# $Ab~initio$ description of collectivity for $sd$ shell nuclei

## TL;DR

This paper applies ab initio shell model methods to sd-shell nuclei, demonstrating their effectiveness in describing nuclear deformation and comparing results with phenomenological interactions and experimental data.

## Contribution

It introduces ab initio approaches (IM-SRG and CC) to sd-shell nuclei, showing their validity in capturing deformation phenomena in open-shell nuclei.

## Key findings

- IM-SRG and CC results agree with experimental data for most isotopes.
- Ab initio methods effectively describe deformation in sd-shell nuclei.
- Comparison with SDPF-MU highlights the role of pf orbitals in nuclear structure.

## Abstract

In the present work, we have reported shell model results for open shell nuclei Ne, Mg and Si isotopes with $10 \leq N \leq 20$ in $sd$-shell model space. We have performed calculations in $sd$ shell with two $ab~initio$ approaches: in-medium similarity renormalization group (IM-SRG) and coupled-cluster (CC) theory. We have also performed calculations with phenomenological USDB interaction and chiral effective field theory based CEFT interaction. The results for rotational spectra and $B(E2;2_1^+\rightarrow 0_1^+)$ transitions are reported for even-mass isotopes. The IM-SRG and CC results are in reasonable agreement with the experimental data except at $N$ =20. This demonstrates a validity of $ab~initio$ description of deformation for doubly open-shell nuclei for $sd$ shell. To see the importance of $pf$ orbitals, we have also compared our results with SDPF-MU interaction by taking account of $2p-2h$ and $4p-4h$ configurations in $sd$-$pf$-shell model space.