Particle Physics and the Standard Model
Fundamental particles
The Standard Model classifies particles into two families: hadrons (made of quarks) and leptons (fundamental, not made of quarks).
Hadrons
Particles that feel the strong nuclear force. Made of quarks; split into:
- Baryons — three quarks (e.g. protons uud, neutrons udd). The proton is the only stable baryon.
- Mesons — a quark and an antiquark (e.g. pions, kaons).
Leptons
Fundamental particles that don't feel the strong force: the electron, muon and their neutrinos (plus antiparticles). Leptons are conserved in interactions (lepton number).
Quarks
The building blocks of hadrons. The three you need: up (u), down (d), strange (s), with charges +⅔, −⅓, −⅓ respectively. Each has an antiquark. Quarks are never found alone (confinement). Strangeness is conserved in strong interactions but can change in weak interactions.
Antiparticles
Every particle has an antiparticle with the same mass but opposite charge (e.g. positron = anti-electron). When a particle meets its antiparticle they annihilate, producing energy (photons). The reverse, pair production, creates a particle–antiparticle pair from a high-energy photon.
The four fundamental forces and exchange particles
Forces act by exchanging gauge bosons:
| Force | Exchange particle | Acts on |
|---|---|---|
| Strong | gluon (and pions between nucleons) | quarks/hadrons |
| Electromagnetic | photon | charged particles |
| Weak | W⁺, W⁻, Z bosons | all particles (changes quark type) |
| Gravity | (graviton, hypothetical) | mass |
Beta decay is a weak interaction: e.g. β⁻ decay is a down quark → up quark, emitting a W⁻ boson that decays into an electron and antineutrino.
Conservation laws
In any interaction, these are conserved: charge, baryon number, lepton number (and strangeness in strong interactions). Checking these predicts whether a reaction can occur.
Worked example
A neutron (udd) decays into a proton (uud) in β⁻ decay. What quark change occurs and what particle is emitted?
- A down quark changes to an up quark (d → u), emitting a W⁻ boson which decays to an electron + antineutrino — a weak interaction. ✓
Common mistakes
- Confusing baryons (3 quarks) with mesons (quark + antiquark).
- Forgetting leptons are fundamental (not made of quarks).
- Not checking conservation laws (charge, baryon, lepton number) when testing a reaction.
Exam tips
- Learn the hadron/lepton classification and quark compositions (proton uud, neutron udd).
- Know the four forces and their exchange particles.
- Apply conservation of charge, baryon and lepton number to interactions.
Key facts to remember
- Hadrons (feel strong force): baryons = 3 quarks (proton uud, neutron udd), mesons = quark + antiquark; leptons are fundamental (electron, muon, neutrinos).
- Quarks u/d/s (charges +⅔/−⅓/−⅓); antiparticles annihilate/pair-produce; forces via exchange particles (photon, W/Z, gluon).
- β⁻ decay = d → u quark + W⁻; conserve charge, baryon and lepton number.