Pentaquark baryon in anisotropic lattice QCD

The pentaquark (5Q) baryon is studied in anisotropic quenched lattice QCD with renormalized anisotropy a(s)/a(t)=4 for a high-precision mass measurement. The standard Wilson action at beta=5.75 and the O(a) improved Wilson quark action with kappa=0.1210(0.0010)0.1240 are employed on a 12(3)x96 lattice. Contribution of excited states is suppressed by using a smeared source. We investigate both the positive- and negative-parity 5Q baryons with I=0 and spin J=1/2 using a non-NK-type interpolating field. After chiral extrapolation, the lowest positive-parity state is found to have a mass, m(5Q)=2.25 GeV, which is much heavier than the experimentally observed Theta(+)(1540). The lowest negative-parity 5Q state appears at m(5Q)=1.75 GeV, which is near the s-wave NK threshold. To distinguish spatially-localized 5Q resonances from NK-scattering states, we propose a new general method imposing a "Hybrid Boundary Condition (HBC)," where the NK threshold is artificially raised without affecting compact five-quark states. The study using the HBC method shows that the negative-parity state observed on the lattice is not a compact 5Q state but an s-wave NK-scattering state.