Lightest scalars as chiral partners of the Nambu--Goldstone bosons

Abstract

We review the spectrum of lightest scalar resonances recently determined using dispersion techniques. The conceptual difference between the pole mass and the bare mass (or the line--shape mass) is stressed. The nature of the lightest scalars are discussed and we argue, without relying on any model details, that the $\sigma (500)$, $\kappa (700)$, $a_0(980)$ and $f_0(980)$ may be understood as chiral partners of the Nambu--Goldstone bosons in the linear realization of chiral symmetry. But there remains some difficulties in understanding the role of $f_0(980)$ in this picture.

Comments: Invited talk given by Zheng at QCD06, Montpellier, France, July 3--7, 2006