The lowest resonance in QCD from low--energy data
Abstract
We show that a generalization of su(2) Chiral Perturbation Theory, including a perturbative singlet scalar field, converges faster towards the physical value of sensible low--energy observables. The physical mass and width of the scalar particle are obtained through a simultaneous analysis of the pion radius and the gamma gamma -> pi0 pi0 cross-section. Both values are statistically consistent with the ones obtained by using Roy equations in pi-pi scattering. In addition we find indications that the photon-photon-singlet coupling is quite small.
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