Low-lying Λ baryons with spin 1/2 in two-flavor lattice QCD

Low-lying Lambda baryons with spin 1/2 are analyzed in full (unquenched) lattice QCD. We construct 2 x 2 cross correlators from flavor SU(3) octet and singlet baryon operators, and diagonalize them so as to extract information of two low-lying states for each parity. The two-flavor CP-PACS gauge configurations are used, which are generated in the renormalization-group improved gauge action and the O(a)-improved quark action. Three different beta's, beta = 1.80, 1.95, and 2.10, are employed, whose corresponding lattice spacings are a = 0.2150, 0.1555, and 0.1076 fm. For each cutoff, we use four hopping parameters, (kappa(val), kappa(sea)), which correspond to the pion masses ranging about from 500 MeV to 1.1 GeV. Results indicate that there are two negative-parity Lambda states nearly degenerate at around 1.6 GeV, while no state as low as Lambda(1405) is observed. By decomposing the flavor components of each state, we find that the lowest (1st-excited) negative-parity state is dominated by flavor-singlet (flavor-octet) component. We also discuss meson-baryon components of each state, which has drawn considerable attention in the context of multiquark pictures of Lambda(1405).