  Inspired by the model of Nambu and Jona-Lasinio, various Lagrangians are
considered for a system of interacting quarks. Employing standard techniques of
many-body theory, the scalar part of the quark self-energy is calculated
including terms up to second-order in the interaction. Results obtained for the
single-particle Green's function are compared with those which only account for
the mean-field or Hartree-Fock term in the self-energy. Depending on the
explicit form of the Lagrangian, the second-order contributions range between 4
and 90 percent of the leading Hartree-Fock term. This leads to a considerable
momentum dependence of the self-energy and the effective mass of the quarks.
