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Taking the sum rule for the rho meson as an example, we study the possibilityof explaining the phenomenological enhamcement factors for certain terms in thevacuum expectation value of the operator product expansion in the QCD sum rule.We take a QCD motivated extended Nambu-Jona-Lasinio (ENJL) model as the lowenergy effective Lagrangian for QCD with which we calculate the nonperturbativecontributions to the vacuum condensate expansion to obtain the enhancementfactors. Our result shows that such nonperturbative contributions can causelarge enough enhancement factors which can be consistent with thephenomenological values.Comment: 17 pages in RevTex with 5 figure

Nonperturbative explanation of the enhancement factors in the QCD sum rule for the ρ meson