Study of the Pygmy Resonance within a New Microscopic Multiphonon Approach
Nicola Lo Iudice
Universita' di Napoli Deferico II
Dipartimento di Scienze Fisiche, Universita' di Napoli Federico II,
Monte S. angelo via Cintia, 80126 Napoli Italy
We developed an equation of motion phonon method (EMPM) [1] which generates a
basis of microscopic multiphonon states built of Tamm-Dancoff phonons and uses such
a basis to solve the nuclear eigenvalue problem for a realistic Hamiltonian.
The method extends the random-phase-approximation (RPA) or higher RPA and is free
of approximations. In particular, it avoids the quasi-boson approximation and yields a
ground state correlated explicitly.
The method was developed first in the particle-hole formalism and applied to 16O,
where the important role of multiphonon states was emphasized.
More recently, it was applied to 208 Pb [2], object of several recent
investigations [3], with special attention at the E1 low-energy excitations corresponding
to the pygmy resonance. The calculation provided a satisfactory description of the lowlying dipole spectrum. The good agreement with experiments was ascribed
to the coupling to the two-phonon configurations.
We have now developed a quasiparticle version of the method and adopted this scheme
to investigate the electric dipole response in neutron rich oxygen isotopes where a
considerable
dipole strength associated to the pygmy resonance was detected [4].
The calculations were performed in a space spanned by up to three-phonon states. To
our knowledge, this is the first microscopic multiphonon calculation which diagonalizes
a realistic Hamiltonian in a space covering up to three-phonon states and includes
explicitly correlations
in the ground state.
The first results show that the two and three phonons aff ect strongly the fine structure of
the giant dipole resonance as well as the
distribution and magnitude of the strength near the neutron emission threshold.
[1] D. Bianco, F. Knapp, N. Lo Iudice, F. Andreozzi, and A. Porrino, Phys. Rev. C 85,
014313 (2012).
[2] D. Bianco, F. Knapp, N. Lo Iudice, F. Andreozzi, A. Porrino, and P. Vesely, Phys.
Rev. C, submitted to.
[3] A. Tamii et al., Phys. Rev. Lett. 107, 062502 (2011) and references therein.
[4] A. Leistenschneider et al., Phys. Rev. Lett. 86, 5442 (2001)