Measurement of Kaon Directed Flow in Au+Au Collisions in the High Baryon Density Region

Abstract

Rapidity-odd directed flow $v_1$ measurements are presented for $K^{\pm}$ and $K^0_S$ in Au$+$Au collisions at $\sqrt{s_{\text{NN}}}$ = 3.0, 3.2, 3.5, and 3.9 GeV with the STAR experiment. For comparison, $v_1$ of $π^{\pm}$, protons, and $Λ$ from the same collisions are also discussed. The mid-rapidity $v_1$ slope $\text{d}v_1/\text{d}y|_{y=0}$ for protons and $Λ$ is positive in these collisions. On the other hand, $v_1$ slope of kaons exhibits a strong $p_\text{T}$ dependence: negative at $p_\text{T} <$ 0.6 GeV/$c$ and positive at higher $p_\text{T}$. A similar $p_\text{T}$ dependence is also evident for the $v_1$ slope of charged pions. Compared to the spectator-removed calculations in Au$+$Au collisions at $\sqrt{s_{\text{NN}}} =$ 3.0-3.9 GeV, the JAM model demonstrates a pronounced shift of the $v_1$ slopes of mesons towards the negative direction. It suggests that the shadowing effect of the spectators plays an important role in the observed kaon anti-flow at low $p_\text{T}$ in the high baryon density region of non-central collisions.

Figures (5)

• Figure 1: The efficiency uncorrected particle density distribution in transverse momentum and particle rapidity for $\pi^+$, $K^+$, $K^-$, and $K_S^0$ measured with STAR detectors TPC and TOF in Au + Au collisions for $\sqrt{s_{NN}}$ from 3.0 to 3.9 GeV. Note that the black arrows show the target rapidity in the center-of-mass frame, and the black window exhibits the measured $p_\text{T}-y$ window.
• Figure 2: Directed flow ($v_1$) of $\pi^{+}$ (solid square), $\pi^{-}$ (open square), $K^{+}$ (solid diamond), $K^{-}$ (open diamond), $K^{0}_S$ (solid star), protons (solid circle), and $\Lambda$ (open circle) as a function of rapidity in mid-central (10-40%) Au + Au collisions for $\sqrt{s_{NN}}$ from 3.0 to 3.9 GeV. Statistical and systematic uncertainties are shown as bars and gray bands, respectively. Data points of $K^{+}$ are shifted horizontally to improve visibility. The JAM calculations for $K^0$ and protons are represented by red and blue lines, with the dashed and solid lines representing cascade and baryonic mean-field (BMF) modes, respectively.
• Figure 3: Collision energy dependence of the $p_\text{T}$ integrated mid-rapidity $v_1$ slope $\text{d}v_1/\text{d}y|_{y=0}$ for $\pi^{\pm}$, $K^{\pm}$, $K^0_S$, protons, and $\Lambda$ in mid-central (10-40%) Au + Au collisions. Statistical and systematic uncertainties are shown as bars and gray bands, respectively. Data points of $\pi^{-}$ and $K^0_S$ are staggered by $\pm$ 0.05 GeV horizontally to improve visibility. The JAM calculations with baryon mean field for protons and $\Lambda$ are shown as solid and dashed lines, respectively. The $p_\text{T}$ ranges for pions, kaons, and protons$/\Lambda$ are $0.2 < p_\text{T} < 1.6$ GeV/$c$, $0.4 < p_\text{T} < 1.6$ GeV/$c$, and $0.4 < p_\text{T} < 2.0$ GeV/$c$, respectively.
• Figure 4: Sideward flow $\langle p_\text{x} \rangle$ of $K^{0}_S$ (star) and $K^{\pm}$ (triangle) as a function of rapidity in Au + Au collisions at $\sqrt{s_{\text{NN}}}$ = 3.9 GeV from STAR and E895 experiments, where statistical uncertainties are shown as bars. The collision centrality applied at STAR are 0-50% for $K^{0}_S$ and $K^{\pm}$. The $p_\text{T}$ ranges for $K^{0}_S$ and $K^{\pm}$ are $0 < p_\text{T} < 0.7$ GeV/$c$ and $0.06 < p_\text{T} < 0.3$ GeV/$c$, respectively. Note that the normalized rapidity from E895 data has been converted to the rapidity in the center-of-mass frame to facilitate a direct comparison.
• Figure 5: Transverse momentum dependence of the mid-rapidity $v_1$ slope $\text{d}v_1/\text{d}y|_{y=0}$ for $\pi^{+}$, $K^{+}$, $K^{-}$, and $K^{0}_{S}$ in mid-central (10-40%) Au + Au collisions for $\sqrt{s_{NN}}$ from 3.0 to 3.9 GeV. Statistical and systematic uncertainties are shown as bars and gray bands, respectively. Data points of $\pi^{+}$ and $K^{0}_{S}$ are shifted $\pm$ 0.05 GeV/$c$ horizontally for better visibility. JAM model calculations of $\text{d}v_1/\text{d}y|_{y=0}$ for $K^0$ (red) and $\pi^+$ (black) are shown as solid and dashed lines, respectively, representing results with and without the spectators. Note that $\text{d}v_1/\text{d}y|_{y=0}$ of $K^{\pm}$ at $p_\text{T} < 0.4$ GeV/$c$ are not measured for 3.0 and 3.2 GeV due to the limited acceptance.