Nearly isentropic flow at sizeable $\eta/s$

Understand

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.