Quark-gluon correlation functions relevant to single transverse spin asymmetries

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 $T_{q,F}(x,x)$ that is responsible for the gluonic pole contribution to the SSAs, as well as $T_{q,F}(0,x)$ and $T_{\mathrm{\Delta }q,F}(0,x)$ 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 $\text{tq}(x,x)$ 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 $\text{tq}(x,x)$. We also discuss the implication of our finding to the phenomenological studies of the SSAs.