Relativistic RPA plus phonon-coupling analysis of pygmy dipole resonances

The relativistic random-phase approximation (RRPA) plus phonon-coupling (PC) model is applied in the analysis of E1 strength distributions in 208Pb and 132Sn, for which data on pygmy dipole resonances (PDR) have recently been reported. The covariant response theory is fully consistent: the effective nuclear interaction NL3 is used both to calculate the spectrum of single-nucleon Dirac states, and as the residual interaction which determines the collective phonon states in the relativistic RPA. It is shown that the picture of the PDR as a resonant oscillation of the neutron skin against the isospin saturated proton-neutron core, and with the corresponding RRPA state characterized by a coherent superposition of many neutron particle-hole configurations, remains essentially unchanged when particle-vibration coupling is included. The effect of two-phonon admixtures is a weak fragmentation and a small shift of PDR states to lower excitation energy. Even though the PDR calculated in the extended model space of ph⊗phonon configurations contains sizeable two-phonon admixtures, it basically retains a one-phonon character and its dynamics is not modified by the coupling to low-lying surface vibrations.