AQA · A-Level · Physics
Course version: 7408 · Version from Sept 2016
+ − Change of quark character in β and in β decay.
Full specification wording
Applications of conservation laws: + − Change of quark character in β and in β decay.
Beta decay is a weak interaction in which a quark changes flavour, so the nucleon changes type.
Checked against the AQA 7408 specification (opens in a new tab) on .
- Point 1 of 3
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Beta decay is a weak interaction in which a quark changes flavour, so the nucleon changes type. In β⁻ decay a neutron (udd) becomes a proton (uud): one d quark transforms into a u quark, emitting an electron and an antineutrino. In β⁺ decay a proton (uud) becomes a neutron (udd): one u quark transforms into a d quark, emitting a positron and a neutrino. The quark character change is d → u in β⁻ and u → d in β⁺. Charge, baryon number and lepton number are conserved overall. Writing the quark-level equation shows the change clearly, for example d → u + e⁻ + ν̄ₑ for β⁻ decay.
What earns the marks
- State that β⁻ decay converts a down quark into an up quark, changing a neutron into a proton.
- State that β⁺ decay converts an up quark into a down quark, changing a proton into a neutron.
- Identify the emitted particles: an electron and an antineutrino in β⁻ decay, a positron and a neutrino in β⁺ decay.
- Show that charge is conserved in the quark-level change, for example the down quark charge of −1⁄3 changes to an up quark charge of +2⁄3 with the electron carrying away −1.
- Show that baryon number is conserved because the number of quarks remains three in each nucleon.
- Show that lepton number is conserved, with the electron and antineutrino balancing lepton number in β⁻ decay and the positron and neutrino balancing it in β⁺ decay.
- Use the quark composition of nucleons, udd for a neutron and uud for a proton, to justify the change.
- Recognise that the change of quark character is a weak interaction, not a strong or electromagnetic one.
Common mistakes and how to put them right
Saying that a whole quark is emitted from the nucleus.
Put it right: The quark changes flavour and leptons are emitted.
Writing the β⁻ change as u → d.
Put it right: d → u for β⁻ decay and u → d for β⁺ decay.
Omitting the antineutrino or neutrino.
Put it right: Include it so that lepton number is conserved.
Treating the electron as a quark-level particle with baryon number.
Put it right: The electron has lepton number 1 and baryon number 0.
Exam tips
- Write the quark-level change as a balanced equation before describing the nucleon change.
- Check charge, baryon number and lepton number line by line after writing each decay.
- Use the words down quark and up quark rather than d and u alone when explaining the change.
- Link each emitted lepton to the conservation law it satisfies.
Try the lesson
The start of the Applications of conservation laws: + − Change of quark character in β and in β decay. lesson, made from checked teaching notes for this specification point.
Have a go first
In β⁻ decay, a neutron changes into a proton. Which quark change occurs?
Listen
Beta decay is a weak interaction in which a quark changes flavour, so the nucleon changes type. In β⁻ decay a neutron (udd) becomes a proton (uud): one d quark transforms into a u quark, emitting an electron and an antineutrino. In β⁺ decay a proton (uud) becomes a neutron (udd): one u quark transforms into a d quark, emitting a positron and a neutrino. The quark character change is d → u in β⁻ and u → d in β⁺. Charge, baryon number and lepton number are conserved overall. Writing the quark-level equation shows the change clearly, for example d → u + e⁻ + ν̄ₑ for β⁻ decay. A strong answer earns marks for these points. State that β⁻ decay converts a down quark into an up quark, changing a neutron into a proton. State that β⁺ decay converts an up quark into a down quark, changing a proton into a neutron. Identify the emitted particles: an electron and an antineutrino in β⁻ decay, a positron and a neutrino in β⁺ decay. Show that charge is conserved in the quark-level change, for example the down quark charge of −1⁄3 changes to an up quark charge of +2⁄3 with the electron carrying away −1. A common slip: Saying that a whole quark is emitted from the nucleus. Instead, the quark changes flavour and leptons are emitted. Exam tip: Write the quark-level change as a balanced equation before describing the nucleon change.
See it
Key words
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Weak interaction
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Conservation of charge
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Conservation of baryon number
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Conservation of lepton number
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See every definition
- Weak interaction
- The change of quark character in beta decay is a weak interaction, not a strong or electromagnetic one.
- Conservation of charge
- Charge is conserved overall, for example the down quark charge of −1⁄3 changes to an up quark charge of +2⁄3 with the electron carrying away −1.
- Conservation of baryon number
- Baryon number is conserved because the number of quarks remains three in each nucleon.
- Conservation of lepton number
- Lepton number is conserved, with the electron and antineutrino balancing lepton number in β⁻ decay and the positron and neutrino balancing it in β⁺ decay.
Label the diagram
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