Speaker
Description
The microscopic structure of the low-energy electric dipole response, commonly denoted as the Pygmy Dipole Resonance (PDR), was studied for $^{120}$Sn in a $^{119}$Sn$(d,p\gamma)^{120}$Sn experiment, using the SONIC@HORUS setup at the University of Cologne. Unprecedented access to the single-particle structure of excited $1^-$ states below and around the neutron-separation threshold was obtained by comparing experimental data to predictions from a novel theoretical approach. The approach combines detailed nuclear structure input from energy-density functional (EDF) plus quasiparticle-phonon model (QPM) theory with reaction theory to obtain a consistent description of both the structure and reaction aspects of the process. Similar to the recently investigated case of $^{208}$Pb [1], the combined results show that the EDF+QPM approach correctly predicts the energies of the relevant neutron single-particle levels in $^{120}$Sn and the fragmentation of the observed spectroscopic strength, and that the understanding of one-particle-one-hole structures of the $1^-$ states in the PDR region is crucial to reliably predict properties of the PDR. Furthermore, the EDF+QPM approach predicts the increasing contribution of complex configurations to the PDR states at higher excitation energies, which has been recently suggested as a cause for the discrepancy between $(\gamma,\gamma')$ and $(p,p')$ experiments [2,3]. This contribution will present the joint experimental and theoretical effort and discuss further applications, allowing a detailed study of the microscopic structure of the PDR along the isotopic chart.
[1] M. Spieker et al., Phys. Rev. Lett. 125, 102503 (2020)
[2] S. Bassauer et al., Phys. Rev. C 102, 034327 (2020)
[3] M. Müscher et al., Phys. Rev. C 102, 014317 (2020)
