Towards the Edge of Reason: Hydrodynamics in Systems at the Femtoscale
In high energy collisions of heavy-ions, experimental findings of collective flow are customarily associated with the presence of a thermalized medium expanding according to the laws of hydrodynamics, which has been dubbed a 'quark-gluon plasma'. Just a few years ago, there was near consensus in the field that systems created in proton-nucleus and proton-proton collisions were too small, too short-lived, and contained too few particles to form a quark-gluon plasma. Then the experimental results started to come in. In all of these small systems, the experimental signals turned out to be similar in type and magnitude to those found in heavy-ion collisions, severely challenging the 'consensus'. In this talk, I will discuss under which conditions hydrodynamics can reasonably be applied to small systems in proton-nucleus and proton-proton collisions, whether it can be used to quantitatively describe experimental data, and what we may learn about QCD transport coefficients and non-equililibrium QCD dynamics along the way.