Structure evolution of ground and excited states in the exotic nucleus $^{22}$Al

TL;DR

This study uses the Gamow shell model to analyze the structure and decay of the weakly bound, proton-rich nucleus $^{22}$Al, revealing a halo-like excited state and insights into continuum effects.

Contribution

It applies advanced Gamow shell model calculations with chiral forces to explore the structure of $^{22}$Al near the proton drip line, highlighting continuum effects and halo phenomena.

Findings

Ground state identified as $4^+$ with a nearby $3^+$ excited state.

The $1_1^+$ excited state shows a significant halo-like structure due to larger s-wave component.

Thomas-Ehrman shift is negligible for ground and first excited states despite weak binding.

Abstract

Recent experimental studies on proton-rich nuclei in the $sd$ shell have revealed intriguing near-threshold phenomena, including exotic structures associated with mirror-symmetry breaking. In particular, a halo-like structure has been suggested for the $1^+$ state of $^{22}$Al based on the large isospin asymmetry observed in the $^{22}$Si/$^{22}$O mirror Gamow-Teller transitions. Recent mass measurements further indicate that the ground state of $^{22}$Al is weakly bound, with a single-proton separation energy of about 100 keV. To investigate how the continuum affects the structure and decay properties of this proton-dripline nucleus, we employ the state-of-the-art Gamow shell model. This approach utilizes valence-space effective interactions and operators derived from chiral forces. Our calculations identify the ground state of $^{22}$Al as a $4^+$ state, with a $3^+$ state as the first excitation. Despite their diffuse nature under weak binding, the Thomas-Ehrman shift for these states is found to be negligible due to their small $s$-wave components. In contrast, the excited $1_1^+$ state possesses a significantly larger $s$-wave component, resulting in a more pronounced halo-like structure.