Directed Flow of Identified Particles in $\mathrm{Au}+\mathrm{Au}$ Collisions at $\sqrt{S_{NN}}=200\mathrm{GeV}$ at RHIC

STAR’s measurements of directed flow ($v_1$) around midrapidity for ${\pi }^{\pm }$, $K^{\pm }$, $K_S^0$, $p$, and $\overline{p}$ in $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$ are presented. A negative $v_1(y)$ slope is observed for most of produced particles (${\pi }^{\pm }$, $K^{\pm }$, $K_S^0$, and $\overline{p}$). In 5%–30% central collisions, a sizable difference is present between the $v_1(y)$ slope of protons and antiprotons, with the former being consistent with zero within errors. The $v_1$ excitation function is presented. Comparisons to model calculations (RQMD, UrQMD, AMPT, QGSM with parton recombination, and a hydrodynamics model with a tilted source) are made. For those models which have calculations of $v_1$ for both pions and protons, none of them can describe $v_1(y)$ for pions and protons simultaneously. The hydrodynamics model with a tilted source as currently implemented cannot explain the centrality dependence of the difference between the $v_1(y)$ slopes of protons and antiprotons.

Figure 1

$v_1$ for ${\pi }^{\pm }$, $K^{\pm }$, $K_S^0$ (left panel), $p$ and $\overline{p}$ (right panel) as a function of rapidity for 10%–70% $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{{\mathrm{s}}_{NN}}=200\mathrm{GeV}$. The lines present the linear fit to the ${\pi }^{\pm }$, $K^{\pm }$, $K_S^0$, $p$, and $\overline{p}$’s $v_1(y)$, respectively. Data points around $y=0.29$ are slightly shifted horizontally to avoid overlapping.

Figure 2

Model calculations of pion (left panel) and proton (right panel) $v_1(y)$ for 10%–70% $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{{\mathrm{s}}_{NN}}=200\mathrm{GeV}$. The QGSM* model presents the basic quark-gluon string model with parton recombination [30]. The hydro* model presents the hydrodynamic expansion from a tilted source [11].

Figure 3

Charged pions (solid stars), protons (solid circles), and antiprotons (solid squares) $v_1(y)$ slope ($d{v}_1/dy$) at midrapidity as a function of centrality for $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{{\mathrm{s}}_{NN}}=200\mathrm{GeV}$.

Figure 4

Proton $v_1(y^{\prime })$ slope ($d{v}_1/d{y}^{\prime }$) at midrapidity as a function of center of mass collision energy, where $y^{\prime }=y/y_{\mathrm{beam}}$.