Small-System Partonic Flow and the Limits of Nominal Size-Based Phase Classification: A Diagnostic Note

Recent measurements by the ALICE Collaboration provide a strong reason to reconsider overly rigid size-based classifications in relativistic collision physics. In high-multiplicity proton–proton and proton–lead collisions, ALICE has observed a characteristic baryon–meson pattern in anisotropic flow that had previously been regarded as a hallmark of partonic collectivity in heavy-ion collisions. The observed grouping and splitting behavior is described by models combining hydrodynamic expansion with quark coalescence, whereas models excluding these ingredients do not reproduce the data adequately. This result does not, by itself, settle the full question of whether small collision systems should simply be identified with the same quark–gluon plasma phase discussed in large nucleus–nucleus collisions. It does, however, challenge a naive classification according to which nominal system size alone should be sufficient to separate baseline systems from systems capable of exhibiting quark-level collective signatures. The main conceptual importance of the result lies precisely here: a robust local signature can emerge even where a stronger global phase attribution remains more delicate. On this reading, small-system flow becomes a diagnostic stress test on classificatory legitimacy rather than a slogan about the collapse of all distinctions. This note argues for a more disciplined interpretive posture: size remains physically relevant, but nominal size alone is no longer a sufficient classifier of the relevant regime. What follows is not the erasure of distinctions between small and large systems, but the need for more condition-sensitive and regime-sensitive criteria.