The Standard Model
Syllabus reference
Unit 4, Topic 3 — 13 hours (including practicals)
Elementary particles
An elementary particle is a particle that is not made up of smaller particles. For every particle there is a corresponding antiparticle with the same mass but opposite charge (e.g. electron and positron).
Quarks (6 types)
| Generation | Quark | Charge | Antiquark | Charge |
|---|---|---|---|---|
| 1st | Up (u) | +⅔ | Anti-up (ū) | −⅔ |
| 1st | Down (d) | −⅓ | Anti-down (d̄) | +⅓ |
| 2nd | Charm (c) | +⅔ | Anti-charm (c̄) | −⅔ |
| 2nd | Strange (s) | −⅓ | Anti-strange (s̄) | +⅓ |
| 3rd | Top (t) | +⅔ | Anti-top (t̄) | −⅔ |
| 3rd | Bottom (b) | −⅓ | Anti-bottom (b̄) | +⅓ |
Leptons (6 types)
| Generation | Lepton | Charge | Neutrino |
|---|---|---|---|
| 1st | Electron (e⁻) | −1 | Electron neutrino (νₑ) |
| 2nd | Muon (μ⁻) | −1 | Muon neutrino (ν_μ) |
| 3rd | Tau (τ⁻) | −1 | Tau neutrino (ν_τ) |
Gauge bosons (force carriers)
| Force | Boson | Acts on |
|---|---|---|
| Strong nuclear | Gluon (g) | Quarks |
| Weak nuclear | W⁺, W⁻, Z⁰ | Quarks and leptons |
| Electromagnetic | Photon (γ) | Charged particles |
| Gravitational | Graviton (hypothetical) | All particles with mass |
Hadrons
Particles made of quarks are called hadrons. There are two types:
Baryons — made of three quarks (e.g. proton = uud, neutron = udd). Baryon number = +1.
Mesons — made of a quark and an antiquark (e.g. pion π⁺ = ud̄). Baryon number = 0.
Worked example: Quark composition
Question: Verify the charge of a proton given its quark composition (uud).
Solution:
Charge = (+⅔) + (+⅔) + (−⅓) = +1 ✓
Conservation laws
In all particle interactions, the following quantities are conserved:
- Charge
- Baryon number (baryons = +1, antibaryons = −1, others = 0)
- Lepton number (leptons = +1, antileptons = −1, others = 0)
- Energy and momentum
Feynman diagrams
Feynman diagrams are visual representations of particle interactions. Time runs from left to right (or bottom to top). Key conventions:
- Straight lines with arrows = fermions (quarks, leptons)
- Wavy lines = photons
- Curly lines = gluons
- Dashed lines = W/Z bosons
Key interactions
Electron–electron interaction — two electrons exchange a virtual photon (electromagnetic repulsion).
Electron–positron annihilation — an electron and positron annihilate, producing two photons.
Beta-negative decay (neutron → proton):
At the quark level: a down quark converts to an up quark by emitting a W⁻ boson, which then decays into an electron and an electron antineutrino.
Symmetry in particle interactions
The same Feynman diagram can represent multiple processes depending on the direction of time — this is crossing symmetry. The significance of symmetry in particle physics reflects deep mathematical properties of nature.
Simulations and videos
External resources:
- CERN — The Standard Model
- Particle Central
- Quantum Diaries — Let's Draw Feynman Diagrams
- Fermilab — Top Quark Activity
- FactMyth — The Standard Model
Crash Course Physics: