Lattice study of K pi scattering in I = 3/2 and 1/2

We report the first lattice QCD results of the scattering amplitudes of the $K\pi$ system for $I = 1/2$ channel together with $I=3/2$ case. We investigate all quark diagrams contributing to these iso-spin states, and find that the scattering amplitudes are expressed as combinations of only three diagrams after setting the masses of $u$-quark and $d$-quark to be the same. The lattice simulations are performed in the quenched approximation at $\beta = 2.23$ on a 12$^3 \times 24$ lattice with an improved Iwasaki gauge action. We employ a new dilution-type noise method to get accuracy of data with reasonable CPU time. A simple method is proposed and applied to eliminate lattice artefact due to the finite extent of lattice along the time direction. A clear difference in the quark mass dependence between $I=3/2$ and $I=1/2$ channels is observed. Although the chiral extrapolation is subtle, we assume $E_{K\pi}^2 \propto m_{u,d}^2$, and obtain the $S$-wave scattering lengths as $a_0(I=3/2) m_{\pi} =-0.084^{+0.051}_{-0.064}$ and $a_0(I=1/2) m_{\pi} = -0.625 \pm0.012$. We show all necessary formulas which make the calculation possible. We argue that $\Lambda N$ is the most appropriate target of the L\"uscher's formula for baryonic system because it has no $\pi$ exchange diagrams and has a scattering length suitable for a lattice QCD simulation.