AQA A Level Physics

Revision Notes

2.2.4 Leptons

Leptons & Antileptons

  • Leptons are a group of fundamental (elementary) particles
    • This means they are not made up of any other particles (no quarks)
  • Leptons interact with other particles via the weak, gravitational or electromagnetic interactions
    • They do not interact via the strong nuclear force
  • The most common leptons are:
    • The electron, e
    • The electron neutrino, ve
    • The muon, μ
    • The muon neutrino, vμ


The most common leptons are the electron, muon and their respective neutrinos

  • The muon is similar to the electron but is slightly heavier
  • Electrons and muons both have a charge of -1e and a mass of 0.0005u
  • Neutrinos are the most abundant leptons in the universe and have no charge and negligible mass (almost 0)
  • Although quarks are fundamental particles too, they are not classed as leptons
    • Leptons do not interact with the strong force, whilst quarks do

Worked Example

Circle all the anti-leptons in the following decay equation.

Worked example - leptons, downloadable AS & A Level Physics revision notes

Lepton Number

  • Similar to baryon number, the lepton number, L is the number of leptons in an interaction
  • L depends on whether the particle is a lepton, anti-lepton or neither
    • Leptons have a lepton number L = +1
    • Anti-leptons have a lepton number L = –1
    • Particles that are not leptons have a lepton number L = 0
  • Lepton number is a quantum number and is conserved in all interactions
  • This is helpful for knowing whether an interaction is able to happen


The lepton number depends if the particle is a lepton, anti-lepton or neither

Worked Example

If the lepton number is conserved in the following decay, identify whether particle X should be a neutrino or anti-neutrino

Lepton Number Worked Example

Step 1: Determine the lepton number of all the particles on both sides of the equation

0 + (–1) = 0 + X

Step 2: Identify the lepton number of X

             If the lepton number must be conserved, X must also have a lepton number of –1

Step 3: State the particle X

             Particle X is an anti-neutrino

Exam Tip

No need to fret about memorising all the different lepton numbers – the lepton number of each lepton and anti–lepton is provided on the datasheet.

Remember that although quarks are fundamental particles, they have a lepton number of 0 and are classed as baryons.

Muon Decay

  • Muons are leptons that are slightly heavier than the electron
  • Muons (μ) typically decay into an electron
  • Anti-muons (μ+) typically decay into positrons

2.2.4 Muon Decay Feynman Diagram

  • Muon decay occurs through the weak interaction
  • This can be recognised by the exchange of the W boson on a Feynman diagram

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