Meson loops in the Nambu-Jona-Lasinio model

We calculate the effects of meson loops in the vacuum sector of the Nambu-Jona-Lasinio model. 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 $\langle \overline{q}q \rangle$ and $F_\pi$ are calculated, and are shown to be substantial, at the level of $\sim 30\%$, consistent with the $1/N_c$ expansion.