Soft-fermion-pole contribution to single-spin asymmetry for pion production in pp collisions

We study the single transverse spin asymmetry for the inclusive pion production in the nucleon-nucleon collision, p^\uparrow p\to\pi X, based on the twist-3 mechanism in the collinear factorization. We derive the soft-fermion-pole (SFP) contribution to the twist-3 single-spin-dependent cross section associated with the twist-3 quark-gluon correlation functions in the polarized nucleon. We find that the SFP can give rise to a large effect to the asymmetry A_N owing to the large partonic hard cross sections with the large color factor, if the SFP function has a similar magnitude as the soft-gluon-pole (SGP) function, in spite of the absence of the "derivative term" for the SFP function unlike for the SGP function.