Particles

What particles covers

What matter is made of at the smallest scale A-level reaches, and the conservation rules its constituents obey. The unit is mostly recall and careful checking rather than calculation. It supplies the annihilation physics the medical physics option later uses for PET.

The main ideas

• Proton, neutron and electron with their charges and masses, specific charge, nuclide notation and isotopes.
• The strong nuclear force, with its short attractive range and its repulsion at very small separations.
• Alpha and beta-minus decay as balanced equations, and the neutrino hypothesised from the beta energy spectrum.
• Antiparticles, annihilation and pair production, including the threshold energy and the role of a nearby nucleus.
• The four interactions and their exchange particles, with diagrams for beta decay of both signs, electron capture and electron-proton collisions.
• Hadrons and leptons classified, with baryon number, the two lepton numbers and strangeness.
• Quark and antiquark compositions built from the data table, and decays taken apart as a change of flavour.

Key equations

specific charge = Qm$\text{specific charge} = \frac{Q}{m}$specific charge = Q/m

charge per kilogram, for a particle, nucleus or ion

E = hf = hcλ$E = hf = \frac{hc}{\lambda}$E = hf = hc/λ

photon energy, usually converted to or from MeV here

minimum photon energy = the rest energy of the particle$\text{minimum photon energy} = \text{the rest energy of the particle}$minimum photon energy = the rest energy of the particle

annihilation of a slow particle and antiparticle

threshold energy = twice the rest energy$\text{threshold energy} = \text{twice the rest energy}$threshold energy = twice the rest energy

pair production from one photon

charge, baryon number, lepton number and strangeness equal before and after

the audit any proposed interaction must pass

Every equation the particles notes display is collected, with its conditions, on the particles equations page.

Common errors

• Lepton number is counted separately for the electron and muon families, so an equation can balance on the total and still fail.
• Strangeness is conserved exactly by the strong interaction but may change by one in a weak one, which is often the evidence a question wants.
• Choosing between the W-plus and the W-minus is settled by balancing charge at the vertex where it is emitted, not by which particle decays.
• An interaction can satisfy every quantum number and still be forbidden, because energy and momentum must also work out. The standard example is a free proton, which cannot decay to a neutron because the neutron is heavier.

Suggested order

Constituents of the atom first, then stable and unstable nuclei. Antimatter next, since annihilation and pair production are needed for the interaction diagrams. Classification and quarks follow, and conservation laws last, because that lesson turns the unit into one checking procedure.