QCE Physics - Unit 4 - The Standard Model
The Standard Model | QCE Physics
Learn the QCE Physics Standard Model topic, including elementary particles, antiparticles, quarks, leptons, baryons, mesons and gauge bosons.
Updated 2026-06-15 - 4 min read
QCAA official coverage - Physics 2025 v1.3
Exact syllabus points covered
- Describe the concepts of elementary particles and antiparticles.
- Identify the six types of quarks.
- Describe baryons and mesons.
- Identify the six types of leptons.
- Identify the four gauge bosons.
- Compare the strong nuclear, weak nuclear and electromagnetic forces in terms of the gauge bosons.
- Contrast the fundamental forces experienced by quarks and leptons.
The Standard Model is the framework QCE Physics uses to classify fundamental particles and the interactions between them. It does not include gravity as a quantum force, but it is extremely successful for electromagnetic, weak and strong interactions.
Original Sylligence diagram for physics standard model table.
Elementary particles and antiparticles
An elementary particle is treated as not being made from smaller components in the Standard Model. Each matter particle has an antiparticle with the same mass and opposite charge-like properties. For example, the electron's antiparticle is the positron.
When a particle and its corresponding antiparticle interact, annihilation can occur, converting mass-energy into other particles or photons while conserving quantities such as charge, energy and momentum.
Do not confuse elementary particles with composite particles. Protons and neutrons are not elementary in the Standard Model because they are made of quarks.
Quarks
The six quark flavours are:
| Generation | Quarks | | --- | --- | | 1 | up, down | | 2 | charm, strange | | 3 | top, bottom |
Quarks have fractional electric charge. Up, charm and top have charge $+\frac{2}{3}e$. Down, strange and bottom have charge $-\frac{1}{3}e$.
Quarks are affected by the strong nuclear force, the weak nuclear force and the electromagnetic force if charged. They are not observed as isolated particles under normal conditions because of confinement; they appear inside composite particles called hadrons.
Baryons and mesons
Hadrons are composite particles made of quarks. The two main groups in this syllabus are baryons and mesons.
Baryons contain three quarks. Protons and neutrons are baryons:
- proton: $uud$
- neutron: $udd$
Mesons contain a quark and an antiquark. They are usually unstable and appear in particle interactions and decay processes.
Leptons
The six leptons are:
| Generation | Charged lepton | Neutrino | | --- | --- | --- | | 1 | electron | electron neutrino | | 2 | muon | muon neutrino | | 3 | tau | tau neutrino |
Charged leptons experience electromagnetic and weak interactions. Neutrinos have no electric charge, so they do not experience the electromagnetic force, but they do experience the weak interaction.
Leptons are not made of quarks. This is a common classification boundary in Standard Model questions.
Gauge bosons and forces
Gauge bosons are force carriers. The QCE syllabus names four gauge bosons:
| Interaction | Gauge boson | | --- | --- | | Electromagnetic | photon | | Strong nuclear | gluon | | Weak nuclear | $W^+$, $W^-$ and $Z^0$ bosons |
The electromagnetic force acts between electrically charged particles and is mediated by photons. The strong nuclear force acts between quarks and is mediated by gluons. The weak nuclear force acts on quarks and leptons and is mediated by $W$ and $Z$ bosons.
QCE asks you to compare strong, weak and electromagnetic forces in terms of gauge bosons. A concise comparison should include what particles experience the force, the mediator and the relative role of the force.
Quarks compared with leptons
Quarks experience all three Standard Model interactions named in this topic: strong, weak and electromagnetic if charged. Leptons do not experience the strong force. Charged leptons experience electromagnetic and weak interactions. Neutrinos experience the weak interaction and gravity, but gravity is outside the Standard Model force comparison used here.