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

It is shown that gauge theories give contributions to neutrinoless double-$\beta$ decay [${\left(\beta \beta \right)}_{0\nu }$] which are not covered by the standard parametrizations. While probably small, their existence raises the question of whether the observation of ${\left(\beta \beta \right)}_{0\nu }$ implies the existence of a Majorana mass term for the neutrino. For a "natural" gauge theory we argue that this is indeed the case.