Interaction of massless minimally coupled scalar field with spinor field in de Sitter universe

The interaction of massless minimally coupled scalar field and spinor field $ \left( \Psi + \Phi_m \rightarrow \Psi + \Phi_m \right) $ is investigated and the behavior of interaction and scattering matrix in Minkowski limit is studied. It is seen that the massless minimally coupled scalar field operator, the interaction Lagrangian and the scattering matrix become zero. Also the incoming and outgoing spinor fields, which in de Sitter space-time are different and are interacted with massless minimally coupled scalar field, in null curvature limit are the same and no interaction seen. In other word, the massless minimally coupled scalar field is dependent on curvature and can be considered as a part of a gravitational field.
Comment: 7 pages, 1 figure