Transverse single-spin asymmetries inp↑p→γXfrom quark-gluon-quark correlations in the proton

Abstract

We analyze the transverse single-spin asymmetry in direct photon production from proton-proton collisions, denoted ${A}_{N}^{\ensuremath{\gamma}}$, 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 ${A}_{N}^{\ensuremath{\gamma}}$, 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 ${A}_{N}^{\ensuremath{\gamma}}$ 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.