Measurement of directed flow of $K^{*0}$ and $\phi$ resonances in Au+Au collisions at RHIC BES energies

Abstract

We report measurements of the directed flow ($v_{1}$) for charged kaons, $\phi$ mesons, and $K^{*0}$ resonances in Au+Au collisions at $\sqrt{s_{NN}}$ = 14.5, 19.6, and 27 GeV. This analysis includes the first-ever $v_{1}$ measurement for the $K^{*0}$ resonance in heavy-ion collisions. Our results reveal a centrality-dependent difference in directed flow between charged kaons and $K^{*0}$ resonances, with the difference increasing toward more central collisions.In contrast, the $v_{1}$ difference between kaons and $\phi$ mesons remains nearly constant across centralities. The observed kaon-$K^{*0}$ difference can be qualitatively understood within a hydrodynamic framework that incorporates a hadronic afterburner and an asymmetric loss of $K^{*0}$ yields in momentum-space. Since hadronic rescattering depends strongly on the system size and scattering cross sections among hadrons, the measured $K^{*0}$ $v_{1}$ offers valuable constraints on phase-space-dependent rescattering effects in heavy-ion collisions, thereby providing important input for transport-based models of QCD matter.