Lepton

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	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.

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

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