Directed flow from parton spin-orbit coupling in $pp$ and $pA$ collisions

TL;DR

This paper proposes a new mechanism for directed flow in unpolarized proton-proton and proton-nucleus collisions, linked to gluon spin-orbit coupling, offering a novel probe into proton structure.

Contribution

It introduces a novel mechanism for azimuthal correlations based on gluon spin-orbit coupling, connecting flow phenomena with proton spin structure at small-x.

Findings

Identifies a new directed flow mechanism in $pp$ and $pA$ collisions.

Links azimuthal correlations to gluon spin-orbit coupling.

Suggests a probe for double helicity parton distributions.

Abstract

We point out a novel mechanism to generate $\cos \phi$ two-particle azimuthal correlation (`directed flow') in unpolarized proton-proton and proton-nucleus collisions in the forward rapidity region of the projectile proton. This is a direct consequence of the recently discovered strong spin-orbit coupling in gluons at small-$x$. The observable simultaneously serves as a unique probe into the double helicity parton distribution functions of the proton.