Single-transverse spin asymmetry in prompt photon production

Based on a few standard assumptions we calculate the single-spin asymmetry for the collision of a transverse polarized proton and an unpolarized proton for xF < 0 (with respect to the polarized proton). In this target fragmentation region the relevant matrix elements are three-gluon correlators. The imaginary phase, mandatory to achieve a non-zero asymmetry, comes from an on-shell inner quark line leaving a related gluon momentum soft and giving rise to different pole structures. The three-gluon correlators are finally estimated as a numerical constant times the unpolarized twist-2 gluon distribution xG(x). Using the first moment of the three-gluon correlators, we further derived a relation between the direction of the colour magnetic field inside the polarized nucleon and the nucleon spin which was also found in sum-rule calculations for the higher twist contributions to nucleon structure functions. (author)