AQA AS Level Physics Paper 1, November 2020: Question 1

10 marks · Medium difficulty · Extended Answer

Determine the lepton number, strangeness, charge, and identity of particle X in a proton collision, identify the missing particle in a pion decay, and discuss the nature, quark structure, and stability of hadrons.

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Question

Question text

01 One strong interaction that occurs when two high-energy protons collide is

p + p → p + π+ + π− + X

01.1 Determine the lepton number, strangeness and charge of particle X.

[2 marks]

lepton number =

strangeness =

charge =

01.2 Identify particle X.

[1 mark]

01.3 A possible decay of a negative pion is

π− → e− + Y

What is particle Y?

Tick ( ) one .

[1 mark]

ν�e

π0

01.4 Some subatomic particles are classified as hadrons. There are two classes of

hadrons.

Discuss the nature of hadrons.

*02* Your answer should include:

• the identifying properties of hadrons

• the structure of a hadron in each class

• a discussion of the stability of free hadrons.

[6 marks]

Mark scheme

Show the mark scheme

Question Answers Additional Comments/Guidance Mark

details

accept (+) 1.6 x 10–19 (C)

Lepton number = 0 and Strangeness = 0 condone lack of unit

01.1 charge = (+)1(e) 2 AO2.1b

AO2.1b

Proton / p / 1H Apply ECF to answers any particle other than a

1 AO2.1b

proton.

01.2 1

The particle must be correct for the given L, S

and Q. (clip with 2.1)

01.3 Tick in first box only (electron) antineutrino ν̅ 1 AO1.1a

(e)

7407_1 Version post-stand

details

The marking scheme for this question includes an overall assessment for

the quality of written communication (QWC). There are no discrete marks for

the assessment of QWC but the candidate’s QWC in this answer will be one

of the criteria used to assign a level and award the marks for this question.

Mark Criteria QWC

6 All 3 areas A, B and C covered The student

Only allow minor omissions presents the relevant

52 complete descriptions with one information

6 partial from A, B and C. coherently, The following statements are likely to be present.

employing structure,

style and SP&G to Area A

render meaning Hadrons properties:

clear. The text is • Identifies hadrons as consisting of quarks

legible. • May interact via the strong nuclear force

4 Full description of one area, with The student AO1.1a

partial description of two other. presents relevant Area B AO1.1a

OR information in a way General structure:

Full description of two areas with which assists the AO1.1a

01.4 • Two classes are mesons and baryons 6

very little on third or nothing at all. communication of AO1.1a

3 A full description of one area and meaning. The text is • quark-antiquark: meson

• Quark, quark, quark: baryon AO1.1a

a partial description of one area. legible. SP&G are

OR sufficiently accurate Area C AO1.1a

A partial discussion of all three not to obscure Stability of free hadron:

areas. meaning. • Only stable free baryon is proton

2 A full description of one area. The student • Example of decay of a free meson or baryon

OR presents some

A partial discussion of two areas. relevant information e.g. kaon decay into pions / states neutron

in a simple form. The decays into a proton

1 Only one area covered, and that text is usually

partially. legible. SP&G allow

meaning to be

derived although

errors are

sometimes

obstructive.

0 No relevant information

– SICS – – JUNE 2020

Total

How to answer it

Particles and Interactions Study Guide

What this question tests

This exam question evaluates your understanding of fundamental particle conservation laws (lepton number, strangeness, and charge), particle identification via conservation rules, weak interaction decays involving leptons, and a structured extended-writing assessment of hadron properties, classifications (baryons vs. mesons), and free particle stability.

Part 01.1: Conservation Laws in a Strong Interaction

Determine the lepton number, strangeness, and charge of particle X.

✅ Correct Answer

Lepton number = 0
Strangeness = 0
Charge = +1 (or +1e / 1.6×10⁻¹⁹ C)

💡 Key Knowledge

In any particle interaction, lepton number, strangeness, and charge must be conserved (along with baryon number).
Protons have charge +1 , lepton number 0 , strangeness 0 .
Pions ( π⁺ , π⁻ ) have charge +1 and -1 respectively, and strangeness 0 .

🧠 Exam Technique

Set up balancing equations for each conservation property across the reaction arrow: Initial = Final. Do them one by one to avoid calculation clutter.

❌ Common Errors

Confusing baryon number with lepton number, or forgetting that strangeness is conserved in strong interactions (unlike weak interactions where strangeness can change by 0, +1, or -1).

🎯 [2 marks] 1 mark for Lepton number = 0 and Strangeness = 0; 1 mark for charge = +1.

Part 01.2: Identifying Particle X

Identify particle X based on your determined properties.

✅ Correct Answer

Proton (Accept p or ¹₁H )

💡 Key Knowledge

A particle with charge +1 , lepton number 0 , strangeness 0 , and a baryon number of 1 (since two protons enter and one leaves alongside pions) matches the properties of a proton.

🧠 Exam Technique

Examiners apply Error Carried Forward (ECF) here. If you calculated incorrect values in 01.1, you will still get credit in 01.2 as long as your named particle logically matches your 01.1 values.

🎯 [1 mark] For correctly identifying the proton with proper notation.

Part 01.3: Pion Decay and Lepton Conservation

Identify particle Y in the decay: π⁻ → e⁻ + Y

✅ Correct Answer

Tick the first box: ν̄ₑ (electron antineutrino)

💡 Key Knowledge

Pions are mesons (lepton number = 0). Electrons are leptons (lepton number = +1).
To conserve lepton number overall, an antilepton (lepton number = -1) must be created on the right-hand side.
Lepton family numbers must also be conserved, requiring an electron antineutrino ( ν̄ₑ ).

❌ Common Errors

Choosing the electron neutrino ( νₑ ) instead of the antineutrino ( ν̄ₑ ), which violates lepton number conservation ( 0 ≠ +1 + (+1) ).

🎯 [1 mark] For ticking the first box only.

Part 01.4: Extended Writing - Nature of Hadrons

Discuss the nature of hadrons, covering identifying properties, structure within classes, and stability of free hadrons.

💡 Key Knowledge (What top-level answers include)

Area A (Identifying Properties): Hadrons are particles that experience the strong nuclear force and consist of combinations of quarks.
Area B (Classes & Structure): Divided into two classes: Baryons (consisting of 3 quarks, e.g., protons, neutrons) and Mesons (consisting of a quark-antiquark pair, e.g., pions, kaons). Note: Baryon number for baryons is +1, for mesons is 0.
Area C (Stability): The only stable free hadron in the universe is the proton. All other free hadrons (like neutrons, pions, and kaons) are unstable and decay via the weak interaction into other particles (ultimately ending up as protons, electrons, and photons). Free neutrons decay into protons via beta-minus decay.

🧠 Exam Technique & QWC

This is assessed using levels of response based on Quality of Written Communication (QWC) and coverage of Areas A, B, and C. Structure your response into distinct paragraphs matching each area so the examiner can easily tick off all required points.

🎯 [6 marks] Level-based mark scheme (Level 6 requires full coverage of all three areas with coherent structure and clear scientific terminology).

Topics

Physics · 3.2 Particles and radiation

Question and mark scheme from the AQA AS Level Physics examination, Paper 1, November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.