Hydrogen atom on null-plane and Melosh transformation

The null-plane dynamics of hydrogen-like atoms is studied in approximation depending on c, the velocity of light, being large. Neglecting terms in the Hamiltonian of order c/sup -3/ (relative to electron rest energy) a symmetry SU(2)/sub W/ appears which is analogous to the SU(6)/sub W/ of hadron classification. This symmetry, if accurate, would dictate zero ground state magnetic moment. The symmetry is broken by terms of third order, which can, however, be transformed away by the appropriate approximation to the Melosh transformation. There then emerges a better symmetry, SU(2)/sub M/, broken only at fourth order. (8 refs).