Ultrarelativistic nuclear collisions: Direction of spectator 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 or inward directions—outward or toward the center line of the collision. In this Communication we discuss how the measurements of the directed flow at midrapidity, especially in asymmetric collision such as $\mathrm{Cu}+\mathrm{Au}$, 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 outward.

Figure 1

Schematic view of the collision. Arrows indicate the direction of the spectator flow, outward from the center line.

Figure 2

$〈cos\left({\mathrm{\Psi }}_{13}\right)〉$ as function of the difference in number of target and projectile nucleon participants in $\mathrm{Pb}+\mathrm{Pb}$ collision in the impact parameter range $2<b<3\mathrm{fm}$.

Figure 3

$〈cos({\mathrm{\Psi }}_{13}-{\mathrm{\Psi }}_{sp})〉$ and $〈cos\left({\mathrm{\Psi }}_{13}\right)〉$ as function of the impact parameter for $\mathrm{Pb}+\mathrm{Pb}$ collisions at $\sqrt{s_{{}_{\mathrm{NN}}}}=2.76\mathrm{TeV}$ (open markers) and $\mathrm{Cu}+\mathrm{Au}$ collisions at 200 GeV (filled markers). In $\mathrm{Cu}+\mathrm{Au}$ collisions the Au nucleus is defined as the projectile; ${\mathrm{\Psi }}_{sp}$ is calculated using Au spectators.

Figure 4

Participant distribution in $\mathrm{Cu}+\mathrm{Au}$ collisions in the impact parameter range $2<b<3\mathrm{fm}$. Positive $x$ direction is toward the Au nucleus.