Scalar Tetraquark Currents With Application to the QCD Sum Rule
  We study the light scalar mesons in the QCD sum rule. We construct both the diquark-antidiquark currents $(qq)(\bar q \bar q)$ and the meson-meson currents $(\bar qq)(\bar qq)$. We find that there are five independent currents for both cases, and derive the relations between them. For the meson-meson currents, five independent currents are formed by products of color singlet $\bar q q$ pairs or color octet pairs. However, they can be related to each other, and the relations are derived. We obtain the masses of the light scalar mesons which are consistent with the experiments. 
Hua-Xing Chen, Atsushi Hosaka, and Shi-Lin Zhu