leptonflavor

Lepton flavor is a quantum property used to distinguish the three families of leptons: the electron, muon, and tau, along with their corresponding neutrinos (ν_e, ν_μ, ν_τ). In many contexts it is convenient to refer to lepton flavor numbers L_e, L_μ, and L_τ, which count the number of electrons, muons, and taus (and their neutrinos) in a process.

In the Standard Model, lepton flavor is conserved in renormalizable interactions, leading to approximate conservation of each flavor in charged-lepton processes. However, the observation of neutrino oscillations shows that neutrinos have mass and that flavor eigenstates are not static; a neutrino produced as ν_e can later be detected as ν_μ or ν_τ, indicating that lepton flavor is not strictly conserved in the neutrino sector.

Flavor and mass eigenstates are related by the PMNS matrix, which describes how the three neutrino flavors mix and propagate. The phenomenon of oscillations depends on neutrino mass-squared differences and mixing angles, and experiments studying solar, atmospheric, reactor, and accelerator neutrinos have measured these parameters. This reveals that lepton flavor is a meaningful, but not absolute, label for neutrinos.

Experimentally, charged-lepton flavors appear to be conserved to extremely high precision, and searches for charged-lepton flavor violation (such as μ → e γ or μ → 3e) have yielded very stringent upper limits. Lepton flavor remains an important area for tests of lepton flavor universality and for probing physics beyond the Standard Model, where new mechanisms could induce observable flavor-changing effects in both the neutrino and charged-lepton sectors.