Two-particle photodisintegration of He-4: He-4(gamma,d)d,   He-4(gamma,p)T, He-4(gamma,n)He-3
  Using a covariant diagram technique and the concept of a nucleus as an elementary particle we calculated differential cross-sections for two-particle photodisintegration reactions of the He-4. The only functional parameter was the vertex structure function, which described the "collapse" of a He-4 nucleus and the nucleon remnants. The interaction of a real photon was determined by the value of particles charge, since the electromagnetic form factors were calculated in the photon point. The inseparability property of the electric charge from the particle mass allowed us to match the energy-momentum and charge conservation laws in the interaction. Therefore, the requirement of gauge symmetry was immediately satisfied. The covariant amplitude of the process equaled to the sum of pole diagrams and regular part. 
O. E. Koshchii, P. E. Kuznietsov, Yu. A. Kasatkin
nucl-th nucl-ex