System-size dependence of the viscous attenuation of anisotropic flow in p+Pb and Pb+Pb collisions at LHC energies

Abstract

The elliptic and triangular flow coefficients ($\mathrm{v_n, \, n=2,3}$) measured in Pb+Pb ($\sqrt{s_{_{\rm NN}}} = 2.76$ TeV) and p+Pb ($\sqrt{s_{_{\rm NN}}} = 5.02$ TeV) collisions, are studied as a function of initial-state eccentricity ($\varepsilon_n$), and dimensionless size characterized by the cube root of the mid-rapidity charged hadron multiplicity density $\mathrm{\left< N_{ch} \right>^{1/3}}$. The results indicate that the influence of eccentricity ($\mathrm{v_n} \propto \varepsilon_n$) observed for large $\mathrm{\left< N_{ch} \right>}$, is superseded by the effects of viscous attenuation for small $\mathrm{\left< N_{ch} \right>}$, irrespective of the colliding species. Strikingly similar acoustic scaling patterns of exponential viscous modulation, with a damping rate proportional to $\mathrm{n^2}$ and inversely proportional to the dimensionless size, are observed for the eccentricity-scaled coefficients for the two sets of colliding species. The resulting scaling parameters suggest that, contrary to current predilections, the patterns of viscous attenuation, as well as the specific shear viscosity $\left<\frac{\eta}{s}(\text{T})\right>$ for the matter created in p+Pb and Pb+Pb collisions, are comparable.