Scalar mesons in QCD and tests of the gluon content of the sigma

Abstract

We summarize the different features of the scalar mesons from QCD spectral sum rule analyses of the two- and three-point functions. The results do not favour the u-bar u + d-bar d interpretation of the broad and low mass sigma (0.6), and the u-bar s resonance nature of the eventually observed kappa(0.9) meson. We also discuss some OZI-violating and classic semileptonic and radiative decay processes which can reveal in a model-independent way the eventual gluon component sigma_B of the sigma. In a meson-gluonium mixing scenario, one also expects an observation of the K K-bar final states from the sigma_B which may compete (if phase space allowed) with the one from a low mass s-bar s state assumed in the literature to be the SU(3) partner of the $\sigma (0.6)$ if this latter is a u-bar u + d-bar d state