On the existence of a proton halo in ²²Al

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 ²²Al is revealed by the large isospin asymmetry in ²²Si/²²O 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 ²²Si and the doubly-magic nucleus ²²O, which largely explains the observed asymmetry in the transitions. Furthermore, these transitions involve only the 1⁺ excited states of the daughter nuclei ²²Al and ²²F. The 1⁺ state of ²²Al cannot be considered a halo state due to its proton-unbound nature. An analysis of the spin-parity suggests that a weakly bound 2s1/2 valence proton in the ground-state ²²Al is improbable. To investigate the shell structure for the ground state of ²²Al, 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 ²²Al 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 ²²Al, slightly larger 2s1/2 occupations have been reported in other recent theoretical results. The question of how many low- 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 ²²Al is a halo nucleus.