Do nuclear collisions create a locally equilibrated quark-gluon plasma?

Understand

Experimental results on azimuthal correlations in high energy nuclear collisions (nucleus-nucleus, proton-nucleus and proton-proton) seem to be well described by viscous hydrodynamics.

• It is often argued that this agreement implies either local thermal equilibrium or at least local isotropy.
• In this note, I present arguments why this is not the case.
• Neither local near-equilibrium nor near-isotropy are required in order for hydrodynamics to offer a successful and accurate description of experimental results.