Editing Lepton (section)

=== Mass ===
In the [[Standard Model]], each lepton starts out with no intrinsic mass. The charged leptons (i.e. the electron, muon, and tau) obtain an effective mass through interaction with the [[Higgs field]], but the neutrinos remain massless. For technical reasons, the masslessness of the neutrinos implies that there is no mixing of the different generations of charged leptons as [[CKM matrix|there is for quarks]]. The zero mass of neutrino is in close agreement with current direct experimental observations of the mass.<ref>{{harvnb|Peskin|Schroeder|1995|p=27}}</ref>

However, it is known from indirect experiments—most prominently from observed [[neutrino oscillation]]s<ref>{{harvnb|Fukuda|1998}}</ref>—that neutrinos have to have a nonzero mass, probably less than {{val|2|ul=eV/c2}}.<ref name="Neutrino" /> This implies the existence of physics [[beyond the Standard Model]]. The currently most favoured extension is the so-called [[seesaw mechanism]], which would explain both why the left-handed neutrinos are so light compared to the corresponding charged leptons, and why we have not yet seen any right-handed neutrinos.