Viscous Properties of Strongly Interacting Matter at High Temperature

Understand

Substantial collective flow is observed in collisions between large nuclei at high energy, and the data are well-reproduced by perfect fluid dynamics.

• In a separate development, calculation of the dimensionless ratio of shear viscosity $\eta$ to entropy density within AdS/CFT yields $\eta/s = 1/4\pi$, which has been conjectured to be a lower bound for any physical system.
• It is shown that the transition from hadrons to quarks and gluons has behavior similar to helium, nitrogen, and water at and near their phase transitions in the ratio $\eta/s$.
• Conversely, there are indications that the ratio of bulk viscosity $\zeta$ to entropy density may have a maximum in the vicinity of the phase transition.