Meson-loop effects in the NJL model at zero and nonzero temperature

Two different possibilities to include meson-loop corrections in the Nambu-Jona-Lasinio model: a strict 1/Nc expansion in next-to-leading order and a nonperturbative scheme corresponding to a one-meson-loop approximation to the effective action are compared. Both schemes are considering with chiral symmetry, in particular with the Goldstone theorem and the Gell-Mann-Oakes-Renner relation. The numerical part at zero temperature focuses on the pion and the ρ-meson sector. For the latter the meson-loop corrections are crucial in order to include the dominant ρ → ππ-decay channel, while the standard Hartree + RPA approximation only contains unphysical qq-bar-decay channels. It is found that mπ, fπ, <Ψ-barΨ>, and quantities related to the ρ-meson self-energy can be described reasonably with one parameter set in the 1/Nc-expansion scheme, whereas we did not succeed to obtain such a fit in the nonperturbative scheme. It is also investigated the temperature dependence of the quark condensate. Here it is found consistency with chiral perturbation theory to lowest order. Similarities and differences of both schemes are discussed