Observation of Strong $\phi$-meson Directed Flow at High Baryon Density

Abstract

We report the first observation of a large directed flow ($v_1$) of $\phi$ mesons in mid-central Au+Au collisions at $\sqrt{s_{\mathrm{NN}}}=3.0$-4.5 GeV, together with the $\sqrt{s_{\mathrm{NN}}}=7.7$ GeV collider-energy result for the collision-energy dependence, measured by the STAR experiment at RHIC. Despite its mesonic nature, the $\phi$-meson $v_{1}$ exhibits a magnitude comparable to that of protons and $\Lambda$ baryons, and is significantly larger (by a factor of $\gtrsim3$) than that of $K^0_S$ mesons at the lowest energies. The midrapidity slope $dv_1/dy$ shows a pronounced energy dependence similar to that observed for protons and $\Lambda$ baryons, in contrast to lighter mesons, in the high-baryon-density region. Comparisons with hadronic transport calculations indicate that the observed $\phi$-meson directed flow is consistent with production via high-mass baryon resonances and the collective motion of baryons influenced by baryonic mean-field interactions. Due to its long lifetime and relatively weak effective $\phi$-$N$ interaction, the $\phi$ meson can retain sensitivity to collective dynamics established during the dense baryonic stage of the collision. These results provide new experimental constraints on baryon-resonance production and strange-hadron transport dynamics in dense QCD matter.