Negative parity baryons in quenched anisotropic lattice QCD

We investigate the negative-parity baryon spectra in quenched lattice QCD. We employ the anisotropic lattice with a standard Wilson gauge and $O(a)$ improved Wilson quark actions at three values of lattice spacings with a renormalized anisotropy $\ensuremath{\xi}{=a}_{\ensuremath{\sigma}}{/a}_{\ensuremath{\tau}}=4,$ where ${a}_{\ensuremath{\sigma}}$ and ${a}_{\ensuremath{\tau}}$ are spatial and temporal lattice spacings, respectively. The negative-parity baryons are measured with the parity projection. In particular, we pay much attention to the lowest SU(3) flavor-singlet negative-parity baryon, which is assigned as the $\ensuremath{\Lambda}(1405)$ in the quark model. For the flavor octet and decuplet negative-parity baryons, the calculated masses are close to the experimental values of corresponding lowest-lying negative-parity baryons. In contrast, the flavor-singlet baryon is found to be about 1.7 GeV, which is much heavier than the $\ensuremath{\Lambda}(1405).$ Therefore it is difficult to identify the $\ensuremath{\Lambda}(1405)$ to be the flavor-singlet three-quark state, which seems to support an interesting picture of the pentaquark $(udsq\overline{q})$ state or the $N\overline{K}$ molecule for the $\ensuremath{\Lambda}(1405).$