On the difficulties a lonely lepton
In conversions of elementary particles into each other, the total electric charge of the particles before conversion is equal to that after. If electrons, muons, tau-leptons or neutrinos are involved in the conversion, similarly their lepton numbers (charges), unique to each of the three lepton generations, stay unchanged. A muon, say, does not live long, so in a few microseconds an electron is flying instead, but also a muon neutrino and an electron antineutrino are emitted in this decay. The disappeared muon has passed its lepton number to the former while the latter carries the lepton number of the newborn electron but with the opposite sign. Thus, the muonic as well as electronic lepton number is conserved in the process. However, this conservation law is only approximate as evidenced by the recent discovery of transitions of different-generation neutrinos to each other. To clarify the situation, large-scale experiments are already being prepared to search for the neutrinoless transformation of electrons to muons.