Meson loops in the Nambu-Jona-Lasinio model

The effects of meson loops in the vacuum sector of the Nambu-Jona-Lasinio model are calculated. Using the effective action formalism we take consistently all next-to-leading-order 1/N-c terms into account. This leads to a symmetry-conserving approach, in which all features of spontaneously broken chiral symmetry, such as the Goldstone theorem, the Goldberger-Treiman and the Gell-Mann-Oakes-Renner relations are preserved. Contributions to [(q) over bar q] and F-pi are calculated, and are shown to be substantial, at the level of similar to 30%, consistent with the 1/N-c expansion. The leading non-analytic terms in the chiral expansion of [(p) over bar q], F-pi and m(pi) have the same form as the one-loop results of chiral perturbation theory.