Violation of lepton number in 3 units.

The number of leptons may or may not be a conserved quantity. The Standard Model predicts that it is (in perturbative processes), but there is the well known possibility that new physics violates lepton number in one or two units. The first case ($\Delta L=1$) is associated to proton decay into mesons plus a lepton or an anti-lepton, while the second one ($\Delta L=2$) is usually associated to Majorana neutrino masses and neutrinoless double beta decay. It is also conceivable that leptons can only be created or destroyed in groups of three ($\Delta L=3$). Colliders and proton decay experiments can explore this possibility.