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We consider a two-flavor Nambu \& Jona-Lasinio model in Hartree approximationinvolving scalar-isoscalar and pseudoscalar-isovector quark-quark interactions.Average meson fields are defined by minimizing the effective Euklidean action.The fermionic part of the action, which contains the full Dirac sea, isregularized within Schwinger's proper-time scheme. The meson fields arerestricted to the chiral circle and to hedgehog configurations. The onlyparameter of the model is the constituent quark mass $M$ which simultaneouslycontrols the regularization. We evaluate meson and quark fieldsself-consistently in dependence on the constituent quark mass. It is shown thatthe self-consistent fields do practically not depend on the constituent quarkmass. This allows us to define a properly parameterized reference field whichfor physically relevant constituent masses can be used as a good approximationto the exactly calculated one. The reference field is chosen to have correctbehaviour for small and large radii. To test the agreement betweenself-consistent and reference fields we calculate several observables likenucleon energy, mean square radius, axial-vector constant and delta-nucleonmass splitting in dependence on the constituent quark mass. The agreement isfound to be very well. Figures available on request.Comment: 12 pages (LATEX), 3 figures available on request, report FZR 93-1

On size and shape of the average meson fields in the semibosonized Nambu & Jona-Lasinio model