AQA A-Level Physics Paper 1, June 2025: Question 9

1 mark · Medium difficulty · Multiple Choice

Determine the quark configuration of particle X in the strong interaction X + p → n + π⁰.

Practise this question

Question

Question 09 shows the strong interaction equation: X + p → n + π⁰. Below, the question asks 'What is the quark configuration of X?' Four options are given: A is u-bar u-bar d-bar, B is s u-bar, C is d u-bar, and D is u d-bar, each accompanied by a selection box.
Question text

09 The following equation represents a strong interaction between particle X and a proton.

X + p → n + π0

What is the quark configuration of X?

[1 mark]

A u u d

B s u

C d u

D u d

Mark scheme

Show the mark scheme Mark scheme for question 09 showing the correct answer as option C with content 'd u-bar', categorised under assessment objective AO1.

09 C d u AO1

How to answer it

Strong Interactions & Quark Composition of Mesons

📌 What this question tests

This question assesses your ability to apply conservation laws (charge, baryon number, and strangeness) to deduce unknown particles in strong interactions, and subsequently determine the correct quark-antiquark composition of the resulting meson.

Question 09 Analysis

Identifying Particle X in: X + p → n + π⁰

💡 Key Knowledge: Conservation Rules

  • In a strong interaction, the following quantum numbers are strictly conserved:
    • Charge (Q)
    • Baryon number (B)
    • Strangeness (S)
  • Particle properties:
    • Proton (p): Q = +1, B = +1, S = 0
    • Neutron (n): Q = 0, B = +1, S = 0
    • Neutral pion (π⁰): Q = 0, B = 0, S = 0
  • Quark charges: u = +2/3, d = -1/3, s = -1/3
  • Antiquark charges: ū = -2/3, d̄ = +1/3, s̄ = +1/3

📐 Step-by-Step Quantum Number Balancing

  1. Baryon Number (B):
    B(X) + 1 = 1 + 0  ⇒  B(X) = 0
    Since B = 0, X must be a meson (quark-antiquark pair), ruling out baryons like option A.
  2. Strangeness (S):
    S(X) + 0 = 0 + 0  ⇒  S(X) = 0
    No strange quarks or antiquarks involved, ruling out option B.
  3. Charge (Q):
    Q(X) + (+1) = 0 + 0  ⇒  Q(X) = -1
    Particle X has a charge of -1 (it is a π⁻ meson).
  4. Find Quark Composition with Q = -1:
    • Option C ( dū ): (-1/3) + (-2/3) = -1 ✔️
    • Option D ( ud̄ ): (+2/3) + (+1/3) = +1 ✖️

✅ Correct Answer & Mark Allocation

Correct Option: C ( dū )

Mark Scheme: [1 Mark] for selecting C (AO1 - recall and apply knowledge of quarks and conservation laws).

🧠 Exam Technique: Elimination Strategy

  • Check Baryon Number first: Right side has B = 1 + 0 = 1. The proton has B = 1, so X must have B = 0. Instantly eliminate A ( ūūd̄ is an antibaryon, B = -1).
  • Check Strangeness: Right side has no strange particles (S = 0). Strangeness is conserved in strong interactions, so eliminate B ( sū has S = -1).
  • Compare the remaining two (C and D): Just add the charges of the quarks to see which gives -1.

❌ Common Pitfalls & Traps

  • Confusing π⁺ and π⁻: Many students confuse ud̄ (which is π⁺, Q = +2/3 + 1/3 = +1) with dū (which is π⁻, Q = -1/3 - 2/3 = -1). Always write down the fractional charges explicitly to verify.
  • Forgetting Strangeness is Conserved: Strangeness is only allowed to change by ±1 in weak interactions. The prompt explicitly specifies this is a strong interaction, so S must be conserved.
  • Sign Errors on Antiquarks: Remember an anti-up quark ( ū ) has charge -2/3, not +2/3. An anti-down quark ( d̄ ) has charge +1/3, not -1/3.

Summary of Candidate Particles

Option Configuration Type Charge (Q) Baryon No. (B) Strangeness (S) Status
A ūūd̄ Antibaryon (p̄) -1 -1 0 Violates B
B sū Meson (K⁻) -1 0 -1 Violates S
C dū Meson (π⁻) -1 0 0 Correct ✔
D ud̄ Meson (π⁺) +1 0 0 Violates Q

Topics

Physics · 3.2 Particles and radiation

Question and mark scheme from the AQA A-Level Physics examination, Paper 1, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.