AQA · A-Level · Physics
Course version: 7408 · Version from Sept 2016
Lepton number as a quantum number; conservation of lepton number for muon…
Full specification wording
Classification of particles: Lepton number as a quantum number; conservation of lepton number for muon leptons and for electron leptons.
Lepton number L is a quantum number that tracks leptons.
Checked against the AQA 7408 specification (opens in a new tab) on .
- Point 9 of 15
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Lepton number L is a quantum number that tracks leptons. Assign L = 1 to each lepton (e⁻, μ⁻, νₑ, ν_μ), L = −1 to each antilepton (e⁺, μ⁺, ν̄ₑ, ν̄_μ) and L = 0 to all non-leptons such as protons and photons. In any allowed interaction the total Lₑ before equals the total Lₑ after, and the total L_μ before equals the total L_μ after.
For example: In beta-minus decay a neutron becomes a proton while an electron and an electron antineutrino are emitted; the electron and antineutrino have opposite electron lepton numbers, so the total remains zero.
What earns the marks
- Lepton number is a quantum number with value 1 for a lepton, −1 for an antilepton and 0 for non-leptons.
- Electron lepton number Lₑ and muon lepton number L_μ are conserved separately in particle interactions.
- In beta-minus decay, the emitted electron and electron antineutrino have opposite electron lepton numbers, so total Lₑ is unchanged.
- In beta-plus decay, the emitted positron and electron neutrino have opposite electron lepton numbers, so total Lₑ is unchanged.
- A reaction that changes an electron lepton into a muon lepton without producing a matching antilepton is forbidden by separate conservation.
- Lepton number conservation is used alongside conservation of charge, baryon number and energy to test whether a proposed reaction is allowed.
Common mistakes and how to put them right
Using a single combined lepton number and allowing an electron to turn into a muon.
Put it right: Conserve electron lepton number and muon lepton number separately.
Giving an antineutrino lepton number 1 instead of −1.
Put it right: Antineutrinos have lepton number −1.
Forgetting that a neutron and proton have lepton number 0.
Put it right: Assign 0 to all baryons and mesons in the tally.
Exam tips
- Draw a before-and-after table with columns for charge, baryon number, Lₑ and L_μ to make conservation checks systematic.
- Show the substitution of values explicitly so the examiner can see that each quantum number balances.
- If a reaction fails one conservation rule, state which rule is violated rather than only saying the reaction is impossible.
Try the lesson
The start of the Classification of particles: Lepton number as a quantum number; conservation of lepton number for muon leptons and for electron leptons. lesson, made from checked teaching notes for this specification point.
Have a go first
What is the lepton number of an electron?
Listen
Lepton number L is a quantum number that tracks leptons. Assign L = 1 to each lepton (e⁻, μ⁻, νₑ, ν_μ), L = −1 to each antilepton (e⁺, μ⁺, ν̄ₑ, ν̄_μ) and L = 0 to all non-leptons such as protons and photons. In any allowed interaction the total Lₑ before equals the total Lₑ after, and the total L_μ before equals the total L_μ after. For example, in beta-minus decay a neutron becomes a proton while an electron and an electron antineutrino are emitted; the electron and antineutrino have opposite electron lepton numbers, so the total remains zero. A strong answer earns marks for these points. Lepton number is a quantum number with value 1 for a lepton, −1 for an antilepton and 0 for non-leptons. Electron lepton number Lₑ and muon lepton number L_μ are conserved separately in particle interactions. In beta-minus decay, the emitted electron and electron antineutrino have opposite electron lepton numbers, so total Lₑ is unchanged. In beta-plus decay, the emitted positron and electron neutrino have opposite electron lepton numbers, so total Lₑ is unchanged. A common slip: Using a single combined lepton number and allowing an electron to turn into a muon. Instead, conserve electron lepton number and muon lepton number separately. Exam tip: Draw a before-and-after table with columns for charge, baryon number, Lₑ and L_μ to make conservation checks systematic.
Key words
Say what each word means before you turn the card.
Lepton number
Try answering aloud or in your head before turning the card.
Electron lepton number
Try answering aloud or in your head before turning the card.
Muon lepton number
Try answering aloud or in your head before turning the card.
Conservation laws
Try answering aloud or in your head before turning the card.
See every definition
- Lepton number
- A quantum number with value 1 for a lepton, −1 for an antilepton and 0 for non-leptons.
- Electron lepton number
- A quantum number, Lₑ, that is conserved separately in particle interactions.
- Muon lepton number
- A quantum number, L_μ, that is conserved separately in particle interactions.
- Conservation laws
- Lepton number conservation is used alongside conservation of charge, baryon number and energy to test whether a proposed reaction is allowed.
There is more to this lesson
The full lesson goes on to check your understanding and help you earn the marks. Create a free account to carry on, with your progress saved.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge
Test your understanding of Classification of particles: Lepton number as a quantum number; conservation of lepton number for muon leptons and for electron leptons.
Active recall beats re-reading. Take an instant quiz now to test what you’ve learned and lock it into long-term memory.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge