AQA A-Level Physics Paper 1, June 2023: Question 11

1 mark · Medium difficulty · Multiple Choice

Identify the correct particles and quarks E, F, and G in a Feynman diagram representing a quark change producing an electron antineutrino.

Practise this question

Question

A Feynman diagram showing an incoming quark E changing into an outgoing quark F via a virtual exchange particle, resulting in an outgoing G and an electron antineutrino. Below the diagram, a multiple-choice table lists four options (A, B, C, D) for the identities of E, F, and G: A) up quark, down quark, beta minus; B) down quark, up quark, beta minus; C) up quark, down quark, beta plus; D) down quark, up quark, beta plus.
Question text

11 The diagram represents a quark change in which an electron antineutrino is produced.

What are E, F and G?

[1 mark]

E F G

A up quark down quark β−

B down quark up quark β−

C up quark down quark β+

D down quark up quark β+

Mark scheme

Show the mark scheme The mark scheme shows that the correct answer for question 11 is B, corresponding to down quark for E, up quark for F, and beta minus for G.

11 B down quark up quark β−

How to answer it

Interpreting Quark Changes in Feynman Diagrams

What this question tests

This question assesses your understanding of particle interactions, specifically weak interaction Feynman diagrams for beta decay (neutron decay / quark transitions). You need to interpret particle/antiparticle lines, arrow directions for fermions, charge conservation at vertices, and identify incoming/outgoing quarks and virtual exchange particles.

Question 1.1

Identifying Quark Transitions and Exchange Particles

✅ Correct Answer

Option B

E: down quark
F: up quark
G: β⁻ (electron)

💡 Key Knowledge

  • Beta-minus decay quark equation: d → u + e⁻ + ν̄ₑ (A down quark changes into an up quark, emitting a W⁻ boson which decays into an electron and an electron antineutrino).
  • Fermion arrows: Incoming particles and outgoing anti-particles have arrows pointing forward in time (upwards/rightwards), while outgoing particles have arrows pointing forward along their path. Note that antiparticles conventionally follow time backwards in Feynman diagrams, meaning their arrow points backwards relative to motion, but standard AQA diagrams show incoming/outgoing paths clearly.

🧠 Exam Technique

Break the diagram down vertex by vertex:

  1. Left Vertex (Quark change): An incoming quark E turns into an outgoing quark F while emitting a gauge boson (the wavy line). A charge of -1/3 (down) turning into +2/3 (up) requires the emission of a W⁻ boson (charge -1 ). Therefore, E must be a down quark and F an up quark. This immediately eliminates options A and C.
  2. Right Vertex (Lepton production): The W boson decays into an electron antineutrino ( ν̄ₑ ) and particle G. To conserve lepton number and charge, particle G must be an electron ( β⁻ ).

❌ Common Errors

  • Confusing β⁻ and β⁺ . Remember that beta-minus decay involves a down quark becoming an up quark via a W⁻ boson, yielding an electron and an electron antineutrino.
  • Mixing up the order of up and down quarks by failing to check charge conservation at the first vertex ( -1/3 = +2/3 + (-1) ).
Mark Allocation: 1 mark for selecting the correct option combination (B).

Topics

Physics · 3.2 Particles and radiation

Question and mark scheme from the AQA A-Level Physics examination, Paper 1, June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.