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 $N_{\rm ch}$ 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 $κ_0(p_\mathrm{T})/κ_0$ largely independent of the fluctuation amplitude. Identified-hadron mass ordering and viscous model comparisons demonstrate sensitivity to bulk viscosity ($ζ/s$). 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.