model. These constraints could be important for a more precise determination of $\eta \mathrm{/}s$.