- | generation | charged lepton | charge | neutrino |- |---|---|---|---|- | 1st | [[Electron]] | −1 e | electron neutrino (νₑ) |- | 2nd | muon (μ) | −1 e | muon neutrino (ν_μ) |- | 3rd | tau (τ) | −1 e | tau neutrino (ν_τ) |+ | generation | charged lepton | charge | mass | mean life | neutrino |+ |---|---|---|---|---|---|+ | 1st | [[Electron]] | −1 e | 0.510 998 950 00 MeV | stable — see below | electron neutrino (νₑ) |+ | 2nd | muon (μ) | −1 e | 105.658 375 5 MeV | 2.196 981 1 × 10⁻⁶ s | muon neutrino (ν_μ) |+ | 3rd | tau (τ) | −1 e | 1776.93 ± 0.09 MeV | 290.3 × 10⁻¹⁵ s | tau neutrino (ν_τ) |+ Masses and lifetimes are PDG 2024. Each generation is heavier than the last: the tau outweighs the electron by a factor of about 3 480, and outweighs a whole [[Proton]] nearly twice over.+- The [[Electron]] is the only charged lepton that is stable; muons and taus decay in microseconds or less. Neutrinos barely interact at all — trillions pass through you each second.+ ## How long they last++ | lepton | mean life | distance light covers in one lifetime |+ |---|---|---|+ | electron | > 6.6 × 10²⁸ years (lower limit, 90 % CL) | — |+ | muon | 2.197 μs | 658.6 m |+ | tau | 290.3 fs | 87.03 μm |++ The [[Electron]] is the only charged lepton that is stable. Nobody has ever seen one decay, and the experimental floor is already 6.6 × 10²⁸ years — nearly 5 × 10¹⁸ times the age of the universe.++ The other two are not close. A muon lives roughly **7.6 million times** longer than a tau, and both are gone in a flash by human standards.++ That 658.6 m figure is worth a second look. Muons made by cosmic rays in the upper atmosphere reach the ground in large numbers, far further than 658 m of travel should allow. They arrive because they are moving near light speed and their clocks run slow. Cosmic-ray muons are the classic everyday proof that time dilation is real.++ ## Why exactly three generations?++ Nobody knows. It is one of the open questions of the Standard Model — the pattern repeats three times, with no explanation for why it starts or stops.++ But we do know that it *does* stop, at least for light neutrinos. A Z boson can decay into any neutrino type that is light enough, and each available type makes it decay a little faster. Measure the Z's width, and you count the neutrino species without ever detecting one:++ > N = 2.996 ± 0.007 (Standard Model fits to LEP–SLC data)++ Three. Not three and a half, not four.++ ## Where to read next++ - [[neutrino]] — the three neutral leptons, and why they barely interact+ - [[Quark]] — the other family of matter particles, which also comes in three generations+ - [[Electron]] — the only stable charged lepton, and the one doing all the chemistry
History of Lepton
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- A **lepton** is a fundamental particle that does not feel the strong nuclear force. Leptons come in three generations, each pairing a charged particle with its neutrino — six in all, plus their antiparticles.+ A **lepton** is a fundamental particle that does not feel the strong nuclear force. Leptons come in three generations, each pairing a charged particle with its [[neutrino]] — six in all, plus their antiparticles.- - no color charge — the strong force ignores them+ - no color charge — the [[strong force]] ignores them
+ A **lepton** is a fundamental particle that does not feel the strong nuclear force. Leptons come in three generations, each pairing a charged particle with its neutrino — six in all, plus their antiparticles.+ ++ ## The six leptons++ | generation | charged lepton | charge | neutrino |+ |---|---|---|---|+ | 1st | [[Electron]] | −1 e | electron neutrino (νₑ) |+ | 2nd | muon (μ) | −1 e | muon neutrino (ν_μ) |+ | 3rd | tau (τ) | −1 e | tau neutrino (ν_τ) |++ Three things every lepton shares:++ - spin ½ — they are fermions+ - no color charge — the strong force ignores them+ - lepton number +1, conserved in every interaction observed so far++ The [[Electron]] is the only charged lepton that is stable; muons and taus decay in microseconds or less. Neutrinos barely interact at all — trillions pass through you each second.