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The chiral limit of the rho and sigma masses and widths is discussed. We workwithin the inverse amplitude method to one loop in SU(2) ChPT and analyze theconsequences that all chiral logarithms cancel out in the rho-channel, whilethey do not cancel for the sigma case, and how they strongly influence theproperties of this latter resonance. Our results confirm and explain thedifferent behavior of the sigma and rho poles for NC not far from 3, but weextend the analysis to very large NC, where the behavior of these tworesonances is re-analyzed. We note that the rather natural requirement ofconsistency between resonance saturation and unitarization imposes usefulconstraints. By looking only at the rho-channel, and within the singleresonance approximation, we find that the masses of the first vector and scalarmeson nonets, invoked in the single resonance approximation, turn out to bedegenerated in the large NC limit. On the contrary we show that, forsufficiently large NC, the scalar meson evolution lies beyond the applicabilityreach of the one-loop inverse amplitude method and if the scalar channel isalso incorporated in the analysis, it may lead, in some cases, tophenomenologically inconsistent results.

Properties of the rho and sigma Mesons from Unitary Chiral Dynamics