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.
Highlights
Single spin asymmetries (SSAs) are an important tool to understand the spin and tomographic structures of the polarized proton [1,2,3,4]
This difference is due to the additional t-channel gluon exchanges in the qg contribution, dominating the high energy pp cross section, while such analogous contributions are absent in the qγ contribution to the ultraperipheral pA collisions (UPCs) cross section
We have studied single spin asymmetries in ultraperipheral p↑A collisions
Summary
Single spin asymmetries (SSAs) are an important tool to understand the spin and tomographic structures of the polarized proton [1,2,3,4]. Inclusive hadron production in p↑p collisions, where p↑ denotes the transversely polarized proton, is one of the simplest ways to study SSA. We instead use the collinear twist-3 framework and calculate the polarized and the unpolarized cross section for p↑ðp; SÞA → hðPhÞX production, with the nuclei described by an equivalent photon flux. In this setup, the process essentially becomes a pγ collision, similar to deep inelastic scattering (DIS) in the photoproduction limit. PffiWffi e consider inclusive pion production, p↑A → πX, at s 1⁄4 200 GeV and numerically calculate the asymmetry assuming that the forward region is dominated by the twist-3 fragmentation contribution.
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