Observation of partonic collectivity via $p_{\rm T}$-differential radial flow fluctuations in Au+Au collisions at $\sqrt{s_{\rm NN}} = 200$ GeV
We report the observation of partonic radial collectivity in Au+Au collisions at $\sqrt{s_{\rm NN}} = 200$~GeV via the $p_{\rm T}$-differential flow observable $v_{0}(p_{\rm T})$ using the \texttt{AMPT} String Melting model. For inclusive charged hadrons, we establish three signatures of collectivity: long-range pseudorapidity correlations, the factorization of two-particle correlations, and a centrality-independent scaling of $v_{0}(p_{\rm T})$ normalized by its $p_{\rm T}$-integrated value $v_{0}$, analogous to anisotropic flow. For identified particles ($\pi^{\pm}, K^{\pm}, p + \overline p$), the $v_{0}(p_{\rm T})$ spectra show mass ordering at low-$p_{\rm T}$ and meson-baryon separation at intermediate-$p_{\rm T}$. In \textit{central} collisions, $v_{0}(p_{\rm T})/n_{q}$ exhibits robust \textit{Number of Constituent Quark} (NCQ) scaling with $(m_{\rm T} - m_{0})/n_{q}$, a scaling that breaks down in \textit{peripheral} collisions and is more precise at RHIC than at LHC energies, consistent with earlier $v_{2}$ studies. These findings provide strong evidence that radial collectivity originates predominantly at the partonic stage, extending the paradigm of quark-level dynamics from anisotropic to isotropic flow.