Engineering the shapes of quark-gluon plasma droplets by comparing anisotropic flow in small symmetric and asymmetric collision systems

Abstract

The observation of collective flow phenomena in small collision systems challenges our understanding of quark-gluon plasma (QGP) formation and evolution. This complexity lies in the initial geometries, which are influenced by both nucleon configuration and subnucleonic fluctuations, introducing uncertainties in interpreting flow patterns. We disentangle these contributions through comparative measurements of elliptic ($v_2$) and triangular ($v_3$) flow in asymmetric $d$+Au and symmetric $^{16}$O+$^{16}$O collisions at $\sqrt{s_{NN}}=200$ GeV, which produce medium of comparable sizes but with vastly different initial geometries. The larger $v_2$ in $d$+Au reflects its dominant elliptic geometry, while the similar $v_3$ in both systems is better explained by considering subnucleonic fluctuations. These contrasting flow patterns are quantitatively described by a state-of-the-art hydrodynamic model tuned to large-system Au+Au data, indicating efficient transformation of initial geometries to final-state anisotropies. These results provide evidence for droplet formation in small systems with transport properties that are similar to those observed in large collision systems, consistent with QGP-like behavior.

Figures (2)

• Figure 1: Contrasting initial geometry in $d$+Au and O+O collisions. Illustration of one ultra-central $d$+Au (left) or O+O (right) collision for nucleon-Glauber model (top) and quark-Glauber model where each nucleon is replaced with three constituent quarks (bottom). Red blobs indicate participating nucleons (top) or quarks (bottom). The typical ordering of eccentricities $\varepsilon_n$ between the systems and models is shown Huang:2025cjm. Black-dotted circles indicate Wood-Saxon radii for Au (6.38 fm) and O (2.61 fm). Purple-dotted circles show the locations of two nucleons of the deuteron.
• Figure 2: Flow coefficients from two-particle correlation method. Raw correlators $\left\langle v_n^2\right\rangle^{\mathrm{obs}}$ (left), non-flow subtracted correlators $\left\langle v_n^2\right\rangle$ (middle), and flow coefficients $v_n\{2\}$ (right) versus $N_{\mathrm{ch}}$ for $n=2$ (top) and $n=3$ (bottom) in O+O and $d$$+$Au collisions. Error bars denote statistical uncertainties, shaded bands denote systematic uncertainties. The $v_n\{2\}$ values are compared to hydrodynamic model predictions Jahan:2025cbp, whose error band represents model uncertainties.