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

TL;DR

This paper reviews the controversies surrounding the light sigma-particle, arguing against the universality interpretation and proposing a revised classification based on a simple field theoretical model.

Contribution

It challenges the conventional universality argument for the sigma-particle and suggests a new classification based on independent production process analysis.

Findings

The universality argument is not correct for sigma production.

Production processes have their own value independent of scattering.

Proposal to update the sigma classification in PDG from f_0(400-1200) to sigma(400-800).

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.