Particle physics

Revision notes

Particle physics uses a small number of fundamental particle types to describe a much larger family of observed particles. Quarks carry fractional electric charge and combine to form hadrons. Baryons contain three quarks, while antibaryons contain three antiquarks; mesons contain a quark-antiquark pair. Protons and neutrons are baryons. Leptons, such as the electron and neutrinos, are not made of quarks in the standard model used at this level.

Every particle has an antiparticle with the same mass and opposite values of additive quantum numbers such as electric charge where applicable. Particle reactions must obey conservation laws. Electric charge is conserved. Baryon number is +1 for baryons, -1 for antibaryons and 0 for non-baryons. Electron-family lepton number, and other lepton-family numbers where used, are conserved across reactions when particles and antiparticles are counted with opposite signs.

Mass-energy can be converted into other forms provided total energy and momentum are conserved. Pair production creates a particle-antiparticle pair when sufficient energy and momentum conditions are met; annihilation converts particle-antiparticle rest energy and kinetic energy into other particles such as photons. Accelerators give charged particles high energies and detectors infer particle properties from tracks, curvature, deposited energy and decay products.

Particle families

The standard school-level particle model divides matter particles into quarks and leptons. Quarks experience the strong interaction and combine to form hadrons. Baryons contain three quarks; antibaryons contain three antiquarks. Mesons contain a quark-antiquark pair.

Leptons, including electrons and neutrinos, are not made from quarks in the model. Each particle has an antiparticle with the same mass and opposite relevant additive quantum numbers.

Quark structure and charge

Up-type quarks carry charge +\(\dfrac{2}{3}e\) and down-type quarks carry -\(\dfrac{1}{3}e\). Combining quark charges reproduces hadron charge. For example, proton uud has charge +e and neutron udd has charge zero.

Quark composition is a powerful check on proposed reactions but must be used alongside conservation laws.

Conservation laws

Allowed reactions conserve energy, momentum, electric charge and relevant quantum numbers such as baryon number and lepton number. A reaction that conserves charge but violates baryon number or lepton number is not allowed in the simple model.

Apply conservation law by law rather than relying on whether the reaction 'looks plausible'.

Beta decay

In beta-minus decay, a neutron changes to a proton at quark level when a down quark changes to an up quark, accompanied by electron and antineutrino emission. In beta-plus decay an up quark changes to a down quark, turning a proton into a neutron, and a positron and an electron neutrino are emitted.

Neutrinos are essential for conservation of energy, momentum and lepton number in beta processes.

Annihilation and pair production

A particle and antiparticle can annihilate, converting rest energy and kinetic energy into photons or other allowed products. Pair production converts photon energy into particle-antiparticle rest energy and kinetic energy, subject to conservation laws. A nearby nucleus must take part so that momentum is conserved, which is why a single photon cannot produce a pair in empty space.

Exam technique

Create a small conservation table for charge, baryon number and lepton number before and after a reaction. Keep particle symbols and antiparticle signs precise. Do not assume charge conservation alone is sufficient.