Existence of sigma(600)/kappa(900) and Possible Classification of Chiral Scalar Nonet

Recently, by reanalyzing the phase shift data of I=0 pipi$ and of I=1/2 Kpi scatterings, we showed the evidences for existence of light scalar mesons, sigma(600) and kappa(900), respectively, which had been sought but missing for a long time. The sigma(600) and kappa(900) together with the established resonances, a_0(980) and f_0(980), are shown to be consistently classified as the members of a single scalar sigma-nonet, appearing in the SU(3) linear sigma model. Especially the mass value of the iso-singlet flavor-octet member satisfies, together with those of kappa(900) and a_0(980), the Gell-Mann Okubo mass formula. The repulsive background phase shift delta_{BG}, which was essential to lead the sigma/kappa-existence in our phase shift analyses, is also shown to be quantitatively describable in the framework of linear sigma model. Thus, the origin of delta_{BG} is reduced to the ``compensating lambda phi^4-interaction," necessary from the viewpoint of chiral symmetry.