Quark distribution functions in a diquark spectator model

Quark distribution functions appear in the field-theoretical description of hard scattering processes as the parts that connect the quark and gluon lines to hadrons in the initial state. These parts are defined through (connected) matrix elements of nonlocal operators built from quark and gluon fields. The simplest, but most important ones, are the quark-quark correlation functions [1–3]. For each quark flavor one can write down