Exotic Tetraquark ud bar[s] bar[s] of J^P=0^+ in the QCD Sum Rule
  We study a QCD sum rule analysis for an exotic tetraquark ud bar[s] bar[s] of J^P=0^+ and I = 1. We construct q q bar[q] bar[q] currents in a local product form and find that there are five independent currents for this channel. Due to high dimensional nature of the current, it is not easy to form a good sum rule when using a single current. This means that we do not find any sum rule window to extract reliable results, due to the insufficient convergence of the OPE and to the exceptional important role of QCD continuum. Then we examine sum rules by using currents of linear combinations of two currents among the independent ones. We find two reasonable cases that predict a mass of the tetraquark around 1.5 GeV. 
Hua-Xing Chen, Atsushi Hosaka, Shi-Lin Zhu