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 mid-rapidity region. We support our argument by RQMD model calculations for Au+Au collisions at $\sqrt s$ = 200 AGeV.