Ultra-relativistic nuclear collisions: where the spectators flow?

TL;DR

This paper investigates the deflection direction of spectators in ultra-relativistic nuclear collisions, using directed flow measurements at mid-rapidity, and finds evidence that spectators tend to deflect outward.

Contribution

It proposes a method to determine spectator deflection direction in high-energy collisions and provides evidence favoring outward deflection in asymmetric collisions.

Findings

Spectators likely deflect outward in ultrarelativistic collisions.

Mid-rapidity flow measurements can reveal spectator deflection direction.

Existing data supports outward deflection of spectators.

Abstract

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.