Precision Mass Measurement of the Proton Dripline Halo Candidate Al22

We report the first mass measurement of the proton-halo candidate ²²Al performed with the low energy beam ion trap facility's 9.4 T Penning trap mass spectrometer at facility for rare isotope beams. This measurement completes the mass information for the lightest remaining proton-dripline nucleus achievable with Penning traps. ²²Al has been the subject of recent interest regarding a possible halo structure from the observation of an exceptionally large isospin asymmetry [J. Lee et al., Large isospin asymmetry in Si22/O22 Mirror Gamow-Teller transitions reveals the halo structure of ²²Al, Phys. Rev. Lett. 125, 192503 (2020).]. The measured mass excess value of ME=18 092.5(3) keV, corresponding to an exceptionally small proton separation energy of Sₚ=100.4(8) keV, is compatible with the suggested halo structure. Our result agrees well with predictions from $sd$-shell USD Hamiltonians. While USD Hamiltonians predict deformation in the ²²Al ground state with minimal 1s1/2 occupation in the proton shell, a particle-plus-rotor model in the continuum suggests that a proton halo could form at large quadrupole deformation. These results emphasize the need for a charge radius measurement to conclusively determine the halo nature.