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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 nextto-leading-order 1 Nc 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 qq and F π are calculated, and are shown to be substantial, at the level of ∼ 30%, consistent with the 1 Nc expansion. The leading nonanalytic terms in the chiral expansion of qq , F π and m π have the same form as the one-loop results of chiral perturbation theory.

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