We explore whether ''f 0 (600) or '' exists as a pole in QCD, by a full lattice QCD simulation on the 8 3 ϫ16 lattice using the plaquette action and Wilson fermions. It is shown that there exists a pole, whose mass is as low as that of the , owing to the disconnected diagram of the meson propagator. A discussion is given on the physical content of the .

Using lattice QCD simulation in the quenched approximation, we study the κ meson, which is 3 P0 in the quark model, and compare experimental and other lattice data. The κ is the lowest scalar meson with strangeness and constitutes the scalar nonet. The obtained mass is much higher than the recent experimental value, and therefore the κ(800) is difficult to consider as a simple two-body constituent-quark structure, and may have another unconventional structure.

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We study the possible significance of four-quark states in the iso-singlet scalar mesons (J P C = 0 ++ , I = 0) by performing two-flavor full lattice QCD simulations on an 8 3 × 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.

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