Experimental studies of the microscopic structure of the pygmy dipole resonance and comments on a possible pygmy quadrupole resonance

Abstract

The pygmy dipole resonance (PDR) has been observed on the low-energy tail of the isovector giant dipole resonance (IVGDR) below and above the neutron-separation threshold. While the additional strength is recognized as a feature of the electric dipole response of many nuclei with neutron excess, its microscopic structure, which intimately determines its contribution to the overall strength, is still poorly understood. This makes reliable predictions of the PDR in neutron-rich nuclei far off stability difficult. It has been shown that the coupling to complex configurations drives the strength fragmentation for both the IVGDR and the PDR, and that more strength gets fragmented to lower energies when including such configurations. The wavefunctions of \(J^{\pi }= 1^-\) states belonging to the PDR are, however, expected to be dominated by one-particle-one-hole (1p-1h) excitations of the excess neutrons. First experiments were performed to access these parts of the wavefunction via inelastic proton scattering through isobaric analog resonances and via one-neutron transfer reactions. The results, obtained from these experiments, will be discussed in this contribution and some new results added. Given the special occasion of Professor Bracco’s university retirement, I will also comment on experimental studies of a possible pygmy quadrupole resonance (PQR).