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Title: Why small collision systems show collective flow
Plain: 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: 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.
Settled by: 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.
Kind: phenomenon
Answer type: mechanism
References: (empty)
Literature status: contested
Status 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.
Posed since: 2010