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

TL;DR

Problem: How the hadronic phase and early-time dynamics shape collective flow in heavy-ion collisions, particularly for short-lived resonances. Approach: The study measures rapidity-odd directed flow $v_{1}$ for $K^{*0}$, $φ$, and charged kaons in Au+Au collisions at $\sqrt{s_{NN}}$ = 14.5, 19.6, and 27 GeV, using STAR with TPC/TOF for PID, an Event Plane Detector–based $\Psi_1$, and the invariant-mass method to extract $v_{1}$. Contributions: First $v_{1}$ measurement for $K^{*0}$ and a clear centrality dependence showing a sign change for $K^{*0}$ and a growing $\langle v_1 \rangle$ difference with $K$, with hydrodynamic calculations including the hadronic UrQMD afterburner reproducing the trend. Significance: Demonstrates that $K^{*0}$ $v_{1}$ is sensitive to phase-space–dependent hadronic rescattering and provides important constraints for transport-based descriptions of QCD matter.

Abstract

We report measurements of the directed flow ($v_{1}$) for charged kaons, $φ$ 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 $φ$ 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 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.

Figures (4)

• Figure 1: Rapidity (y) dependence of directed flow ($v_1$) for $K^{*0}, \phi~\text{and} ~K$ meson for 10-40% (lower pannel) and 40-80% (upper pannel) Au+Au collisions at $\sqrt{s_{NN}}$ = 14.6, 19.6 and 27 GeV.
• Figure 2: Transverse momentum ($p_{T}$) dependence of directed flow ($v_1$) for $K^{*0}$ (left panel) and $\phi$ (right panel) meson for 10-40% and 40-80% Au+Au collisions at $\sqrt{s_{NN}}$ = 19.6 GeV.
• Figure 3: Rapidity and $p_{T}$-integrated directed flow ($v_1$) as a function of number of participating nucleons ($\langle N_{part}\rangle$) for $K^{*0}, \phi~\text{and} ~K$ meson in Au+Au collisions at $\sqrt{s_{NN}}$ = 14.6, 19.6 and 27 GeV.
• Figure 4: The difference between $\langle v_1 \rangle$ of $K^{*0}~\text{and}~K$ meson as a function of number of participating nucleons ($\langle N_{part}\rangle$) in Au+Au collisions at $\sqrt{s_{NN}}$ = 14.6, 19.6 and 27 GeV. The hydrodynamic model calculations are shown with shaded bands.