AQA A-Level Physics Paper 1, June 2024: Question 2
9 marks · Medium difficulty · Extended Answer
Identify particle classifications, apply conservation rules to particle interactions, compare rest energies in neutral pion decay, explain gamma photon production from decay, and suggest reasons for slow progress in particle physics.
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Question text
02 A positive pion collides with a neutron and the following interaction is observed:
π+ + n → Κ+ + Σ0
Σ0 is a neutral sigma particle with a strangeness of −1
The interaction can be used to deduce the classifications of the Σ0.
02.1 Identify the classifications of each particle in Table 1.
Tick ( ) the appropriate boxes for each particle.
[2 marks]
Table 1
Particle Baryon Hadron Lepton Meson
π+
n
K+
Σ0
02.2 A conservation rule predicts that the following interaction cannot occur:
π− + n → Κ− + Σ0
State the conservation rule.
Go on to explain your answer.
[3 marks]
One way in which neutral pions decay is
π →0 e + e +−+ γ
*0032.*3 Compare the rest energies of the particles involved in this decay.
[2 marks]
02.4 The decay of the neutral pion leads to the production of further gamma photons.
Explain why.
[1 mark]
02.5 The Standard Model is a theory that classifies elementary particles.
Evidence for the theory has been collected since about 1950. However, the term
Standard Model has only been used since 1973.
Suggest why progress in particle physics is slow.
[1 mark]
Mark scheme
Show the mark scheme
Question Answers Additional comments/Guidelines Mark AO
02.1 2 1 × AO1
All four rows correct 1 2 1 × AO2
Any two rows correct 1
Accept any reasonable notation for
02.2 Identifies strangeness as the consideration as interaction Do not award MP1 for suggestion that any 3 AO3
would be strong/not weak other quantum number is not conserved.
– 0
Κ and Σ have same strangeness of -1
Evidence for MP2 and MP3 can be seen in a
correct use of strangeness values e.g.
Demonstration that LHS and RHS strangeness not equal
AND that the LHS is zero 0 + 0 → -1 -1
02.3 2 AO1
8 Electron AND positron rest energy =
clear assignment of each particle to its correct rest mass
including the photon 0.510999 MeV
π0 rest energy = 134.972 MeV
idea that LHS mass > RHS mass
Gamma/photon rest mass = 0
Allow rounded values for rest mass.
02.4 annihilation of the positron with an electron 1 AO1
Do not allow answers such as ‘elimination’
Accept idea that (people with)
02.5 Idea that international collaboration/co- 1 AO1
particular/specialist
operation/verification is required
talents/technology/infrastructure must be in
OR place.
(investment in) expensive equipment/hardware/ Ignore references to peer review or vague
infrastructure is required statements such as ‘it takes a long time’ or
‘it/the research is expensive’.
Total 9
How to answer it
Particle Interactions and Conservation Laws Study Guide
What this question tests
This question assesses your understanding of particle classifications (badrons, leptons, mesons, hadrons), conservation laws in particle interactions (specifically strangeness), rest energies in particle decays, pair annihilation processes, and the practical contexts of high-energy physics research.
Classifying Subatomic Particles
✅ Correct Answer / Table Completion
- pi+ : Hadron (tick), Meson (tick)
- n (neutron): Baryon (tick), Hadron (tick)
- K+ : Hadron (tick), Meson (tick)
- Sigma-0 : Baryon (tick), Hadron (tick)
💡 Key Knowledge
- Hadrons are particles that feel the strong nuclear force (made of quarks). Both baryons and mesons are hadrons.
- Baryons consist of 3 quarks (e.g. neutrons, protons).
- Mesons consist of a quark-antiquark pair (e.g. pions, kaons).
❌ Common Errors
Students often fail to tick "Hadron" when they tick "Baryon" or "Meson", forgetting that baryons and mesons are sub-categories under the overarching family of hadrons.
Explaining Conservation Rules in Interactions
✅ Correct Answer
- Identify conservation of strangeness as the relevant rule.
- State that K⁻ and Sigma⁰ both have a strangeness of -1 .
- Demonstrate mathematically that strangeness is not conserved: LHS strangeness = 0 + 0 = 0 , whereas RHS strangeness = -1 + (-1) = -2 .
🧠 Exam Technique
When an examiner asks why an interaction cannot occur, always check conservation laws in this priority order: Charge, Baryon number, Lepton number, and Strangeness (for strong/em interactions). Show the arithmetic clearly for both sides (LHS vs RHS).
❌ Common Errors
Losing the first mark by suggesting that charge, baryon number, or lepton number is violated (they are conserved in this specific equation; only strangeness fails).
Comparing Rest Energies in Decays
✅ Correct Answer
- Correctly assign rest energies: Electron and positron ≈ 0.511 MeV each, neutral pion ( pi⁰ ) ≈ 135 MeV , and gamma photon ( γ ) = 0 .
- State clearly that the rest energy of the left-hand side (LHS) is greater than the total rest energy of the right-hand side (RHS).
💡 Key Knowledge
In particle decays, total energy and momentum are always conserved. The "missing" rest energy is converted into the kinetic energy of the decay products.
Further Gamma Photon Production
✅ Correct Answer
The production of further gamma photons is due to the annihilation of the positron with an electron.
❌ Common Errors
Using vague terminology like "elimination" or "neutralisation". You must use the precise physics term: annihilation.
Contextualizing Progress in Particle Physics
✅ Correct Answer
Progress is slow because particle physics requires massive international collaboration/co-operation and extremely expensive specialized equipment, hardware, or infrastructure (such as large particle accelerators).
🧠 Exam Technique
Avoid generic answers like "it takes a long time" or "it is expensive". Examiners award credit for structural and logistical realities of modern scientific mega-projects, such as shared global funding and massive accelerator facilities.
Topics
Physics · 3.2 Particles and radiation
Question and mark scheme from the AQA A-Level Physics examination, Paper 1, June 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.