On the Quark and Gluon Substructure of the σ and Other Scalar Mesons(§4. Meson Spectroscopy and Quark Model,Possible Existence of the σ-Meson and Its Implications to Hadron Physics)

Abstract

Within the increasing experimental evidence of the existence of the I = 0 scalar low mass and wide σ meson, we review the first analysis of its quark and gluon substructure from QCD spectral sum rules and some low-energy theorems. The present data favour equal components of u^^-u + d^^-d and of gg in its wave function, which make the wide σ and the narrow f_0(980) as η'-like particles. A coherent picture of the other I = 0 scalar mesons spectra within this mixing scheme is shortly discussed. We also expect the a_0(980) to be the lowest isovector u^^-d state, and the K^*_0 (1430) its d^^-s partner.