The multiphonon version of the quasiparticle-phonon nuclear model: application to the problem of existence of low-lying two-phonon states in deformed nuclei

The multiphonon version of the quasiparticle-phonon nuclear model (QPNM) conventient for an analysis of an interaction between configurations with a different number of phonons is proposed. The properties of the matrix elements coupling these configurations are discussed in detail. It is shown that if violation of the Pauli principle shifts the strengths of low-lying two-phonon states towards higher excitation energies, the interaction with complex configurations (three-phonon, etc.) provides the opposite effect of the same order. As a result, the multiphonon version of the QPNM admits the existence of low-lying two-phonon states in deformed nuclei thus removing the discrepancy between the QPNM and other microscopic models. 16 refs.; 1 fig.; 2 tabs