Suppression of elliptic flow in a minimally viscous quark-gluon plasma

Abstract

We compute the time evolution of elliptic flow in non-central relativistic heavy-ion collisions, using a (2+1)-dimensional code with longitudinal boost-invariance to simulate viscous fluid dynamics in the causal Israel-Stewart formulation. We show that even ``minimal'' shear viscosity eta/s=hbar/(4pi) leads to a large reduction of elliptic flow compared to ideal fluid dynamics, raising questions about the interpretation of recent experimental data from the Relativistic Heavy Ion Collider.

Figures (4)

• Figure 1: (Color online) Time evolution of the central entropy density (a) and average radial flow velocity (b) in central Cu+Cu collisions (see text for details). Here and later stars indicate the time when all matter is frozen out.
• Figure 2: (Color online) Time evolution of the dominant components of the shear viscous pressure tensor, normalized by $e{+}p$ and averaged over the transverse plane, for two different initial conditions (red and green, respectively). Note that the normalization factor $e{+}p{\,\sim\,}T^4$ decreases rapidly with time.
• Figure 3: (Color online) Time evolution of the spatial eccentricity $\epsilon_x$ (a) and momentum anisotropy $\epsilon_p$ (b) for non-central Cu+Cu collisions at $b{\,=\,}7$ fm. See text for details.
• Figure 4: (Color online) (a) The elliptic flow $v_2(p_T)$ for pions, kaons and protons from ideal fluid dynamics (solid lines) and viscous hydrodynamics (dotted and dashed lines). Dotted lines account only for viscous effects on the flow pattern that enters the equilibrium part of the distribution function; dashed lines additionally include viscous (non-equilibrium) corrections to the latter. (b) Effects of different choices for the kinetic relaxation time $\tau_\pi$ and for the initial viscous pressure $\pi^{mn}$ on pion elliptic flow (with the same separation of equilibrium and non-equilibrium contributions as in part (a)).