We investigate the mass spectrum of the pentaquark baryon ($udud\overline{s}$) in quenched lattice QCD with exact chiral symmetry. Using three different interpolating operators for $\Theta$, we measure the $3\times 3$ correlation matrix and obtain the eigenvalues $A^{\pm }(t)$ with $\pm$ parity for 100 gauge configurations generated with Wilson gauge action at $\beta =6.1$ on the ${20}^3\times 40$ lattice. For the lowest-lying $J^P=1/2^{-}$ state, its mass is almost identical to that of the $KN$ $s$ wave, while for the lowest-lying $J^P=1/2^{+}$ state, its mass is smaller than that of the $KN$ $p$ wave, especially for the regime $m_u<m_s$. By chiral extrapolation (linear in $m_{\pi }^2$) to $m_{\pi }=135\mathrm{MeV}$, we obtain the masses of the lowest-lying states: $m(1/2^{-})=1424(57)\mathrm{MeV}$ and $m(1/2^{+})=1562(121)\mathrm{MeV}$, in agreement with the masses of $m_K+m_N\simeq 1430\mathrm{MeV}$ and ${\Theta }^{+}(1540)$, respectively.

• Received 24 January 2005

DOI:https://doi.org/10.1103/PhysRevD.72.034505