AQA · A-Level · Physics
Course version: 7408 · Version from Sept 2016
Simple diagrams to represent the above reactions or interactions in terms…
Full specification wording
Particle interactions: Simple diagrams to represent the above reactions or interactions in terms of incoming and outgoing particles and exchange particles.
Particle interactions are represented by simple Feynman diagrams.
Checked against the AQA 7408 specification (opens in a new tab) on .
- Point 3 of 3
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Particle interactions are represented by simple Feynman diagrams. In AQA standard diagrams, time runs upwards: incoming particles are at the bottom, outgoing at the top. A wavy line represents the exchange particle (virtual photons for electromagnetic, W⁺/W⁻ bosons for weak). At each vertex, charge, baryon and lepton numbers are conserved. For example, in beta-minus decay, a neutron enters and a proton leaves, emitting a W⁻ boson that decays into an electron and antineutrino. Beta-plus decay involves a proton becoming a neutron, emitting a W⁺ that decays into a positron and neutrino. Electron capture and electron-proton collisions both involve a proton and electron interacting via a W boson to produce a neutron and electron neutrino.
What earns the marks
- Incoming particles are drawn at the bottom and outgoing particles at the top, with time increasing upwards.
- The exchange particle is drawn as a wavy line connecting two vertices, not as a solid particle line.
- Each vertex conserves charge, baryon number and lepton number; the diagram must be consistent with these conservation rules.
- The correct exchange particle is chosen: virtual photon for electromagnetic, W⁺ or W⁻ for weak charged-current interactions.
- For beta-plus decay, a proton changes into a neutron, emitting a W⁺ that decays into a positron and an electron neutrino.
- For electron-proton collisions, a proton and an electron interact via a W⁻ boson to produce a neutron and an electron neutrino.
Common mistakes and how to put them right
Drawing the exchange particle as a solid line like a fermion.
Put it right: Use a wavy line for bosons so it is visually distinct.
Confusing electron capture and electron-proton collisions.
Put it right: Note that electron capture involves an atomic electron and a W⁺ boson from the proton, whereas a collision involves a free electron and a W⁻ boson from the electron.
Using a photon as the exchange particle for beta decay.
Put it right: Use a W⁻ or W⁺ boson because beta decay is a weak interaction.
Exam tips
- Sketch the diagram roughly first, then check charge and baryon number at every vertex before finalising it.
- Label each particle with its symbol and ensure the W boson has the correct charge sign (W⁺ or W⁻) to conserve charge at the first vertex.
Try the lesson
The start of the Particle interactions: Simple diagrams to represent the above reactions or interactions in terms of incoming and outgoing particles and exchange particles. lesson, made from checked teaching notes for this specification point.
Have a go first
In a Feynman-style diagram of beta-minus decay, which particle is shown as the exchange particle emitted at the weak interaction vertex?
Listen
Particle interactions are represented by simple Feynman diagrams. In AQA standard diagrams, time runs upwards: incoming particles are at the bottom, outgoing at the top. A wavy line represents the exchange particle (virtual photons for electromagnetic, W⁺/W⁻ bosons for weak). At each vertex, charge, baryon and lepton numbers are conserved. For example, in beta-minus decay, a neutron enters and a proton leaves, emitting a W⁻ boson that decays into an electron and antineutrino. Beta-plus decay involves a proton becoming a neutron, emitting a W⁺ that decays into a positron and neutrino. Electron capture and electron-proton collisions both involve a proton and electron interacting via a W boson to produce a neutron and electron neutrino. A strong answer earns marks for these points. Incoming particles are drawn at the bottom and outgoing particles at the top, with time increasing upwards. The exchange particle is drawn as a wavy line connecting two vertices, not as a solid particle line. Each vertex conserves charge, baryon number and lepton number; the diagram must be consistent with these conservation rules. The correct exchange particle is chosen: virtual photon for electromagnetic, W⁺ or W⁻ for weak charged-current interactions. A common slip: Drawing the exchange particle as a solid line like a fermion. Instead, use a wavy line for bosons so it is visually distinct. Exam tip: Sketch the diagram roughly first, then check charge and baryon number at every vertex before finalising it.
Key words
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Conservation laws at vertices
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Exchange particles as carriers of forces
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Weak and electromagnetic interactions
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See every definition
- Conservation laws at vertices
- At each vertex, charge, baryon number and lepton number are conserved, and the diagram must be consistent with these conservation rules.
- Exchange particles as carriers of forces
- A wavy line represents the exchange particle, which is a virtual photon for electromagnetic interactions and a W⁺ or W⁻ boson for weak interactions.
- Weak and electromagnetic interactions
- The correct exchange particle is chosen for each: a virtual photon for electromagnetic and W⁺ or W⁻ for weak charged-current interactions.
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