INITIAL GEOMETRY FLUCTUATIONS AND THEIR IMPACT ON ELLIPTIC FLOW IN SMALL AND LARGE COLLISION SYSTEMS

Abstract

Collective behavior of relativistic heavy-ion collisions is now one of the most significant fields of study in high-energy nuclear physics, especially in the study of the properties of strongly interacting matter in extreme conditions. Elliptic flow (v₂) is one of the several observables to describe this behavior, and it can directly give information on the anisotropic expansion of the system that is created right after the collision. Although the elliptic flow is well established and well known in large collision systems (Au +Au and Pb +Pb), recent experimental results have shown that the flow-like signals can still be present even in small systems (d+Au and p+Pb) collisions. This has brought some fundamental questions into the origin of collectivity in such systems, in which the emergence of a highly interacting medium is less assured.

We examine the importance of initial geometry fluctuations of events in the production of elliptic flow in both small and large collision systems through a Monte Carlo Glauber model in the current work. Each event is simulated with the initial spatial distribution of nucleons and the participant eccentricity (ε2) is computed as a measure of anisotropy of the overlap region. This is then used to estimate v₂ through a linear mapping between eccentricity and elliptic flow, which permits a direct relationship between initial geometry and final-state observables without having to evolve the full hydrodynamics. These findings show that the small systems have much bigger variation in the eccentricity, and hence the distribution of the elliptic flow is much broader, whereas the large systems have much more smooth and stable behavior, because of averaging over a large population. These results qualitatively recreate experimental tendencies at RHIC and LHC and emphasize initial geometry fluctuations as an essential mechanism of collective phenomena at various system scales.