Calculation of the Four-Quark Condensates in the QCD Motivated   Nambu-Jona- Lasinio Model
  The four-quark condensates appearing in QCD sum rule are calculated nonperturbatively in a realistic QCD motivated extended Nambu-Jona-Lasinio model. The calculation is in the framework of the effective potential for local composite operators up to next-to-the-leading order in the 1/N expansion in which the non-factorized parts of the condensates are included. We show in this paper the possibility of explaining the phenomenological enhancement factor $~\kappa=2.4~$, needed for fitting the data in the factorization approximation, as the contribution of the non-factorized parts of the four- quark condensates. 
Qing Wang, Yu-Ping Kuang, Yu-Ping yi, and Tian-Fang Cai