AQA AS Level Physics Paper 1, June 2018: Question 1

11 marks · Medium difficulty · Short Answer

Describe nuclear interactions, conservation of baryon number in alpha and beta decay, deduce the quark structure of a lambda particle from a strong interaction equation, identify a feature of weak decay, and explain the need for collaboration in particle physics.

Practise this question

Question

Question text

01.1 Describe the interaction that is responsible for keeping protons and neutrons together

in a stable nucleus.

You should include details of the properties of the interaction in your answer.

[3 marks]

01.2 Nuclei can decay by alpha decay and by beta decay.

In alpha decay only one particle is emitted but in beta decay there are two emitted

particles.

Explain how baryon number is conserved in alpha and beta decay.

[3 marks]

01.3 Kaons are mesons that can be produced by the strong interaction between pions and

protons.

The equation shows a reaction in which a kaon and a lambda particle are produced.

π–+ p → K0+ Λ0

*02* Deduce the quark structure of the Λ0

[2 marks]

quark structure =

01.4 The kaon decays by the weak interaction.

The equation shows an example of kaon decay.

K0 → π++ π–

State one feature of this decay that shows it is an example of the weak interaction.

[1 mark]

01.5 There have been considerable advances in our understanding of particle physics over

the past 100 years.

Explain why it is necessary for many teams of scientists and engineers to collaborate

in order for these advances to be made.

[2 marks]

Mark scheme

Show the mark scheme

Question Answers Additional Comments/Guidelines Mark

THREE FROM: If wrong interaction identified then zero marks

the strong interaction If refer to strong interaction correctly then ignore

has short range OR mention range (less than 5 fm) any subsequent reference to other interactions 1

attraction up to 5 fm 1

01.1

repulsive (any distance below 1fm) 1

is zero/negligible beyond 5 fm (3 MAX)

only affects hadrons/ baryons and mesons

mediated by gluons/pions

in alpha decay number of nucleons/protons and neutrons is If only refer to baryon number/nucleon number of

unchanged OR baryons in parent nucleus equals the total number of alpha particle then do not award first mark

baryons in daughter nucleus and the alpha particle Can be shown by equations

𝐴 𝐴−4 4 1

01.2 in beta decay a neutron changes into a proton (and both have same e.g. 𝑍X → 𝑍−2Y + 2𝛼

baryon number) Second marking point can also be shown in

beta (-) particle and antineutrino have zero baryon number/beta(+) equation

and neutrino have zero baryon number

quark structure 𝜋− = 𝑢 ̅ d and p = uud if two of the quark structures correct then 1 mark

01.3 quark structure kaon = d𝑠̅ any correct answer (uds) full marks 1

hence as strong interaction quark structure 0 = uds 1

01.4 strangeness is not conserved/lost 1

TWO FROM:

results of experiments must be independently checked/validated/peer 1

reviewed before they are accepted/can be confirmed

01.5 particle accelerators are very expensive and collaboration helps to

spread the cost of building them 1

many skills and disciplines are required (which one team are unlikely (2 MAX)

to have)

– – –

lots of data to process (so more teams needed)

Total 11

How to answer it

Particle Physics Study Guide

What this question tests

This exam question tests your core understanding of particle interactions, conservation laws (specifically baryon number and strangeness), quark structures of hadrons, and the collaborative nature of modern scientific research. You need to apply fundamental properties of the strong and weak nuclear forces to physical decays and reactions.

Question 01.1

The Strong Nuclear Interaction

✅ Correct Answers (Choose 3)

It is the strong interaction.
It has a short range (up to approximately 5 fm).
It is attractive up to 5 fm (and down to about 0.5 fm).
It is repulsive at very short distances (below ~1 fm).
It is effectively zero beyond 5 fm.
It only affects hadrons (baryons and mesons).
It is mediated by gluons (or pions between nucleons).

❌ Common Errors & Pitfalls

A major trap is confusing the strong force with electrostatic repulsion or gravitational attraction. If you identify the wrong interaction initially, examiners award zero marks for the entire part. Always explicitly state "strong interaction" first before listing its properties.

Maximum Marks: 3

Question 01.2

Conservation of Baryon Number in Radioactive Decays

💡 Key Knowledge

Baryon number ($B$) is strictly conserved in all particle reactions and decays. Each nucleon (proton or neutron) has a baryon number of +1, while leptons and exchange bosons have $B = 0$.

✅ Correct Answer Breakdown

Alpha decay: Total number of nucleons (protons and neutrons) remains unchanged; the total baryon number in the parent nucleus equals the sum of baryon numbers in the daughter nucleus and the alpha particle.
Beta decay: A neutron changes into a proton (both possess a baryon number of +1, so $B$ is conserved).
Emitted particles: The beta particle (electron/positron) and antineutrino/neutrino have a baryon number of zero.

Maximum Marks: 3

Question 01.3

Deducing Quark Structure

📐 Step-by-Step Deduction

Given the reaction: π⁻ + p → K⁰ + Λ⁰

Recall or deduce the quark structure of the proton ( uud ) and the negative pion π⁻ ( ūd ).
Recall the quark structure of the neutral kaon K⁰ ( ds̄ ).
Apply conservation laws across the equation to find the lambda particle ( Λ⁰ ) quarks: it must contain one up, one down, and one strange quark ( uds ).

🧠 Exam Technique

Even if you forget the kaon composition, knowing that the lambda particle is a neutral baryon containing a strange quark allows you to instantly write down uds for full marks.

Maximum Marks: 2

Question 01.4

Features of the Weak Interaction

✅ Correct Answer

Strangeness is not conserved (or strangeness changes by ±1 during the decay).

💡 Key Knowledge

One of the hallmark indicators of a weak interaction decay is that strangeness is not conserved. The strong interaction conserves strangeness, but the weak interaction allows quark flavor changes.

Maximum Marks: 1

Question 01.5

Collaboration in Modern Physics

✅ Correct Answers (Choose 2)

Results of experiments must be independently checked, validated, and peer-reviewed before acceptance.
Particle accelerators and detectors (like CERN) are exceptionally expensive, so international collaboration spreads the financial cost.
Diverse skills, technical specialties, and engineering disciplines are required—far beyond the scope of a single research team.
There are massive volumes of collision data to process, requiring distributed computing and large analysis teams.

🧠 Exam Technique

Keep your points concise and business-like. Avoid vague statements like "scientists need friends to help." Instead, use professional terminology such as peer review, cost-sharing, and multidisciplinary expertise.

Maximum Marks: 2

Topics

Physics · 3.2 Particles and radiation

Question and mark scheme from the AQA AS Level Physics examination, Paper 1, June 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.