Leptons – Elementary Particles

1. What are Leptons?

Leptons are a class of fundamental (elementary) particles that do not participate in strong nuclear interaction. They are point-like particles with no internal structure (as far as we know).

Key Idea: Leptons are not made of quarks and hence are not hadrons.

2. Types of Leptons

There are six leptons, grouped into three generations:

Generation	Lepton	Symbol	Charge
1st	Electron	e⁻	−1
	Electron Neutrino	ν_e	0
2nd	Muon	μ⁻	−1
	Muon Neutrino	ν_μ	0
3rd	Tau	τ⁻	−1
	Tau Neutrino	ν_τ	0

3. Properties of Leptons

(a) Electric Charge

Charged leptons (e⁻, μ⁻, τ⁻) have charge −e, while neutrinos are electrically neutral.

(b) Spin

All leptons have spin ½ and hence obey Fermi–Dirac statistics.

(c) Mass

Mass increases with generation:

Electron < Muon < Tau

Neutrinos have extremely small but non-zero mass.

4. Interactions of Leptons

Interaction	Participate?
Gravitational	Yes
Electromagnetic	Only charged leptons
Weak	Yes (all leptons)
Strong	No

5. Lepton Number

Each lepton family conserves a quantum number called lepton number.

Lepton → +1 Antilepton → −1

Example:

β-decay conserves lepton number:

n → p + e⁻ + ν̄_e

6. Importance of Leptons

Electrons form atomic structure
Neutrinos play a key role in nuclear reactions
Muon and tau confirm particle generations
Essential to the Standard Model of particle physics

7. Summary

✔ Leptons are elementary particles ✔ They do not experience strong interaction ✔ Exist in three generations ✔ Obey Fermi–Dirac statistics ✔ Fundamental to modern physics