Neutrinoless double-βdecay in SU(2)×U(1) theories

It is shown that gauge theories give contributions to neutrinoless double-β decay [(ββ)_0ν] which are not covered by the standard parametrizations. While probably small, their existence raises the question of whether the observation of (ββ)_0ν implies the existence of a Majorana mass term for the neutrino. For a "natural" gauge theory we argue that this is indeed the case.