Collision energy and system size dependence of $p_{\mathrm{T}}$-differential radial flow fluctuations $v_0(p_{\mathrm{T}})$ at RHIC

Abstract

We report the first RHIC measurements of $v_0(p_{\mathrm{T}})$ in Au+Au collisions at $\sqrt{s_{\mathrm{NN}}}=7.7$--$200$ GeV and O+O collisions at $\sqrt{s_{\mathrm{NN}}}=200$ GeV. The integral fluctuation $v_0$ follows a common $N_{\mathrm{ch}}$ dependence in large and small systems, suggesting that the fluctuation magnitude is predominantly controlled by event-by-event fluctuations of the initial transverse size. The normalized response $v_0(p_{\mathrm{T}})/v_0$ factorizes across centralities and systems, revealing a universal hydrodynamic response ${\kappa }_0(p_{\mathrm{T}})/{\kappa }_0$ largely independent of the fluctuation amplitude. Identified-hadron mass ordering and viscous model comparisons demonstrate sensitivity to bulk viscosity ($\zeta /s$). These results establish $v_0(p_{\mathrm{T}})$ as a probe of the radial hydrodynamic response, collectivity across large and small collision systems, and QGP transport properties.

Conference Proceedings: The 22nd International Conference on Strangeness in Quark Matter (SQM 2026). 4 Figures. 4 Pages. Submitted on behalf of the STAR collaboration