  Neutrinoless double beta decay allows to constrain lepton number violating
extensions of the standard model. If neutrinos are Majorana particles, the mass
mechanism will always contribute to the decay rate, however, it is not a priori
guaranteed to be the dominant contribution in all models. Here, we discuss
whether the mass mechanism dominates or not from the theory point of view. We
classify all possible (scalar-mediated) short-range contributions to the decay
rate according to the loop level, at which the corresponding models will
generate Majorana neutrino masses, and discuss the expected relative size of
the different contributions to the decay rate in each class. Our discussion is
general for models based on the SM group but does not cover models with an
extended gauge. We also work out the phenomenology of one concrete 2-loop model
in which both, mass mechanism and short-range diagram, might lead to
competitive contributions, in some detail.
