Lattice QCD study of four-quark components of the isosinglet scalar   mesons: Significance of disconnected diagrams
  We study the possible significance of four-quark states in the iso-singlet scalar mesons ($J^{PC}=0^{++}$, $I=0$) by performing two-flavor full lattice QCD simulations on an $8^3 \times 16$ lattice using the improved gauge action and the clover-improved Wilson quark action. In particular, we evaluate the propagators of molecular and tetra-quark operators together with singly disconnected diagrams. In the computation of the singly disconnected diagrams we employ the $Z_2$-noise method with the truncated eigenmode approach. We show that the quark loops given by the disconnected diagrams play an essential role in propagators of tetraquark and molecular operators. 
Masayuki Wakayama, Teiji Kunihiro, Shin Muroya, Atsushi Nakamura,   Chiho Nonaka, Motoo Sekiguchi and Hiroaki Wada
hep-lat