Transverse single-spin asymmetries in $p^\uparrow p \to \gamma X$ from quark-gluon-quark correlations in the proton

TL;DR

This paper calculates the transverse single-spin asymmetry in direct photon production from proton-proton collisions, emphasizing quark-gluon-quark correlations, to aid in understanding the Qiu-Sterman function and the Sivers function's process dependence.

Contribution

It provides a comprehensive twist-3 calculation of $A_N^ ext{gamma}$ including all relevant pole contributions, estimating its size and uncertainties for RHIC experiments.

Findings

$A_N^ ext{gamma}$ can be used to extract the Qiu-Sterman function.

The study suggests a potential resolution to the sign mismatch crisis.

It links the Sivers function from SIDIS to proton-proton collisions.

Abstract

We analyze the transverse single-spin asymmetry in direct photon production from proton-proton collisions, denoted $A_N^\gamma$, within collinear twist-3 factorization. We provide a calculation of the contribution due to quark-gluon-quark correlations in the unpolarized proton as well as summarize previous studies on those effects in the polarized proton. Both soft-gluon poles and soft-fermion poles are considered. From this complete result we then estimate $A_N^\gamma$, including error bands due to uncertainties in the non-perturbative inputs, at kinematics relevant for planned measurements of this observable at the Relativistic Heavy Ion Collider. We find $A_N^\gamma$ can allow for a "clean" extraction of the Qiu-Sterman function, which could lead to a definitive solution to the so-called "sign mismatch" crisis. Since we use the Sivers function extracted from semi-inclusive deep-inelastic scattering to develop our input for the Qiu-Sterman function, this reaction can also make a statement about the process dependence of the Sivers function.