Single spin asymmetries in ultra-peripheral $p^\uparrow A$ collisions

TL;DR

This paper proposes using ultra-peripheral proton-nucleus collisions to study single spin asymmetries, providing new formulas and numerical estimates that could test the role of twist-three fragmentation functions.

Contribution

It introduces a novel channel for SSA investigation in UPCs and derives simple collinear twist-three formulas for polarized cross sections.

Findings

A few percent SSA asymmetry in forward region at 200 GeV

New formulas for polarized UPC cross sections

Proposes UPCs as a test for twist-three fragmentation dominance

Abstract

We suggest inclusive hadron production in ultra-peripheral proton-nucleus collisions (UPCs) $p^\uparrow A \to h AX$ as a new channel to investigate single spin asymmetries (SSAs), in particular, to test the assumed dominance of the contribution from twist-three fragmentation functions. The UPC cross sections are obtained by considering the photoproduction limit of semi-inclusive deep inelastic scattering (SIDIS). In particular, we find simple formulas for the polarized UPC cross sections in the collinear twist-three framework. We then numerically calculate the fragmentation contribution to SSA in $p^\uparrow A \to \pi A X$ at $\sqrt{s} = 200$ GeV and find a few percent asymmetry in the forward region.