Missed prediction of the neutron halo in ³⁷Mg

Halo phenomena have long been an important frontier in both experimental and theoretical nuclear physics. ³⁷Mg was identified as a halo nucleus in 2014 and remains the heaviest nuclear halo system to date. While the halo phenomenon in ³⁷Mg was not predicted before the discovery, its description has been still challenging afterwards. In this Letter, we report a microscopic and self-consistent description of the neutron halo in ³⁷Mg using the deformed relativistic Hartree-Bogoliubov theory in continuum (DRHBc) that was developed in 2010. The experimental neutron separation energies and empirical matter radii of neutron-rich magnesium isotopes as well as the deformed p-wave halo characteristics of ³⁷Mg are well reproduced without any free parameters. In particular, the orbital occupied by the halo neutron in ³⁷Mg, exhibiting p-wave components comparable to those suggested in experiments, remains consistent across various employed density functionals including PC-F1, PC-PK1, NL3*, and PK1. The DRHBc theory investigated only even-even magnesium isotopes in previous works and for that reason missed predicting ³⁷Mg as a halo nucleus before 2014. Although the core and the halo of ³⁷Mg are both prolate, higher-order shape decoupling on the hexadecapole and hexacontatetrapole levels is predicted.