Nearly isentropic flow at sizeable η/s

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 (∼50% centrality) PbPb or central pPb collisions. As a non-vanishing mean free path is indicative of non-minimal dissipation, this challenges the perfect fluid paradigm of ultra-relativistic nucleus-nucleus and hadron-nucleus collisions.