EMMI Workshop: Light-ion collisions 2026

In large collision systems, the initial elliptical geometry generated at finite impact parameter gives rise to the elliptic flow coefficient $v_2$, whose centrality dependence is largely determined by this geometry. In contrast, intermediate and small systems exhibit a much weaker centrality dependence. To investigate the origin of $v_2$ in oxygen-oxygen collisions, we decompose the three-dimensional initial energy density profiles generated by McDipper into an average state and uncorrelated fluctuation modes, followed by a 3D non-boost invariant hydrodynamic evolution with MUSIC. We find that the average state contributes only marginally to the final $v_2$, indicating that elliptic flow in this system is predominantly driven by event-by-event fluctuations. Furthermore, despite the large event-by-event fluctuations within a given centrality class, we show that a reduced basis containing only a few selected modes provides a satisfactory description of the probability distribution of $v_2$.