Resonances and weak interactions in $D^+ topi^+ pi^- pi^+$ decays

We describe the $pipi$ $S$-wave in $D^+ topi^+ pi^- pi^+$ decays using a unitary model for the $pipi$ Final State Interactions (FSI). The three body decay is treated as a quasi two-body process where, at the weak vertex, the D meson decays into a resonance and a pion. The weak part of the decay amplitude is evaluated using the effective weak Hamiltonian within the factorization approximation.