Measurement of directed flow of K * 0 and ϕ resonances in Au+Au collisions at RHIC BES energies

We report measurements of the directed flow ( v 1 ) for charged kaons, ϕ mesons, and K *0 resonances in Au+Au collisions at √ 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 towards more central collisions. In contrast, the v 1 difference between kaons and ϕ 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 with respect to first order event plane. 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.