Controversies on and a Reasoning for Existence of the light sigma-particle

Abstract

The light sigma-particle is, regardless of the strong criticism, reviving recently due to the works done from various sides. I review essential points of the controversies (especially related to our works) and of their answer: Conventionally a large concentration of the iso-scalar S-wave 2 pi events below 1 GeV (being, correctly, due to the sigma-production), which is observed in most of production processes, is interpreted as a mere background from the viewpoint of, so called, universality argument. However, I show, by resorting to a simple field theoretical model, that the argument is not correct and the production process has ``its own value'' independent of the scattering process. Thus it is suggested that the present index ``f_0(400-1200) or sigma'' in PDG'98 is to be changed as ``sigma(400-800)'' in the PDG 2000.