# Electromagnetic Excitations and Responses in Nuclei from First Principles

## TL;DR

This paper explores how nuclear clustering affects electromagnetic excitations in nuclei using an ab initio symmetry-adapted no-core shell model, providing insights into nuclear responses and collective behaviors.

## Contribution

It introduces a symmetry-based ab initio approach capable of modeling electromagnetic responses in nuclei up to calcium, emphasizing clustering effects and benchmarking against exact solutions.

## Key findings

- Accurate modeling of electromagnetic sum rules in light nuclei.
- Identification of clustering effects on giant resonances.
- Benchmarking results show agreement with hyperspherical harmonics for 4He.

## Abstract

We discuss the role of clustering on monopole, dipole, and quadrupole excitations in nuclei in the framework of the ab initio symmetry-adapted no-core shell model (SA-NCSM). The SA-NCSM starts from nucleon-nucleon potentials and, by exploring symmetries known to dominate the nuclear dynamics, can reach nuclei up through the calcium region by accommodating ultra-large model spaces critical to descriptions of clustering and collectivity. The results are based on calculations of electromagnetic sum rules and discretized responses using the Lanczos algorithm, that can be used to determine response functions, and for 4He are benchmarked against exact solutions of the hyperspherical harmonics method. In particular, we focus on He, Be, and O isotopes, including giant resonances and monopole sum rules.