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

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