Speaker
Description
Directed flow $v_1$ and particle yield ratios provide complementary probes of
particle-production mechanisms and the evolution of baryon-rich QCD matter. The $\phi$ meson, with its hidden-strangeness $(s\bar{s})$ structure, long lifetime, and limited late-stage rescattering, is a sensitive probe of early collective dynamics.
In this talk, we report measurements of $\phi$-meson yields and directed
flow in Au+Au collisions at $\sqrt{s_{NN}}=3.0$, 3.2, 3.5, 3.9, and 4.5~GeV
from the RHIC Beam Energy Scan II fixed-target program. The collision-energy
dependence of the $\phi/K^{-}$ yield ratio and the midrapidity slope of the
directed flow, $\left.dv_1/dy\right|_{y=0}$, is presented. The $\phi$-meson
$v_1$ slope is positive and exhibits a baryon-like energy dependence, with
magnitudes comparable to those of protons and $\Lambda$ baryons and
substantially larger than that of $K^0_S$ mesons at the lowest energies.
UrQMD comparisons indicate important contributions from baryonic mean fields
and high-mass baryon-resonance decays in describing both observed behaviors.
These measurements provide new constraints on $\phi$-meson production
mechanisms and strange-hadron transport properties in dense QCD matter.
