Observation of partonic collectivity via $p_{\mathrm{T}}$-differential radial flow fluctuations in Au+Au collisions at $\sqrt{s_{\mathrm{N}\mathrm{N}}}=200$ GeV

We report the observation of partonic radial collectivity in Au+Au collisions at $\sqrt{s_{\mathrm{N}\mathrm{N}}}=200$~GeV via the $p_{\mathrm{T}}$-differential flow observable $v_0(p_{\mathrm{T}})$ using the $\mathtt{\text{AMPT}}$ 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 $v_0(p_{\mathrm{T}})$ normalized by its $p_{\mathrm{T}}$-integrated value $v_0$, analogous to anisotropic flow. For identified particles (${\pi }^{\pm },K^{\pm },p+\bar{p}$), the $v_0(p_{\mathrm{T}})$ spectra show mass ordering at low-$p_{\mathrm{T}}$ and meson-baryon separation at intermediate-$p_{\mathrm{T}}$. In $\mathit{\text{central}}$ collisions, $v_0(p_{\mathrm{T}})\mathrm{/}{n}_q$ exhibits robust $\mathit{\text{Number}}\mathit{\text{of}}\mathit{\text{Constituent}}\mathit{\text{Quark}}$ (NCQ) scaling with $(m_{\mathrm{T}}-m_0)\mathrm{/}{n}_q$, a scaling that breaks down in $\mathit{\text{peripheral}}$ 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.