We study the generation of directed flow in the hydrodynamic expansion of the
hot matter formed in ultrarelativistic heavy-ion collisions at 200GeV. The
experimentally observed negative directed flow in a wide range of central
pseudorapidities isreproduced assuming that the fireball is tilted away from
the collision axis. The tilt of the source is consistent with a preferential
emission in the forward/backward hemisphere from forward/backward participating
nucleons. The model reproduces the experimentally observed scaling of the
directed flow when going from Au-Au to Cu-Cu systems.Comment: figure adde
