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 $K_S^0$ 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 ${\pi }^{\pm }$, protons, and $\Lambda$ 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 $\Lambda$ 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.

8 pages, 5 figures

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