Examination of the evidence for a proton halo in $^{22}$Al

TL;DR

This paper investigates the potential proton halo structure in $^{22}$Al by analyzing its nuclear wave functions, shell structure, and density distributions using advanced theoretical models, concluding that a proton halo is unlikely but further experimental verification is needed.

Contribution

The study employs state-of-the-art relativistic Hartree-Bogoliubov theories in continuum to analyze the shell structure and density distributions of $^{22}$Al, providing new insights into its halo nature.

Findings

A small $s$-wave component (~5%) appears only with triaxial deformation.

Density and radius analyses suggest no discernible proton halo in $^{22}$Al.

Further experimental measurements are needed to confirm the halo hypothesis.

Abstract

More and more halo nuclei or candidates have been identified or suggested in experiments in recent years. It was declared that the halo structure of $^{22}$Al is revealed by the large isospin asymmetry in $^{22}$Si/$^{22}$O mirror Gamow-Teller transitions [Phys. Rev. Lett. 125, 192503 (2020)]. We highlight that a significant mirror asymmetry already exists between wave functions of the likely unbound nucleus $^{22}$Si and the doubly-magic nucleus $^{22}$O, which largely explains the observed asymmetry in the transitions. Furthermore, these transitions involve only the $1^+$ excited states of the daughter nuclei $^{22}$Al and $^{22}$F. The $1^+$ state of $^{22}$Al cannot be considered a halo state due to its proton-unbound nature. An analysis of the spin-parity suggests that a weakly bound $2s_{1/2}$ valence proton in the ground state of $^{22}$Al is improbable. To investigate the shell structure for the ground state of $^{22}$Al, we employ the state-of-the-art deformed and triaxial relativistic Hartree-Bogoliubov theories in continuum. We find that a small $s$-wave component of $5\%$ appears for the weakly bound valence proton in $^{22}$Al only when triaxial deformation is considered. While the examination of densities and rms radii indicates that this small $s$-wave component is insufficient to form a discernible proton halo in $^{22}$Al, slightly larger $2s_{1/2}$ occupations have been reported in other recent theoretical results. The question of how many low-$\ell$ components are sufficient to form a proton halo in the presence of the Coulomb barrier remains open. Thus, future measurements of reaction or interaction cross sections and momentum distributions of breakup fragments are highly desirable to verify whether $^{22}$Al is a halo nucleus.