Factorization breaking of four-quark condensates in the Nambu Jona   Lasinio model
  We investigate the factorization hypothesis of the four-quark condensate $\langle q \bar{q} q \bar{q} \rangle = \, A \, \langle q \bar{q} \rangle^2$ with the help of the Nambu Jona-Lasinio Model supplemented with eighth order interactions. For that purpose we use the bosonization method with multiple auxiliary variables. We find that in a simplified U(1) version of the model factorization holds, whereas in the full SU(3)-flavor version of the model factorization is broken by terms which are related to the 't Hooft interactions. 
Fabio L. Braghin and Fernando S. Navarra
hep-ph