Measurements of Kaon Femtoscopy in Au+Au Collisions at $\sqrt{s_{NN}}$ = 3.0-4.5 GeV by the STAR Experiment

TL;DR

This work performs kaon femtoscopy in Au+Au fixed-target collisions at STAR across $\sqrt{s_{NN}}=3.0$–$4.5$ GeV to probe kinetic-freeze-out geometry in a high-baryon-density environment. Charged and neutral kaon correlations are analyzed with $S$-wave-focused parameterizations—Sinyukov-Bowler for charged kaons and Lednicky-Lyuboshitz llfit for neutral kaons—while UrQMD+CRAB provides comparison benchmarks. The results show no clear energy dependence of the source radii $R_G$, kaons do not exhibit the pion-like $m_T$ scaling, and the neutral-kaon abundance asymmetry $\epsilon$ decreases with energy, with UrQMD reproducing these trends within uncertainties. These findings shed light on strangeness production, kaon–pion equilibration, and the space-time evolution of the emitting source in dense baryonic matter, marking the first such measurements in this regime and offering constraints on freeze-out dynamics.

Abstract

In these proceedings, we present the measurements of charged $K^{+} - K^{+}$ and neutral $K_{s}^{0} - K_{s}^{0}$ correlation functions from Au+Au fixed-target collisions at $\sqrt{s_{NN}}$ = 3.0, 3.2, 3.5, 3.9 and 4.5 GeV at STAR. This is the first such systematic measurement of correlation functions involving strangeness in the high baryon density region. The source size values do not exhibit a clear energy dependence, and the transverse mass dependence of source size for kaons does not align with the trend observed for pions. Parameters extracted from UrQMD transport model calculations qualitatively capture the measured values.

Figures (4)

• Figure 1: One dimensional correlation functions of $\pi^{+} - \pi^{+}$ (upper panel), $K^{+} - K^{+}$ (middle panel), $K_{s}^{0} - K_{s}^{0}$ (bottom panel) in 0-60% Au + Au collisions at $\sqrt{s_{NN}}$ = 3.0 GeV - 4.5 GeV. Vertical lines and grey vertical band represent the statistical and systematic uncertainties, respectively.
• Figure 2: Energy dependence of extracted parameters of $\pi^{+} - \pi^{+}$ (left panel), $K^{+} - K^{+}$ (middle panel), $K_{s}^{0} - K_{s}^{0}$ (right panel) in 0-60% Au + Au collisions at $\sqrt{s_{NN}}$ = 3.0 - 4.5 GeV.
• Figure 3: Energy dependence of kaon abundance asymmetry of charged kaons (red point) and neutral kaons (black points) in Au + Au collisions at $\sqrt{s_{NN}}$ = 3.0 GeV - 4.5 GeV.
• Figure 4: The average transverse mass dependence of source radii $R_{G}$ of $\pi^{+} - \pi^{+}$, $K^{+} - K^{+}$ , $K_{s}^{0} - K_{s}^{0}$ in 0-60% Au + Au collisions at $\sqrt{s_{NN}}$ = 3.0 GeV - 3.9 GeV. The $R_{G}$ of $\pi^{+} - \pi^{+}$ is represented by black points, while the $K^{+} - K^{+}$ and $K_{s}^{0} - K_{s}^{0}$ are represented by green and brown points, respectively. The black dashed line represents a power-law fit to all the data points for pions, with the power-law exponent b is -0.25.