Lattice Study of the ExoticS=+1Baryon

We propose $S=+1$ baryon interpolating operators, which are based on an exotic description of the antidecuplet baryon, like the diquark-diquark-antiquark structure. By using one of the new operators, the mass spectrum of the spin-$1/2$ pentaquark states is calculated in quenched lattice QCD at $\beta =6/g^2=6.2$ on a ${32}^3\times 48$ lattice. It is found that the $J^P$ assignment of the lowest $\Theta (uudd\overline{s})$ state is most likely $(1/2{)}^{-}$. We also calculate the mass of the charm analog of the $\Theta$ and find that the ${\Theta }_c(uudd\overline{c})$ state lies much higher than the $DN$ threshold, in contrast to several model predictions.