QCD sum rule approach for the light scalar mesons as four-quark states
  We study the two point-function for the scalar mesons $\sigma, \kappa, f_0(980)$ and $a_0(980)$ as diquak-antidiquark states. We also study the decays of these mesons into $\pi\pi$, $K\pi$ and $K\bar{K}$. We found that the couplings are consistent with existing experimental data, pointing in favor of the four-quark structure for the light scalar mesons. 
Thiago V. Brito, Fernando S. Navarra, Marina Nielsen and Mirian E.   Bracco