Abstract. Photonuclear reaction research is of great interest to obtain information about the structure of nuclei. The investigation of structural effects requires certain insights into the reaction mechanisms, that have to be identified on the basis of the fundamental principles of covariance and gauge invariance. The major achievement of the chosen model is the ability to reproduce the cross-section dependence using the minimal necessary set of parameters. We analyze the two-particle disintegration of ‫݁ܪ‬ ଷ nuclei by photons. Our interest was raised by the fact that ‫݁ܪ‬ ଷ is the simplest many-particle system which admits an exact solutions. We also consider the process ‫݁ܪ‬ሺߛ, ݀ሻ݀ ସ . This process comes at the expense of the quadrupole absorption of γ-rays, while the dipole transition is suppressed. This property is a consequence of the isospin selection as well as the identity of the particles in the final state. Obtained results describe the energy range from threshold (20 MeV) to 140 MeV. Therefore, the model mentioned in the paper has the peculiarity to be valid not only for the low-energy regime, but also for higher energies. Present paper is devoted to determine the roles of different reaction mechanisms and to solve problems above.

We provide general analysis of electro-break up process of compound scalar system. We use covariant approach with conserved EM current, which gives the ability to include strong interaction into QED. Therefore, we receive the ability to describe disintegration processes on nonlocal matter fields applying standard Feynman rules of QED. Inclusion of phase exponent into wave function receives a physical sense while we deal with the dominance of strong interaction in the process. We apply Green's function (GF) formalism to describe disintegration processes. Generalized gauge invariant electro-break up process amplitude is considered. One is a sum of traditional pole series and the regular part. We explore the deposits of regular part of amplitude, and its physical sense. A transition from virtual to real photon considered in photon point limit . The general analysis for electro-break up process of component scalar system is given. Precisely conserved nuclear electromagnetic currents at arbitrary square of transited momentum are received. The only undefined quantity in theory is vertex function. Therefore, we have the opportunity to describe electron scattering processes taking into account minimal necessary set of parameters.

Using a covariant diagram technique and the concept of a nucleus as an elementary particle, we calculated the differential cross-sections for two-particle photodisintegration reactions of 4 He. The only functional parameter is the vertex structure function, which describes the "collapse" of 4 He nucleus and the nucleon remnants. The interaction of a real photon is determined by the value of particles charge, since the electromagnetic (EM) form factors are calculated at 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 is immediately satisfied. The covariant amplitude of the process equals to the sum of pole diagrams and the regular part, which is added to fulfil the EM current conservation requirement. K e y w o r d s: gauge invariance, vertex function, photodisintegration of 4 He, regular amplitude.

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