Unquenched Lattice QCD study of $\Lambda$(1405)

$\Lambda$(1405) is regarded as one of the most interesting hadrons and attracting much interest from several view points. Especially, the structure of $\Lambda$(1405) remains mysterious. Whereas $\Lambda$(1405) is interpreted as a flavor-singlet three-quark state in quark models, $\Lambda$(1405) could be interpreted as a Kaon-nucleon molecule. $\Lambda$(1405) is then considered to be a bound-state of $\bar K$ and $N$ with binding energy of 30 MeV, and this large binding energy implies strong attraction between $\bar K$ and $N$. Such strong attractive interactions predict a new type of hadronic matter. The property of $\Lambda$(1405) can be therefore an important clue to new paradigm in hadron physics. Lattice QCD calculations are powerful, and expected to cast light on the nature of $\Lambda$(1405). However, few detailed lattice QCD study on $\Lambda$(1405) have been done so far. We study properties of $\Lambda$(1405) with lattice QCD. We make use of the full-QCD gauge configurations generated by CP-PACS/PACS-CS collaboration. We construct correlation matrices from several independent ``octet'' and ``singlet'' operators, and diagonalize them so that we can investigate mass spectra in (J, I, S)=(1/2, 0, -1) channel as well as possible mixing between octet and singlet states, which has not been discussed yet in the context of lattice QCD studies. As a result, we found that the lowest (2nd-lowest) state in this channel is dominated by a flavor-singlet (flavor-octet) state. We also extract the $\bar KN$ and $\pi\Sigma$ components in each observed state.