Quarks

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Quarks Leptons 3Leptons at work

• Unit 1.1b3
• Breithaupt chapter 2.3
• pages 22 23

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AS specification

• Leptons electron, muon, neutrino (electron and
  muon types).
• Leptons are subject to the weak interaction.
• Lepton numbers for the leptons will be given in
  the data booklet.
• Breithaupt chapter 2.3 pages 22 23

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Types of lepton

• Leptons are believed to be fundamental particles.
  That is they do not decay into any other
  particles except leptons.
• Electrons and their neutrinos are stable.
• Muons and tauons eventually decay into electrons
  and neutrinos.
• There are three families of leptons and
  antileptons. In order of increasing rest mass
• electrons and their neutrinos
• muons and their neutrinos
• tauons and their neutrinos
• Tauons are heavier versions of muons (about 20 x
  more massive). You are not expected to know about
  the tauon family in AS physics

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Lepton and antileptons
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Lepton number (L)

• Conservation of lepton number
• In all interactions the total lepton number is
  conserved.
• Lepton numbers are given in the AS examination

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Allowed interactions

• Beta-minus decay
• Lepton number before
• 0
• Lepton number after
• 0 1 1 0
• Muon decay
• Lepton number before
• 1
• Lepton number after
• 1 1 1 1

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Lepton type conservation

• The total electron or muon lepton numbers must
  also be conserved in any interaction.
• Example
• The total lepton number on both sides is the same
  1
• However, in terms of muons and muon antineutrinos
  the number changes from 1 to 1
• In terms of electrons and electron neutrinos the
  number changes from 0 to 2
• Thus interaction is not permitted

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Questions

• 1. Write an equation for beta-plus decay
• 2. Write an equation for µ decay
• 3. Can the interaction shown below occur?

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Answers

• 1. Write an equation for beta-plus decay
• 2. Write an equation for µ decay
• 3. Can the interaction shown below occur?
• Yes, lepton numbers 1 0 0 1
• and it also conserves electron lepton type number

p ? n e ve
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Notes from Breithaupt pages 22 23

• What are the two types of neutrinos?
• Explain the lepton rules in (a) interactions
  between leptons and hadrons and (b) muon decay
• Give and explain an example of a non-allowed
  interaction where lepton number is conserved.
  (see the weak puzzle)
• Try the summary questions on page 23

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Answers to the summary questions