PHENOMENOLOGICAL SIGMA-MODELS

The question of the phenomenological description of the broad scalar resonances compatible with chiral symmetry and unitarity is discussed within the framework of the linear sigma model. It is pointed out that a naive inclusion of the large decay widths in the scalar meson propagators is not an adequate approximation for physical amplitudes. It is shown that the simplest unitarization scheme for the real pipi --> pipi Born amplitude with l = I = 0 leads to a broad resonance arising with a mass m(res) almost-equal-to 420 MeV and a width GAMMA(res)(m(res)2) almost-equal-to 740 MeV at the bare sigma meson mass m(sigma) almost-equal-to 1 GeV. The resonance parameters are largely controlled by the nonresonant background amplitude. Some applications to other reactions are also discussed.