# Excitation of the electric pygmy dipole resonance by inelastic electron scattering

## TL;DR

This study investigates the excitation mechanisms of the electric pygmy dipole resonance (PDR) in $^{140}$Ce via inelastic electron scattering, revealing its predominantly transversal nature and potential to provide new insights into nuclear structure.

## Contribution

It introduces detailed calculations of PDR excitation in electron scattering using QRPA and QPM models, highlighting the reaction's sensitivity to transversal nuclear currents and its complementarity to other methods.

## Key findings

- PDR excitation is mainly transversal at backward angles.
- Electron scattering cross sections for PDR are much smaller than for the first 2+ state.
- At extreme backward angles, PDR can dominate the scattering cross section.

## Abstract

To complete earlier studies of the properties of the electric pygmy dipole resonance (PDR) obtained in various nuclear reactions, the excitation of the 1$^-$ states in $^{140}$Ce by $(e,e')$ scattering for momentum transfers $q=0.1-1.2$~fm$^{-1}$ is calculated within the plane-wave and distorted-wave Born approximations. The excited states of the nucleus are described within the Quasiparticle Random Phase Approximation (QRPA), but also within the Quasiparticle-Phonon Model (QPM) by accounting for the coupling to complex configurations. It is demonstrated that the excitation mechanism of the PDR states in $(e,e')$ reactions is predominantly of transversal nature for scattering angles $\theta_e \approx 90^o-180^o$. Being thus mediated by the convection and spin nuclear currents, the $(e,e')$ like the $(\gamma,\gamma')$ reaction, may provide additional information to the one obtained from Coulomb- and hadronic excitations of the PDR in $(p,p')$, $(\alpha,\alpha')$, and heavy-ion scattering reactions. The calculations predict that the $(e,e')$ cross sections for the strongest individual PDR states are in general about three orders of magnitude smaller as compared to the one of the lowest $2^+_1$ state for the studied kinematics, but that they may become dominant at extreme backward angles.