  Heavy neutral leptons (HNLs) are hypothetical particles able to explain
several puzzles of fundamental physics, first and foremost - neutrino
oscillations. Being sterile with respect to Standard Model interactions, these
particles admit Majorana masses, allowing for violation of the total lepton
number. Lepton number violating (LNV) processes thus become a key signature of
HNLs, pursued by many experiments. In this work, we demonstrate that if HNLs
are the sole origin of neutrino masses, destructive interference between
Majorana states suppresses the same-sign di-lepton signal. In the
phenomenologically interesting case of large HNL couplings, such suppression is
akin to the cancellation of HNLs' contributions to neutrino masses.
Nevertheless, the signal can be much larger than coming from the Weinberg
operator alone. We identify regions of the parameter space of such realistic
HNL models where the LNV signal is maximised at the LHC and future FCC-hh. Our
results are obtained within the effective W approximation which allows for
analytic treatment and gives a clear dependence on the model parameters.
Although approximate, they are argued to be correct within a factor of a few.
