# Deep Inside the Atom

> Quarks, leptons and force-carrying bosons in the Standard Model of matter, and how particle accelerators test it.

From HSC Physics on WhizzNotes, free study notes for NSW students: https://whizznotes.com/physics/deep-inside-the-atom/. Updated 2 November 2026.
Year 11 and Year 12 boxes go deeper than the Year 10 text around them.

### Story time: The machine under two countries
Under the border of France and Switzerland, in a circular tunnel 27 kilometres long, protons race round in opposite directions at 99.999999% of the speed of light. Thousands of magnets bend their paths into the ring. When the beams cross, protons smash head-on.

In 2012, among billions of these collisions, scientists at CERN found traces of a particle that had been predicted 48 years earlier but never seen: the **Higgs boson**. It was the last missing piece of the Standard Model, our best map of what everything is made of.

**In short:** Protons and neutrons are made of smaller particles called quarks: a proton is up, up, down and a neutron is up, down, down. The Standard Model describes all known particles (six quarks, six leptons, the force-carrying bosons and the Higgs boson), and particle accelerators test it.

## Smaller than protons

By the 1930s, physicists thought matter was made of three particles: protons, neutrons and electrons. Then **particle accelerators** and **cosmic ray** detectors revealed hundreds of new particles. Experiments firing electrons at protons showed that protons are not solid balls: they have smaller particles inside them.

These are **quarks**. Protons and neutrons are each made of **three** quarks.

## The Standard Model

The **Standard Model** describes all known **fundamental** particles (particles not made of anything smaller) and three of the four forces.

### Quarks

There are six types (called "flavours") of quark, in three generations:

| Generation | Charge $+\frac{2}{3}$ | Charge $-\frac{1}{3}$ |
|---|---|---|
| 1st | up (u) | down (d) |
| 2nd | charm (c) | strange (s) |
| 3rd | top (t) | bottom (b) |

All ordinary matter uses only **up** and **down** quarks. Quarks are never found alone; they are always bound together.

### Leptons

Leptons are **not** made of quarks and do not feel the strong force:

- **electron**, **muon** and **tau** (charge −1)
- three matching **neutrinos** (no charge, almost no mass)

### Bosons: the force carriers

Forces between particles are carried by exchanging **bosons**:

| Force | Boson | Acts on |
|---|---|---|
| Electromagnetic | photon | charged particles |
| Strong | gluon | quarks |
| Weak | W and Z bosons | all quarks and leptons (causes beta decay) |
| (gives mass) | Higgs boson | |

Gravity is not part of the Standard Model.

### Antimatter

Every particle has an **antiparticle** with the same mass and opposite charge. The positron is the anti-electron. When a particle meets its antiparticle, they **annihilate** and turn into energy.

## Hadrons: particles made of quarks

| Type | Made of | Examples |
|---|---|---|
| **Baryons** | 3 quarks | proton (uud), neutron (udd) |
| **Mesons** | 1 quark + 1 antiquark | pions, such as π⁺ (u, anti-d) |

### Worked example: Checking the charges
Proton, uud: $+\frac{2}{3} + \frac{2}{3} - \frac{1}{3} = +1$

Neutron, udd: $+\frac{2}{3} - \frac{1}{3} - \frac{1}{3} = 0$

π⁺, u and anti-d: $+\frac{2}{3} + \frac{1}{3} = +1$ (the anti-down has charge $+\frac{1}{3}$)

## Beta decay, explained with quarks

In beta minus decay, a **down** quark in a neutron changes into an **up** quark, by the weak force. The neutron (udd) becomes a proton (uud), giving out a W⁻ boson that immediately becomes an electron and an antineutrino.

## Particle accelerators

Accelerators use:

- **electric fields** to speed charged particles up, giving them energy;
- **magnetic fields** to bend them into a ring and focus the beam (as in Charged Particles in Electric and Magnetic Fields).

They smash particles together at enormous energies. By $E = mc^2$, this energy can make **new, heavier particles** that are not found in everyday matter. Detectors track the debris to identify what was made.

Accelerators have found the predicted quarks, the W and Z bosons, the top quark and the Higgs boson, all evidence for the Standard Model. They also confirm special relativity every day: particles near $c$ live longer and need far more energy than classical physics predicts.

