Ultra-relativistic nuclear collisions: where the spectators flow?

In high energy heavy ion collisions, the directed flow of particles is conventionally measured with respect to that of the projectile spectators, which is defined as positive $x$ direction. But it is not known if the spectators deflect in the outward direction or inward -- toward the center line of the collision. In this Letter we discuss how the measurements of the directed flow at mid-rapidity, especially in asymmetric collision such as Cu+Au, can be used to answer this question. We show that the existing data strongly favor the case that the spectators, in the ultrarelativistic collisions, on average deflect outwards.