lepton flavor

Lepton flavor is a finer version of the lepton-counting rule. Not only does the total number of leptons stay balanced, but — in most ordinary reactions — the count within each of the three families also stays balanced separately. There is an electron-type tally (the electron and its neutrino), a muon-type tally (the muon and its neutrino), and a tau-type tally (the tau and its neutrino), and each one seems to be kept on its own little ledger.

Here is the puzzle that revealed it. When a muon decays it could in principle just turn into an electron and a photon, conserving energy, charge, and total lepton number. Yet that never happens. Instead the muon always produces an electron plus two neutrinos — a muon neutrino and an electron antineutrino. The muon neutrino is there to keep the muon-flavor count at +1 (the decaying muon was +1 in muon-flavor), and the electron antineutrino (-1 in electron-flavor) cancels the new electron (+1 in electron-flavor). Each family balances on its own.

Lepton flavor is famous for having a known exception: neutrino oscillation. As neutrinos travel, they slowly change type — an electron neutrino can become a muon neutrino — which means individual lepton-flavor counts are not perfectly conserved after all. This was a major discovery, because it proved neutrinos have a tiny mass, something the original Standard Model did not predict. For charged leptons, though, flavor-changing processes (like muon to electron plus a photon) are still spectacularly rare or absent, and searching for them is a sensitive hunt for new physics.