Novel Rapidity Dependence of Directed Flow in High-Energy Heavy-Ion Collisions

For high-energy nucleus-nucleus collisions, we show that a combination of space-momentum correlations characteristic of radial expansion, together with the correlation between the position of a nucleon in the nucleus and its stopping, results in a very specific rapidity dependence of directed flow: a reversal of sign in the midrapidity region. We support our argument by RQMD model calculations for $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s}=200A\mathrm{GeV}$.