Scaling of the higher-order flow harmonics: implications for initial-eccentricity models and the "viscous horizon"

Understand

The scaling properties of the flow harmonics for charged hadrons $v_{n}$ and their ratios $[ v_n/(v_2)^{n/2}]_{n\geq 3}$, are studied for a broad range of transverse momenta ($p_T$) and centrality selections in Au+Au and Pb+Pb collisions at $\sqrt{s_{NN}}=0.2 \text{and} 2.76$ TeV respectively.

• At relatively low $p_T$, these scaling properties are found to be compatible with the expected growth of viscous damping for sound propagation in the plasma produced in these collisions.
• They also provide important constraints for distinguishing between the two leading models of collision eccentricities, as well as a route to constrain the relaxation time and make estimates for the ratio of viscosity to entropy density $\eta/s$, and the "viscous horizon" or length-scale which characterizes the highest harmonic which survives viscous damping.