Re-analysis of ππ / Kπ Phase Shift and Existence of σ(600) / kappa(900)-Particle

Re-analyzing the old phase shift data of I=0, \pi\pi (I=1/2, K\pi) scattering, we show an evidence for existence of the \sigma-particle (\kappa-particle) with a comparatively light mass, which has been missing for a long time: In the analysis we have applied a new method of interfering Breit-Wigner amplitudes, which makes the scattering amplitude parametrized in terms of only physical quantities, mass and width of resonant particles, in conformity with unitarity. We have introduced phenomenologically a background phase shift \delta_{BG} of repulsive core type. The \chi^2-fits of the both \pi\pi- and K\pi- phase shifts are improved greatly with the \delta_{BG} over the conventional fits without it.