Analysis the $0^{++}$ nonet mesons as four-quark states with the QCD sum   rules
  In this article, we take the point of view that the $0^{++}$ nonet mesons below $1 GeV$ are diquark-antidiquark states $(qq)_{\bar{3}}(\bar{q}\bar{q})_3$, and devote to determine their masses in the framework of the QCD sum rules approach with the interpolating currents constructed from scalar-scalar type and pseudoscalar-pseudoscalar type diquark pairs respectively. The numerical results indicate that the $0^{++}$ nonet mesons may have two possible diquark-antidiquark substructures. 
Zhi-Gang Wang, Wei-Min Yang, Shao-Long Wan