A Model of Radiative Neutrino Mass: with or without Dark Matter

Understand

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, $\lambda\rightarrow0$, the $Z_2$ symmetry becomes exact and the model contains a stable dark-matter candidate.
• However, even for finite $\lambda\ll1$, long-lived dark matter is possible, giving a unified solution to the neutrino mass and dark matter problems that does not invoke a new symmetry.
• Taken purely as a neutrino mass model, the new physics can be at the TeV scale.