  Collider signals of heavy Majorana neutrino mass origin are studied in the
minimal Left-Right symmetric model, where their mass is generated spontaneously
together with the breaking of lepton number. The right-handed triplet Higgs
boson $\Delta$, responsible for such breaking, can be copiously produced at the
LHC through the Higgs portal in the gluon fusion and less so in gauge mediated
channels. At $\Delta$ masses below the opening of the $VV$ decay channel, the
two observable modes are pair-production of heavy neutrinos via the triplet
gluon fusion $gg \to \Delta \to NN$ and pair production of triplets from the
Higgs $h \to \Delta \Delta \to 4N$ decay. The latter features tri- and quad
same-sign lepton final states that break lepton number by four units and have
no significant background. In both cases up to four displaced vertices may be
present and their displacement may serve as a discriminating variable. The
backgrounds at the LHC, including the jet fake rate, are estimated and the
resulting sensitivity to the Left-Right breaking scale extends well beyond 10
TeV. In addition, sub-dominant radiative modes are surveyed: the $\gamma
\gamma$, $Z \gamma$ and lepton flavour violating ones. Finally, prospects for
$\Delta$ signals at future $e^+ e^-$ colliders are presented.
