  A compelling feature of relativistic mean-field phenomenology has been the
reproduction of spin-orbit splittings in finite nuclei after fitting only to
equilibrium properties of infinite nuclear matter. This successful result
occurs when the velocity dependence of the equivalent central potential that
leads to saturation arises primarily because of a reduced nucleon effective
mass. The spin-orbit interaction is then also specified when one works in a
four-component Dirac framework. Here the nature of the spin-orbit force in more
general chiral effective field theories of nuclei is examined, with an emphasis
on the role of the tensor coupling of the isoscalar vector meson (omega) to the
nucleon.
