Transverse single-spin asymmetries in p↑p → γX from quark-gluon-quark correlations in the proton

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 nonperturbative 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.