EFFECTIVE 1 N(C) EXPANSION IN THE NAMBU-JONA-LASINIO MODEL/

The 1/N(c) expansion is developed for the Nambu-Jona-Lasinio model, a commonly used low-energy model of QCD. The next-to-leading order in th is expansion that represents pion and sigma meson exchange in the gap equation is evaluated perturbatively. These contributions are seen to be differing in sign, a feature that is due to their opposite parities . Physically one can interpret this meson exchange as giving rise to s creening (pi) or antiscreening (sigma) of the bare four-point interact ion. As a consequence of the higher isospin degeneracy of the pion, th e contributions from it are found to dominate. In total, we find that the resulting corrections to the usual Hartree approximation are of th e order of 10%, justifying a posteriori the perturbative approach.