AQA · A-Level · Physics
Course version: 7408 · Version from Sept 2016
The muon as a particle that decays into an electron.
Full specification wording
Classification of particles: The muon as a particle that decays into an electron.
The muon is a fundamental lepton, identical to the electron in charge and weak interaction but about 207 times more massive.
Checked against the AQA 7408 specification (opens in a new tab) on .
- Point 10 of 15
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The muon is a fundamental lepton, identical to the electron in charge and weak interaction but about 207 times more massive. Being unstable, a muon decays by the weak interaction. A negative muon (μ⁻) transforms into an electron (e⁻) plus a muon neutrino and an electron antineutrino. A positive muon (μ⁺) gives a positron (e⁺) with an electron neutrino and a muon antineutrino. Charge, lepton number and energy-momentum are conserved. For example, in μ⁻ → e⁻ + ν̄ₑ + ν_μ, the electron antineutrino balances the electron lepton number, while the muon neutrino conserves the muon lepton number. Recognising the muon as a lepton that decays into an electron links particle classification to conservation laws and the weak interaction.
What earns the marks
- Identifies the muon as a lepton, a fundamental particle with charge -1 (or +1 for the antiparticle) and a rest mass about 207 times that of the electron.
- States that the muon is unstable and decays via the weak interaction.
- Writes a correct decay equation such as μ⁻ → e⁻ + ν̄ₑ + ν_μ, or μ⁺ → e⁺ + νₑ + ν̄_μ.
- Checks conservation of charge and specific lepton numbers (electron and muon lepton numbers) in the decay.
- Explains that the decay products include neutrinos, which carry away energy and momentum and are electrically neutral.
Common mistakes and how to put them right
Treating the muon as a hadron or baryon because it is heavy.
Put it right: The muon is a lepton and does not interact via the strong force.
Writing the decay as muon to electron only, which violates lepton-number conservation.
Put it right: Include the correct neutrinos.
Confusing the neutrino flavours in the decay equation.
Put it right: Ensure the electron antineutrino balances the electron's lepton number, and the muon neutrino conserves the initial muon's lepton number (for μ⁻ decay).
Exam tips
- Learn the full decay equation with correct neutrino flavours, then practise checking charge and specific lepton numbers line by line.
- When asked to explain stability or decay, name the interaction explicitly: the weak interaction is responsible for muon decay.
Try the lesson
The start of the Classification of particles: The muon as a particle that decays into an electron. lesson, made from checked teaching notes for this specification point.
Have a go first
A muon is unstable and decays. Which particle is produced when a muon decays?
Listen
The muon is a fundamental lepton, identical to the electron in charge and weak interaction but about 207 times more massive. Being unstable, a muon decays by the weak interaction. A negative muon (μ⁻) transforms into an electron (e⁻) plus a muon neutrino and an electron antineutrino. A positive muon (μ⁺) gives a positron (e⁺) with an electron neutrino and a muon antineutrino. Charge, lepton number and energy-momentum are conserved. For example, in μ⁻ → e⁻ + ν̄ₑ + ν_μ, the electron antineutrino balances the electron lepton number, while the muon neutrino conserves the muon lepton number. Recognising the muon as a lepton that decays into an electron links particle classification to conservation laws and the weak interaction. A strong answer earns marks for these points. Identifies the muon as a lepton, a fundamental particle with charge -1 (or +1 for the antiparticle) and a rest mass about 207 times that of the electron. States that the muon is unstable and decays via the weak interaction. Writes a correct decay equation such as μ⁻ → e⁻ + ν̄ₑ + ν_μ, or μ⁺ → e⁺ + νₑ + ν̄_μ. Checks conservation of charge and specific lepton numbers (electron and muon lepton numbers) in the decay. A common slip: Treating the muon as a hadron or baryon because it is heavy. Instead, the muon is a lepton and does not interact via the strong force. Exam tip: Learn the full decay equation with correct neutrino flavours, then practise checking charge and specific lepton numbers line by line.
See it
Key words
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muon
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weak interaction
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lepton number
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conservation laws
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See every definition
- muon
- A fundamental lepton, identical to the electron in charge and weak interaction but about 207 times more massive, which is unstable and decays by the weak interaction.
- weak interaction
- The interaction responsible for muon decay, in which a negative muon transforms into an electron plus a muon neutrino and an electron antineutrino.
- lepton number
- A quantity conserved in particle interactions, with the electron antineutrino balancing the electron lepton number and the muon neutrino conserving the muon lepton number.
- conservation laws
- Charge, lepton number and energy-momentum are conserved in particle interactions such as the decay of a muon into an electron.
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