Nearly isentropic flow at sizeable $\eta/s$

TL;DR

This paper shows that even minimal interactions in transport theory can produce the observed complex flow patterns in high-energy nuclear collisions, questioning the need for a perfect fluid description.

Contribution

It demonstrates that a simple one-hit transport model with a large shear viscosity to entropy density ratio can reproduce experimental flow data, challenging the perfect fluid paradigm.

Findings

Transport theory with weak interactions explains observed flow structures.

Large η/s ratios are consistent with experimental azimuthal anisotropies.

Transport models can match data without assuming near-perfect fluidity.

Abstract

Non-linearities in the harmonic spectra of hadron-nucleus and nucleus-nucleus collisions provide evidence for the dynamical response to azimuthal spatial eccentricities. Here, we demonstrate within the framework of transport theory that even the mildest interaction correction to a picture of free-streaming particle distributions, namely the inclusion of one perturbatively weak interaction ("one-hit dynamics"), will generically give rise to all observed linear and non-linear structures. We further argue that transport theory naturally accounts within the range of its validity for realistic signal sizes of the linear and non-linear response coefficients observed in azimuthal momentum anisotropies with a large mean free path of the order of the system size in peripheral ($\sim 50 \%$ centrality) PbPb or central pPb collisions. The shear viscosity to entropy density ratio $\eta/s$ of such a transport theory is approximately an order of magnitude larger than that of an almost perfect fluid. The phenomenological success of transport simulations thus challenges the perfect fluid paradigm of ultra-relativistic nucleus-nucleus and hadron-nucleus collisions.