Radial Flow of Strange and multi-strange Hadrons in Heavy-Ion Collisions at RHIC-STAR

We present recent STAR measurements of the radial flow fluctuation coefficient $v_{0}(p_{T})$ for strange and multi-strange hadrons ($\Lambda$, $\bar{\Lambda}$, $K^{0}_{S}$, $\Xi^{-}$, $\bar{\Xi}^{+}$, and $\phi$) in Au+Au collisions at RHIC. Because these particles are less affected by late-stage hadronic interactions, they serve as clean probes of the early partonic stage. The analysis covers both the top RHIC energy and the Beam Energy Scan (BES) program, spanning $\sqrt{s_{\mathrm{NN}}} = 11.5 - 200$ GeV. As the isotropic component of collective expansion, $v_{0}(p_{T})$ provides direct sensitivity to the strength and event-by-event fluctuations of radial flow. We investigate its mass dependence at low $p_{T}$, which is consistent with hydrodynamic expectations, and observe a distinct baryon-mesonsplitting at 200 GeV, suggesting the influence of quark recombination. Furthermore, we test the Number-of-Constituent-Quark (NCQ) scaling of $v_{0}(p_{T})$ to examine whether radial flow originates from partonic collectivity, as established for elliptic flow, and find that it holds within uncertainties across the full BES energy range. The weak energy dependence of $v_{0}(p_{T})$ suggests that radial flow fluctuations are primarily governed by medium properties that evolve slowly with collision energy. Comparisons with hydrodynamic and transport models provide new insights into the development of radial flow and its fluctuations across the QCD phase diagram.