Vertex operators for production of baryon resonances in nucleon-gamma   collisions with preserving gauge invariance and analyticity
  Vertex operators for photo- and electro-production of baryon states with arbitrary spin-parity, $ \gamma + N\to B(J^P)$, are constructed. The operators obey gauge invariance and analyticity constraints. Analyticity is realized as a requirement of the generalized Siegert theorem for vertex form factors. 
A.V. Anisovich, V.V. Anisovich, A.V. Sarantsev
hep-ph