Why small collision systems show collective flow
In plain words
When two lead nuclei collide, the produced particles flow together as if from a drop of liquid. The same flow pattern appears in collisions of single protons, where few expected a liquid to form.
Precise statement
High-multiplicity pp and p-Pb collisions at the LHC show long-range two-particle correlations (the ridge), azimuthal anisotropies $v_{2}$ and $v_{3}$ from multi-particle cumulants, and strangeness enhancement comparable to Pb-Pb. Determine the mechanism (hydrodynamic response of a quark-gluon plasma droplet, initial-state gluon correlations, or kinetic transport with few rescatterings) and the minimum system size for hydrodynamic behavior. Answer: one model reproducing $v_{n}$ versus multiplicity and system size from pp to Pb-Pb with fixed parameters.
What would settle it
Flow-fluctuation and correlation measurements in pp, p-O, O-O, Ne-Ne and Pb-Pb compared with models that include both initial-state and final-state effects.
Status in the literature
Unverified note
LHC O+O and Ne+Ne collisions (July 2025) gave flow consistent with hydrodynamic predictions using ab initio nuclear shapes; the origin of flow in pp collisions remained unsettled as of 2026.