Single transverse spin asymmetries

This is the authors' abstract. We don't add key points for this paper.

Using generalized factorization theorems, we calculate the leading single transverse spin asymmetry for high transverse-momentum direct-photon production in pp collisions, in terms of partonic matrix elements. The leading contribution comes from a ``twist-3'' parton distribution, involving the correlation between quark fields and the gluonic field strength. With simple assumptions for this matrix element, the asymmetry increases with xF, naturally giving effects of 10% or more at large xF.