AQA A-Level Physics Paper 1, November 2020: Question 1

✅ Correct Answer

The reaction is not possible because lepton number is not conserved.

💡 Key Knowledge

• Pions are hadrons (mesons), so their lepton number is 0 .
• Electrons ( e⁻ ) have an electron lepton number of +1 .
• Positrons ( μ⁺ ) are antimuons, carrying a muon lepton number of -1 .
• Electron antineutrinos ( v̄ₑ ) carry an electron lepton number of -1 .

🧠 Exam Technique

Always set out conservation equations clearly. Check each lepton family separately (electron lepton number vs. muon lepton number). Stating that a specific particle or neutrino is missing/imbalanced secures your marks.

❌ Common Errors

Students often lose marks by writing incorrect numerical equations for charge, baryon number, or strangeness when the error actually lies strictly within lepton numbers. Ensure you check every quantum number individually.

✅ Correct Answer

u ū (up anti-up) AND d d̄ (down anti-down)

💡 Key Knowledge

• Pions are mesons made of a quark and an antiquark.
• Neutral pions are superpositions of quark-antiquark pairs of the first generation.
• Ensure you use lowercase letters and indicate antimatter correctly (using a bar or 'anti' prefix).

❌ Common Errors

Do not use capital letters (e.g., writing U Ū will be penalized). Avoid mixing different flavours like u d̄ , which would describe a charged pion ( π⁺ ).

✅ Correct Answer

The suggestion is incorrect. They have different quark compositions ( d s̄ versus d̄ s ) and opposite strangeness values (+1 vs -1).

💡 Key Knowledge

• K⁰ has quark composition d s̄ and strangeness +1 .
• K̄⁰ has quark composition d̄ s and strangeness -1 .
• Particle-antiparticle pairs possess opposite quantum numbers.

🧠 Exam Technique

To score full marks, you must explicitly compare the quark configurations and state their respective strangeness values to prove they are distinct particles.

❌ Common Errors

Vague answers stating "their strangeness is different" without quoting the actual values or listing the correct quark configurations will be capped at 1 mark.

✅ Correct Answer

Only the pion fits all criteria. The kaon has too large a mass/rest energy, and the muon is a lepton so it does not experience the strong nuclear force.

💡 Key Knowledge

• Requirement 1: Must experience the strong nuclear force (meaning it must be a hadron).
• Requirement 2: Must have a rest energy between an electron (~0.511 MeV) and half a nucleon (~469 MeV). Look up values in the Data Booklet!
• Pion rest energy is ~140 MeV (fits range), and it is a meson (hadron).
• Muon rest energy is ~106 MeV (fits range), but it is a lepton, so it does not experience the strong nuclear force.
• Kaon rest energy is ~494 MeV, which exceeds half the mass of a nucleon (~469 MeV).

🧠 Exam Technique

Structure your answer systematically, addressing each of the three particles individually. Check two conditions for each: (1) Does it feel the strong force (is it a hadron)? (2) Is its rest energy within the specified limits?

❌ Common Errors

A common mistake is failing to check the rest energy limits correctly for the kaon, or forgetting that leptons (like the muon) do not feel the strong nuclear force regardless of their mass.