The nature of the lightest scalar meson, its Nc behaviour and semi-local duality

One-loop unitarized Chiral Perturbation Theory (UChPT) calculations, suggest a different Nc behaviour for the sigma or f0(600) and rho(770) mesons: while the rho meson becomes narrower with Nc, as expected for a qqbar meson, the f0(600) contribution to the total cross section below 1 GeV becomes less and less important. Here we review our recent work where we have shown, by means of finite energy sum rules, that a different Nc behaviour for these resonances may lead to a conflict with semi-local duality for large Nc, since local duality requires a cancellation between the f0(600) and rho(770) amplitudes. However, UChPT calculations also suggest a subdominant qqbar component for the f0(600) with a mass above 1 GeV and this can restore semi-local duality, as we show.
Comment: Invited talk at the Hadron2011: XIV International Conference on Hadron Spectroscopy. TU Munich, 13-17 June 2011