A model of radiative neutrino mass: with or without dark matter

We present a three-loop model of neutrino mass whose most-general Lagrangian possesses a softly-broken accidental Z(2) symmetry. In the limit that a single parameter vanishes, A -> 0, the Z(2) symmetry becomes exact and the model contains a stable dark-matter candidate. However, even for finite lambda << 1, long-lived dark matter is possible, giving a unified solution to the neutrino mass and (lark matter problems that does not invoke a new symmetry. Taken purely as a neutrino mass fuodel, the new physics can be at the TeV scale. When dark matter is incorporated, however, only a singlet scalar can remain this light, though the dark matter can be tested in direct-detection experiments.