Flavour stability of the chiral vacuum and scalar meson dynamics

Previous work relating the flavour variation of the chiral order parameters F_π, langleūurangle and S-wave scattering data, based on chiral sum rules and chiral perturbation theory at order p⁴, is extended to include O(p⁶) corrections. The finding of a significant decrease of these order parameters, particularly langleūurangle, with the number of flavours increasing from N_F = 2 to N_F = 3 is confirmed, modulo an assumption on the convergence of the chiral expansion. The connection between scalar resonance physics and the phase structure of the chiral vacuum is also illustrated on the basis of the linear sigma-model. We allow for a very general symmetry breaking sector compatible with softness. The result depends strongly on the input scalar meson masses, in particular, on the presence, or not, of a light sigma.