AQA AS Level Physics Paper 2, June 2025: Question 8

1 mark · Medium difficulty · Multiple Choice

Identify the correct Feynman diagram representing an electron–proton collision.

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Question

Question 8 asks 'Which diagram represents an electron–proton collision?' Four Feynman diagrams labeled A, B, C, and D are shown. Each diagram illustrates an interaction with incoming particles at the bottom and outgoing particles at the top connected by a wavy line representing a W exchange boson. Option A shows an electron producing an antineutrino and a proton producing a neutron via a W+ boson. Option B shows a proton producing a neutron and an electron producing a neutrino via a W+ boson. Option C shows an electron producing an antineutrino and a proton producing a neutron via a W- boson. Option D shows an incoming proton becoming an outgoing neutron on the left and an incoming electron becoming an outgoing neutrino on the right, linked by a W- boson directed from right to left.

Mark scheme

Show the mark scheme Mark scheme table for question 8 showing the correct option is D, with an accompanying Feynman diagram of an electron emitting a W- boson to become an electron neutrino, which is absorbed by a proton to become a neutron. The classification is tagged as AO1.

How to answer it

Feynman Diagrams: Electron–Proton Collisions

What this question tests

This question assesses your understanding of particle interactions governed by the weak interaction, specifically:

  • Identifying reactants and products in an electron–proton collision ( p + e⁻ → n + νₑ ).
  • Applying conservation laws (charge Q, baryon number B, and lepton electron number Lₑ).
  • Reading Feynman diagrams correctly, including time direction, particle arrows, exchange bosons (W⁺ vs W⁻), and charge conservation at each vertex.
Question 08 • Multiple Choice • [1 Mark]

Identifying the Correct Feynman Diagram

✅ Correct Answer

D

1 Mark: Correct option D selected.

In diagram D, an incoming electron interacts with an incoming proton by emitting a W⁻ boson to become an electron neutrino ( νₑ ), and the proton absorbs the W⁻ to become a neutron ( n ).

💡 Key Knowledge

  • Interaction Equation: p + e⁻ → n + νₑ
  • Lepton Conservation: Initial Lₑ = +1 (from e⁻), so the outgoing lepton must have Lₑ = +1 (neutrino, νₑ ), NOT an antineutrino ( ν̄ₑ , which has Lₑ = -1 ).
  • Baryon Conservation: Initial B = +1 (proton), final B = +1 (neutron).
  • Exchange Boson: Weak interaction mediated by W⁺ or W⁻ bosons.

📐 Step-by-Step Diagram Verification

Time flows vertically upward in standard AQA Feynman diagrams.

  1. Check Leptons (Eliminate A and C):
    The collision requires an outgoing neutrino ( νₑ ). Both A and C produce an electron antineutrino ( ν̄ₑ ), violating lepton number conservation ( +1 ≠ -1 ). Hence, A and C are incorrect.
  2. Check Direction of Boson Arrow in B & D:
    In both B and D, the wavy line has an arrow pointing from right to left (from the lepton line towards the baryon line). This indicates the boson is transferred from the electron to the proton.
  3. Test Vertex Charge Conservation for D:
    Vertex Particles Involved Charge Balance
    Right (Lepton) e⁻ emits W⁻ and becomes νₑ -1 = 0 + (-1)  ✔ Conserved
    Left (Baryon) p absorbs W⁻ and becomes n +1 + (-1) = 0  ✔ Conserved

    All vertices conserve charge, confirming D is correct.

❌ Common Traps & Mistakes

  • Confusing νₑ and ν̄ₑ: Forgetting that e⁻ → νₑ conserves lepton number, whereas e⁻ → ν̄ₑ violates it. This immediately ruled out options A and C.
  • Failing to Check Vertex Charge: In option B, the arrow points left with a W⁺ . At the right vertex: e⁻ → νₑ + W⁺ gives -1 ≠ 0 + (+1) = +1 . This blatantly violates charge conservation!
  • Confusing with Electron Capture: In electron capture, a proton in a proton-rich nucleus captures an inner-shell orbital electron. While the overall equation is the same ( p + e⁻ → n + νₑ ), students must always trace the arrow on the exchange line to determine whether a W⁺ or W⁻ is correct.

🧠 Examiner Technique: The 10-Second Elimination Strategy

  1. Step 1: Write down the overall reaction: p + e⁻ → n + νₑ . Look at the products at the top. You need n and νₑ . Immediately eliminate A and C because they show ν̄ₑ .
  2. Step 2: Look at the remaining options (B and D). Note the direction of the boson arrow: it goes from the right (electron side) to the left (proton side).
  3. Step 3: Ask: "What charge leaves the electron?" An electron has charge -1 and leaves behind a neutral neutrino ( 0 ), so it must shed a charge of -1 . Therefore, the carrier going from right to left must be W⁻. This instantly picks D.

Topics

Physics · 3.2 Particles and radiation

Question and mark scheme from the AQA AS Level Physics examination, Paper 2, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.