Radiative Majorana masses for the three known neutrinos can be generated via the exchange of two $W$ bosons if there is a fourth neutrino with both Dirac and Majorana masses. Typical masses of the order ${10}^{-8\mathrm{}}$, ${10}^{-3\mathrm{}}$, and ${10}^2$ eV are obtained.

• Received 6 June 1988

DOI:https://doi.org/10.1103/PhysRevLett.61.674