A level Physics · Edexcel 9PH0 · Topic 8C
The standard model
Key points
- The standard model classifies fundamental particles into quarks, leptons and force-carrying particles.
- Quarks: up (charge ), down (), strange (), and also charm, top and bottom. Leptons include the electron, muon, tau and their neutrinos.
- Hadrons feel the strong force: baryons are three quarks (proton uud, neutron udd) and mesons are a quark and an antiquark (e.g. pion).
- Every particle has an antiparticle with the same mass and opposite charge; a particle and its antiparticle can annihilate into photons, and pair production turns a photon into a particle–antiparticle pair.
- Interactions must conserve charge, baryon number, lepton number and energy–momentum (strangeness is conserved in strong interactions).
- In beta-minus decay a down quark becomes an up quark: n → p + e + .
- Exam tip: check each conservation law in turn to decide whether a reaction is possible.
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Hundreds of particles have been found in accelerators, but they can all be built from a small set of fundamental particles. The standard model sorts them into quarks and leptons, and a few conservation laws decide which interactions are…
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