# Fundamental particles (quarks, hadrons, leptons)

> Physics · CIE A-Level 9702
> Source: https://www.owlsprep.com/study/cie-9702-u11-fundamental-particles/

This module covers Standard Model particle classification, quark properties, hadron sub-types, lepton families, and core conservation rules for AS particle physics exam questions.

**Prerequisites:** [Basic nuclear structure (protons, neutrons, electrons)](https://www.owlsprep.com/study/cie-9702-u7-nuclear-atom-structure/); [Conservation of charge and mass-energy](https://www.owlsprep.com/study/cie-9702-u6-conservation-laws/)

## Learning objectives

- Distinguish between fundamental and composite particles in the Standard Model
- Recall core properties of up, down and strange quarks including charge, baryon number and strangeness
- Classify hadrons into baryons and mesons based on their quark composition
- Apply conservation rules for charge, baryon number and lepton number to validate particle interactions

## Standard Model Particle Classification

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

**Worked example:** 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.

**Check your understanding**

Test your basic classification knowledge:

1. Which of these is a fundamental particle?

   - Proton
   - Electron
   - Neutron
   - Pion

   *Why:* Electron 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.

## Quark Properties and Quantum Numbers

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.

*Notation:* B

$$Q_u = +\frac{2}{3}e, \quad Q_d = Q_s = -\frac{1}{3}e$$

$$B_u = B_d = B_s = +\frac{1}{3}, \quad S_u = S_d = 0, \quad S_s = -1$$

> **Strangeness Exam Rule**
>
> Strangeness S is conserved only in strong nuclear interactions, and can change by ±1 in weak nuclear interactions. This is a very commonly tested rule.

**Worked example:** 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.

## Hadrons: Baryons vs Mesons

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 |

**Worked example:** 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.

*Calculator:* forbidden

## Lepton Families and Conservation Rules

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.

**Exam command terms**

CIE uses specific command terms for this topic with defined expectations:

- **Classify** — Sort the given particle into lepton, baryon or meson, no extra explanation required

- **Deduce** — Use conservation rules to work out an unknown quantum number or quark composition

- **State** — Recall a fact directly from the syllabus, no working needed

**Worked example:** Check if the interaction e⁻ + μ⁻ → e⁻ + μ⁻ 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.

## Common pitfalls

- **Wrong:** Assigning non-zero strangeness to protons or neutrons
  - Why it fails: Protons and neutrons only contain up and down quarks, which both have strangeness 0
  - Correct: Only hadrons that explicitly contain a strange quark have non-zero strangeness
- **Wrong:** Claiming leptons experience the strong nuclear force
  - Why it fails: Leptons have no quark substructure and do not interact via the strong force, this is the defining difference between leptons and hadrons
  - Correct: Only hadrons experience the strong nuclear force
- **Wrong:** Forgetting mesons have a total baryon number of 0
  - Why it fails: A quark has baryon number +1/3 and an antiquark has baryon number -1/3, so their sum is 0
  - Correct: Always sum individual quark baryon numbers to get the total for any hadron
- **Wrong:** Treating total lepton number as a single conserved value
  - Why it fails: Electron lepton number and muon lepton number are conserved separately, not added together
  - Correct: Track lepton numbers per generation, not as a combined total
- **Wrong:** Assuming strangeness is conserved in all interactions
  - Why it fails: Strangeness is only conserved in strong interactions, and can change by ±1 in weak interactions
  - Correct: Confirm the interaction type before applying strangeness conservation

## 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 |

## 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.

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