We investigate the relative size of various twist-3 quark-gluon correlation
functions relevant to single transverse spin asymmetries (SSAs) in a
quark-diquark model of the nucleon. We calculate the quark-gluon correlation
function Tq,F(x,x) that is responsible for the gluonic pole contribution
to the SSAs, as well as Tq,F(0,x) and TΔq,F(0,x)
responsible for the fermionic pole contributions. We find in both cases of a
scalar diquark and an axial-vector diquark that at the first nontrivial order
only the \tq(x, x) is finite while all other quark-gluon correlation
functions vanish. Using the same model, we evaluate quark Sivers function and
discuss its relation to the \tq(x, x). We also discuss the implication of our
finding to the phenomenological studies of the SSAs.Comment: 12 pages, 5 figure