# Observation of excited states in $^{20}$Mg sheds light on nuclear forces and shell evolution

## TL;DR

This study reports the first observation of excited states in the proton-rich nucleus $^{20}$Mg, providing insights into nuclear forces, shell evolution, and potential weakening of the N=8 shell closure at the proton drip-line.

## Contribution

It presents the first experimental detection of resonances and bound states in $^{20}$Mg, challenging current nuclear models and suggesting the need for improved theoretical approaches.

## Key findings

- Observation of proton unbound resonances and bound states in $^{20}$Mg.
- Experimental data challenge existing nuclear structure models.
- Indication of weakening of the N=8 shell closure at the proton drip-line.

## Abstract

The exotic Borromean nucleus $^{20}$Mg with $N$ = 8, located at the proton drip-line provides a unique testing ground for nuclear forces and the evolution of shell structure in the neutron-deficient region. We report on the first observation of proton unbound resonances together with bound states in $^{20}$Mg from the $^{20}$Mg($d$,$d'$) reaction performed at TRIUMF. Phenomenological shell-model calculations offer a reasonable description. However, our experimental results present a challenge for current first-principles nuclear structure approaches and point to the need for improved chiral forces and {\it ab initio} calculations. Furthermore, the differential cross section of the first excited state is compared with distorted-wave Born approximation calculations to deduce a neutron quadrupole deformation parameter of $\beta_n$=0.46$\pm$0.21. This provides the first indication of a possible weakening of the $N$ = 8 shell closure at the proton drip-line.