Quark correlation functions in deep-inelastic semi-inclusive processes

We investigate one-particle semi-inclusive processes in lepton-hadron scattering. In unpolarized scattering order $Q^{-1\mathrm{}}$ corrections appear only when transverse momenta are detected. We consider the twist-two and -three matrix elements and calculate the semi-inclusive structure functions in terms of quark correlation functions. We find that at the twist-three level not only the standard quark distribution and fragmentation function contribute, but also two new transverse "profile functions." We discuss the gauge invariance of the hadronic tensor at the twist-three level. The results of our approach are used to calculate expressions for some cross sections for semi-inclusive processes.