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

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.

Comments: 16 pages, 6 figures; typos corrected, some more discussion/explanation added, a few references added/updated, version to appear in PRD