Effective stable particle behavior of the

We investigate the reason behind the fact that the isobar effectively behaves as a stable particle. We first obtain model-independent conditions for this behavior that depend only on the imaginary part of the delta propagator. We then consider a toy model in which the N system is decoupled from the NN system and check that the approximate behavior of the as a stable particle is not in effect controlled by the coupling to NN channels nor due to the narrow width of the resonance. Finally, we describe the N amplitude in the isospin T=2 channels within a meson-exchange model. Experimental N T=2 phase shifts would be welcome.