Fundamental particles (quarks, hadrons, leptons)
PhysicsΒ· 9702 syllabus Section 27: Particle physicsΒ· 25 min read
1. Standard Model Particle Classificationβ β ββββ± 6 min
All known particles fall into two broad groups: fundamental particles with no substructure, and composite particles made of smaller fundamental units. For the CIE AS syllabus, you only need to focus on the first two generations of matter particles, and exclude gauge bosons and the Higgs boson from this topic.
Fundamental Particle
A particle with no measurable internal components that cannot be split into smaller particles via physical interactions.
All matter particles are either quarks or leptons, the two families of fundamental fermions
Composite particles made of quarks are collectively called hadrons
Particles are also grouped by which of the four fundamental forces they interact with
Classify each of the following particles as lepton, baryon or meson: electron, proton, pion, muon, neutron
- 1
Step 1: Identify fundamental particles first. Electrons and muons have no substructure, so they are leptons.
- 2
Step 2: Identify 3-quark composite particles. Protons and neutrons are each made of 3 quarks, so they are baryons.
- 3
Step 3: Identify quark-antiquark composite particles. Pions are made of one quark and one antiquark, so they are mesons.
Test your basic classification knowledge:
Which of these is a fundamental particle?
Proton
Electron
Neutron
Pion
Reveal answer
Electron βElectron has no substructure, all other options are composite hadrons.
Exam tip:
Exam multiple choice questions almost always include one trick option that incorrectly labels a lepton as a hadron.
2. Quark Properties and Quantum Numbersβ β β βββ± 7 min
The AS syllabus only requires you to know properties of the three lightest quarks: up (u), down (d) and strange (s). Each quark has a corresponding antiquark with opposite values for all quantum numbers.
Baryon Number
A quantum number assigned to each quark equal to +1/3, and each antiquark equal to -1/3. Total baryon number is always conserved in all particle interactions.
Calculate the total charge, baryon number and strangeness of a particle with quark composition uud
- 1
Step 1: Sum individual charge values: 2/3 e + 2/3 e - 1/3 e = +1e
- 2
Step 2: Sum individual baryon numbers: 1/3 + 1/3 + 1/3 = 1
- 3
Step 3: Sum individual strangeness values: 0 + 0 + 0 = 0
- 4
This is the quark composition of a proton.
3. Hadrons: Baryons vs Mesonsβ β β βββ± 6 min
π« No Calculator
All hadrons are made of quarks, but the two sub-groups have very different properties that you must be able to distinguish instantly.
Property | Baryon | Meson |
|---|---|---|
Quark composition | 3 quarks | 1 quark + 1 antiquark |
Total baryon number | +1 | 0 |
Typical charge values | -1, 0, +1 | -1, 0, +1 |
Stable examples | Proton | Pion |
A particle has quark composition udd. Classify it as baryon or meson, and find its total quantum numbers.
- 1
Step 1: Count the quark components. There are 3 quarks total, so this is a baryon.
- 2
Step 2: Calculate total charge: 2/3 e - 1/3 e - 1/3 e = 0
- 3
Step 3: Calculate total baryon number: 1/3 + 1/3 + 1/3 = 1
- 4
This is the quark composition of a neutron.
4. Lepton Families and Conservation Rulesβ β β βββ± 6 min
Leptons are grouped into three generations, but for AS you only need to know the first two: electron and electron neutrino, muon and muon neutrino. Each generation has its own separate lepton number that is always conserved.
Check if the interaction eβ» + ΞΌβ» β eβ» + ΞΌβ» is allowed via conservation rules.
- 1
Step 1: Check charge conservation: -1 + -1 = -1 + -1, total charge -2 on both sides.
- 2
Step 2: Check electron lepton number: 1 + 0 = 1 + 0, conserved.
- 3
Step 3: Check muon lepton number: 0 + 1 = 0 + 1, conserved.
- 4
All rules are satisfied, so the interaction is allowed.
5. Common Pitfalls
Wrong move:
Assigning non-zero strangeness to protons or neutrons
Why:
Protons and neutrons only contain up and down quarks, which both have strangeness 0
Correct move:
Only hadrons that explicitly contain a strange quark have non-zero strangeness
Wrong move:
Claiming leptons experience the strong nuclear force
Why:
Leptons have no quark substructure and do not interact via the strong force, this is the defining difference between leptons and hadrons
Correct move:
Only hadrons experience the strong nuclear force
Wrong move:
Forgetting mesons have a total baryon number of 0
Why:
A quark has baryon number +1/3 and an antiquark has baryon number -1/3, so their sum is 0
Correct move:
Always sum individual quark baryon numbers to get the total for any hadron
Wrong move:
Treating total lepton number as a single conserved value
Why:
Electron lepton number and muon lepton number are conserved separately, not added together
Correct move:
Track lepton numbers per generation, not as a combined total
Wrong move:
Assuming strangeness is conserved in all interactions
Why:
Strangeness is only conserved in strong interactions, and can change by Β±1 in weak interactions
Correct move:
Confirm the interaction type before applying strangeness conservation
6. Quick Reference Cheatsheet
Particle Group | Composition | Baryon Number | Charge Range | Experiences Strong Force? |
|---|---|---|---|---|
Lepton | Fundamental (no substructure) | 0 | -1, 0 | No |
Baryon | 3 quarks | 1 | -1, 0, +1 | Yes |
Meson | 1 quark + 1 antiquark | 0 | -1, 0, +1 | Yes |
Up quark | Fundamental | +1/3 | +2/3 e | N/A |
Down quark | Fundamental | +1/3 | -1/3 e | N/A |
Strange quark | Fundamental | +1/3 | -1/3 e | N/A |
7. Frequently Asked
Do I need to memorise quark combinations for every possible hadron?
No. CIE only requires you to recall the quark structure of protons, neutrons, pions and kaons, and derive combinations for other hadrons using given quantum numbers.
Why are leptons not classified as hadrons?
Leptons have no quark substructure and do not experience the strong nuclear force, which is the defining property of all hadrons.
When this came up on past exams
AI-estimated based on syllabus patterns β cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2024 Β· Paper 22
Quark composition of proton and neutron
- 2023 Β· Paper 12
Classify particles by interaction type
- 2022 Β· Paper 21
Conservation rules for strange particles
Going deeper
What's Next
Mastering fundamental particles is the foundation for all advanced particle physics questions in your AS and A2 CIE 9702 exams. You will next build on these conservation rules to analyse particle annihilation, pair production, and Feynman diagram interactions, which are frequently paired with this topic in Paper 1 multiple choice and Paper 2 structured questions. This content also links directly to your understanding of nuclear decay processes, where weak nuclear interactions mediate quark flavour changes in beta minus and beta plus decay. Ensure you can recall all core quark quantum numbers from memory before moving to the next modules.