  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.
