Third order anharmonic effects in many-body nuclear quantum theory

The quantum theory of third order anharmonic effects in the photon production amplitude developed by V.A. Khodel has been generalized for nuclei with pairing and for a more accurate description of low-energy collective phonons. An expression for the transition amplitude between two- and one-phonon states has been obtained. This expression has been compared to the solution of a similar problem considered within the quasiparticle-phonon model. It has been shown that this approach describes a number of new effects, including three- and four-quasiparticle correlations in the ground state (backward-going graphs).