Ｉ Want to analyze the decay from a excited nucleon to a nucleon and an Omega meson.I have seen a propagator which descripes a process : a→b+c (Eur. Phys. J. A16, 537–547 (2003), Covariant tensor formalism for partial-wave analyses ofψdecay to mesons ,B.S. Zou and D.V. Bugg) But how could it be obtained? Is it general and universal? Or ,how to construct a vertex of the process phenominologically?