STAR's measurements of directed flow ($v_1$) for pions, kaons (charged and $K_s^0$), protons and antiprotons for Au + Au collisions at $\sqrtsNN = 200$ GeV are presented. Negative $v_1(y)$ slope is observed for pions, antiprotons, protons and kaons. The proton $v_1(y)$ slope at midrapidity is found extremely small. Sizable difference is seen between $v_1$ of protons and anti-protons in 5-30% central collisions. Anti-flow can explain the negative slope, however, it has difficulties explaining the centrality dependence of the difference between the $v_1(y)$ slopes of protons and anti-protons. The $v_1$ excitation function is presented. Comparisons to model calculations (RQMD, uRQMD, AMPT and QGSM) are made, and it is found that none of the four models can successfully describe the data.
Categories: nucl-ex