Microscopic description of the proton halo in 12N

The year 2025 marks the 40th anniversary of the discovery of halo nuclei and the 15th anniversary of the development of the deformed relativistic Hartree-Bogoliubov theory in continuum (DRHBc). In this work, we present the first DRHBc description of the proton halo phenomenon. The available experimental proton separation energies and empirical matter root-mean-square (rms) radii are reasonably well reproduced for the N = 5 isotones, ranging from the stable nucleus 9 Be to the drip-line nucleus 12 N. In particular, the DRHBc theory captures the abrupt increase in rms radii at 12 N, unambiguously corroborating its proton halo structure. The formation of this halo is attributed to the occupation of a weakly bound orbital with dominant 1 p components by the valence proton, which contributes approximately 90 % to the diffused proton density of 12 N at large distances from the nuclear center. A shape decoupling between the prolate core and the oblate halo in 12 N is predicted.