Directed flow in ultrarelativistic heavy-ion collisions

We study the generation of the directed flow in the hydrodynamic expansion of hot matter formed in ultrarelativistic heavy-ion collisions at root s = 200 GeV. The experimentally observed negative-directed flow in a wide range of central pseudorapidities is reproduced, 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.