Speaker
Description
Directed flow ($v_1$) and elliptic flow ($v_2$) provide complementary probes of the equation of state, degrees of freedom, and collective dynamics of the produced medium. Here, we present STAR measurements of collective flow for identified strange and multistrange hadrons in heavy-ion collisions at RHIC.
Using high-statistics data from the STAR Beam Energy Scan Phase-II (BES-II) fixed-target and collider programs, together with previously recorded higher-energy Au+Au data, we investigate strange-hadron collective flow over a broad range of collision energies. In Au+Au collisions at $\sqrt{s_{NN}}=3.0$–$4.5$~GeV, directed-flow measurements, including negative directed-flow slopes for low-$p_T$ kaons, provide information on spectator shadowing and collision dynamics. Elliptic-flow ($v_2$) measurements over $\sqrt{s_{NN}}=3.0$–$19.6$~GeV probe the collision-energy evolution of number-of-constituent-quark (NCQ) scaling and the onset of partonic collectivity. Approximate NCQ scaling, with NCQ-scaled baryon-to-meson ratios close to unity, is observed for both particles and antiparticles at $\sqrt{s_{NN}}=7.7$–$19.6$~GeV. Together with the observed mass ordering, these observations are consistent with collective expansion prior to hadronization. As a complementary probe of strange-hadron collectivity, we further present the $p_T$-differential radial-flow fluctuation observable, $v_0(p_T)$, for strange and multi-strange hadrons from $\sqrt{s_{NN}}=11.5$ to $200$~GeV.
Comparisons with transport and hydrodynamic model calculations provide additional constraints on the collision-energy dependence of collective expansion and help interpret its implications for the QCD phase structure.
