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 $\pi^{\pm}$, 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.