Single spin asymmetry in forward $pA$ collisions II: Fragmentation contribution

TL;DR

This paper calculates the twist-three fragmentation contribution to the single spin asymmetry in forward proton-nucleus collisions, incorporating gluon saturation effects, to better understand the underlying mechanisms of SSA dependence on atomic mass number.

Contribution

It provides the first comprehensive calculation of the twist-three fragmentation contribution including gluon saturation effects, completing the analysis of SSA in the hybrid formalism.

Findings

Fragmentation contribution significantly affects SSA in forward $pA$ collisions.

SSA dependence on atomic mass number can distinguish between different twist-three mechanisms.

Results help clarify the role of gluon saturation in spin asymmetries.

Abstract

We compute the twist-three fragmentation contribution to the transverse single spin asymmetry (SSA) in light hadron production $p^\uparrow p\to hX$ and $p^\uparrow A\to hX$ including the gluon saturation effect in the unpolarized nucleon/nucleus. Together with the results in our previous paper, this completes the full evaluation of the SSA in this process in the "hybrid" formalism. We argue that the dependence of SSAs on the atomic mass number in the forward region can elucidate the relative importance of the soft gluon pole contribution from the twist-three quark-gluon-quark correlation in the polarized nucleon and the twist-three fragmentation contribution from the final state hadron.