# Gross, intermediate and fine structure of nuclear giant resonances: Evidence for doorway states

## TL;DR

This paper reviews the fine structure of nuclear giant resonances, analyzing experimental spectra with wavelet techniques to identify decay mechanisms and the role of doorway states in different nuclei.

## Contribution

It provides a microscopic analysis linking the fine structure of resonances to specific decay mechanisms, highlighting the role of doorway states in nuclear excitations.

## Key findings

- Wavelet analysis quantifies fine structure scales.
- Coupling between $1p1h$ and $2p2h$ states is key for ISGQR.
- Fragmentation of $1p1h$ strength explains IVGDR fine structure.

## Abstract

We review the phenomenon of fine structure of nuclear giant resonances and its relation to different resonance decay mechanisms. Wavelet analysis of the experimental spectra provides quantitative information on the fine structure in terms of characteristic scales. A comparable analysis of resonance strength distributions from microscopic approaches incorporating one or several of the resonance decay mechanisms allows conclusions on the source of the fine structure. For the isoscalar giant quadrupole resonance (ISGQR), spreading through the first step of the doorway mechanism, i.e.\ coupling between one particle-one hole ($1p1h$) and two particle-two hole ($2p2h$) states is identified as the relevant mechanism. In heavy nuclei it is dominated by coupling to low-lying surface vibrations, while in lighter nuclei stochastic coupling becomes increasingly important. The fine structure observed for the isovector giant dipole resonance (IVGDR) arises mainly from the fragmentation of the $1p1h$ strength (Landau damping), although some indications for the relevance of the spreading width are also found.