Unstable particles, gauge invariance and the Delta++ resonance parameters
Abstract
The elastic and radiative pion(+)-proton scattering are studied in the framework of an effective Lagrangian model for the Delta++ resonance and its interactions. The finite width effects of this spin-3/2 resonance are introduced in the scattering amplitudes through a complex mass scheme to respect electromagnetic gauge invariance. The resonant pole (Delta++) and background contributions (rho0, sigma, Delta and neutron states) are separated according to the principles of the analytic S-matrix theory. The mass and width parameters of the ++ obtained from a fit to experimental data on the total cross section are in agreement with the results of a model-independent analysis based on the analytic S-matrix approach. The magnetic dipole moment determined from the radiative pion-proton scattering is muDelta++=(6.14 +/- 0.51) nuclear magnetons.
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