Measurement of kaon directed flow in Au+Au collisions in the high baryon density region

Abstract

Rapidity-odd directed flow v₁ measurements are presented for K^± and $K_S^0$ in Au + Au collisions for $\sqrt{s_{NN}}$ from 3.0 to 3.9 GeV with the STAR experiment. For comparison, v₁ of π^±, protons, and Λ from the same collisions are also discussed. The mid-rapidity v₁ slope $\text{d}{v}_1{/\text{d}y|}_{y=0}$ for protons and Λ is positive in these collisions. On the other hand, v₁ 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 v₁ 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 v₁ 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.