Exploring the Origin of Anisotropy in Small Systems: From Symmetric (O+O) to Asymmetric (d+Au) Collisions

TL;DR

This work probes the origin of azimuthal anisotropy in small collision systems by comparing $d+Au$ and $^{16}$O+$^{16}$O at $\,\sqrt{s_{NN}}=200$ GeV to test hydrodynamic response to initial geometry. It combines two-particle correlations with a large $|\,\Delta\eta|$ gap and four-particle cumulants, alongside a subevent approach to assess longitudinal decorrelation, and interprets results using Glauber-based initial-state eccentricities. The findings show that $v_2$ and $v_3$ scale with $\varepsilon_2$ and $\varepsilon_3$, respectively, across systems, consistent with a QGP-like hydrodynamic response and underscoring the role of sub-nucleon fluctuations in shaping $\varepsilon_3$; no strong longitudinal decorrelation is observed after non-flow subtraction. These results reinforce the interpretation of collectivity in small systems as geometry-driven and provide constraints on initial-state fluctuations and non-flow effects relevant for future precision studies.

Abstract

This contribution reports STAR measurements of azimuthal anisotropies in produced particle distributions of the d+Au and $^{16}$O+$^{16}$O collisions at $\sqrt{s_{NN}} = 200$ GeV, probing the origin of collectivity in small systems. We test the hydrodynamic response of the produced medium by comparing these systems with vastly different initial geometries. The measured elliptic ($v_2$) and triangular ($v_3$) anisotropies, and their event-by-event fluctuations, scale robustly with initial-state eccentricities, consistent with the hydrodynamic behavior expected from a Quark Gluon Plasma (QGP) droplet formed in these collisions. The result also helps constrain the role of sub-nucleon fluctuations in determining the initial conditions. Second, the wide pseudorapidity coverage of the STAR detector is used to investigate longitudinal dynamics. Correlations across different rapidity gaps show no significant impact from flow decorrelation, but non-flow contributions are substantial and require careful subtraction.

Figures (3)

• Figure 1: Multiplicity dependence of elliptic ($v_2$) and triangular ($v_3$) flow harmonics and their scaling with initial-state eccentricity ($\varepsilon_n$). Panels (a) and (d) show the measured $v_2\{2\}$ and $v_3\{2\}$. Panels (b) and (e) show the scaling ratio $v_n\{2\}/\varepsilon_n\{2\}$ using eccentricities from a nucleon-Glauber model, while panels (c) and (f) use a sub-nucleon (quark) Glauber model loizidesGlauberModelingHighenergy2016.
• Figure 2: Comparison of elliptic flow fluctuations ($v_2\{4\}/v_2\{2\}$, points) with initial eccentricity fluctuations ($\varepsilon_2\{4\}/\varepsilon_2\{2\}$, bands luEssentialElementsNuclear2019a) in d+Au (left) and O+O (right) collisions.
• Figure 3: Investigation of longitudinal decorrelation and non-flow effects in central d+Au collisions.