Deep Inside the Atom

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)

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 $E = m c^{2}$ , 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.

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 12The 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 ( $1 0^{- 15}$ 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, $n \to p + e^{-} + ν ˉ$ : charge $0 = + 1 - 1 + 0$ , lepton number $0 = 0 + 1 - 1$ .

Energy and mass in particle units. $m = \frac{E}{c ^{2}}$ : 125 GeV is $125 \times 1 0^{9} \times 1.602 \times 1 0^{- 19}$ J, so $m = 2.2 \times 1 0^{- 25}$ 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 $1.44 \times 1 0^{- 14}$ 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, $r = \frac{p}{q B}$ . As the particles gain momentum, $B$ must rise to keep the same radius. At very high energy $p \approx \frac{E}{c}$ , so 6.5 TeV protons in a ring of radius 2800 m need $B = \frac{p}{q r} \approx 7.7$ T.

Relativistic momentum. $p = γ m_{0} v$ . At $0.999998 c$ , $γ = 500$ , so a proton's momentum is 500 times its classical value. Going the other way, a particle with momentum $p$ at $0.60 c$ ( $γ = 1.25$ ) has rest mass $m_{0} = \frac{p}{γ v}$ .

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 $E = m c^{2}$ , testing the Standard Model.

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Protons and neutrons are made of:
1. A Electrons
2. B Quarks
3. C Photons
4. D Neutrinos
Show answer
B: Quarks
Each is made of three quarks.
The quarks in a proton are:
1. A up, up, down
2. B up, down, down
3. C down, down, down
4. D up, up, up
Show answer
A: up, up, down
uud gives charge +2/3 + 2/3 − 1/3 = +1.
The quarks in a neutron are:
1. A up, up, down
2. B up, down, down
3. C up, up, up
4. D strange, up, down
Show answer
B: up, down, down
udd gives charge +2/3 − 1/3 − 1/3 = 0.

Smaller than protons

The Standard Model

Quarks

Leptons

Bosons: the force carriers

Antimatter

Hadrons: particles made of quarks

Beta decay, explained with quarks

Particle accelerators

Quick summary

Quiz

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