**Mind map**

```text
Deep Inside the Atom
  Quarks
    up, down, charm, strange, top, bottom
    +2/3 or −1/3
    Never alone
  Leptons
    Electron, muon, tau
    Three neutrinos
  Bosons
    Photon: electromagnetic
    Gluon: strong
    W and Z: weak
    Higgs: mass
  Hadrons
    Baryons: 3 quarks
    Proton uud, neutron udd
    Mesons: quark + antiquark
  Accelerators
    E fields speed up
    B fields steer
    E = mc² makes new particles
```

### Year 11: Why so much energy?
To make a heavy particle you need at least its rest energy, $mc^2$. The Higgs boson has a rest energy of about 125 GeV, more than 130 times a proton's. That is why the Large Hadron Collider had to accelerate protons to energies of several **tera-electron volts** (TeV, $10^{12}$ eV) before the Higgs could be produced.

### Year 12: The strong force and conservation rules
**The strong force** holds quarks together by exchanging gluons. Between protons and neutrons in a nucleus, a leftover part of the strong force binds the nucleus. It is strongly **attractive** at about 1 femtometre ($10^{-15}$ m), becomes **repulsive** at much shorter distances (stopping the nucleus collapsing), and is almost **zero** beyond about 3 fm. This short range is why very large nuclei become unstable: protons on opposite sides repel electrically but are too far apart to attract by the strong force.

**Conservation rules.** In every particle reaction, **charge** is conserved, and so is the number of baryons (quarks count as $\frac{1}{3}$ each) and the number of leptons (antileptons count as −1). In beta minus decay, $n \rightarrow p + e^- + \bar{\nu}$: charge $0 = +1 - 1 + 0$, lepton number $0 = 0 + 1 - 1$.

**Energy and mass in particle units.** $m = \dfrac{E}{c^2}$: 125 GeV is $125 \times 10^9 \times 1.602 \times 10^{-19}$ J, so $m = 2.2 \times 10^{-25}$ kg. An electron and positron at rest annihilate into **two** gamma photons of 0.511 MeV each, going opposite ways so momentum stays zero. To make an electron-positron pair, a gamma ray needs at least $2 \times 0.511 = 1.022$ MeV.

**Energy from an accelerating voltage.** A proton through 90.0 kV gains 90.0 keV, that is $1.44 \times 10^{-14}$ J.

**Charges and types.** Add the quark charges: sigma-plus, uus, has $\frac{2}{3} + \frac{2}{3} - \frac{1}{3} = +1$. Any particle made of three quarks is a baryon.

**Steering fast particles.** In the ring, $r = \dfrac{p}{qB}$. As the particles gain momentum, $B$ must rise to keep the same radius. At very high energy $p \approx \dfrac{E}{c}$, so 6.5 TeV protons in a ring of radius 2800 m need $B = \dfrac{p}{qr} \approx 7.7$ T.

**Relativistic momentum.** $p = \gamma m_0 v$. At $0.999998c$, $\gamma = 500$, so a proton's momentum is 500 times its classical value. Going the other way, a particle with momentum $p$ at $0.60c$ ($\gamma = 1.25$) has rest mass $m_0 = \dfrac{p}{\gamma v}$.

**Why quarks never appear alone.** The strong force between quarks does not weaken as they are pulled apart. The energy put in becomes a new quark-antiquark pair, so you always get hadrons, never a single quark.

## Quick summary

- Protons and neutrons are made of quarks: proton uud, neutron udd.
- The Standard Model has six quarks, six leptons, force-carrying bosons (photon, gluon, W, Z) and the Higgs boson.
- Baryons have three quarks. Mesons have a quark and an antiquark.
- In beta minus decay, a down quark becomes an up quark through the weak force.
- Accelerators use electric fields to speed particles up and magnetic fields to steer them. Collisions make new particles through $E = mc^2$, testing the Standard Model.

## Quiz

Try these questions from the chapter quiz.

1. Protons and neutrons are made of:
   - A. Electrons
   - B. Quarks
   - C. Photons
   - D. Neutrinos
   - Answer: B. Quarks. Each is made of three quarks.

2. The quarks in a proton are:
   - A. up, up, down
   - B. up, down, down
   - C. down, down, down
   - D. up, up, up
   - Answer: A. up, up, down. uud gives charge +2/3 + 2/3 − 1/3 = +1.

3. The quarks in a neutron are:
   - A. up, up, down
   - B. up, down, down
   - C. up, up, up
   - D. strange, up, down
   - Answer: B. up, down, down. udd gives charge +2/3 − 1/3 − 1/3 = 0.

The whole chapter quiz is on the page, for signed-in students: https://whizznotes.com/physics/deep-inside-the-atom/#quiz