QCD Sum Rules and 1/$N_c$ expansion: On the low energy dominance and   separation of scattering backgrounds
  In multiquark correlator analyses with 1/Nc classifications, it is possible to separate the scattering background and to justify the factorization of condensates, which allows us to achieve an isolated peak saturation in the QCD sum rules for multiquark currents. Then we can extract leading Nc properties of the ground state. An application to the sigma meson is demonstrated. 
Toru Kojo and Daisuke Jido