Particle physics

Learning objectives

• Distinguish leptons, baryons and mesons.
• Describe quark composition of nucleons.
• Conserve charge in particle processes.

The big idea

Quarks combine into hadrons: baryons contain three quarks and mesons contain a quark–antiquark pair. Leptons, including electrons and neutrinos, are not made of quarks in this model.

A proton contains two up quarks and one down quark; a neutron contains one up and two down. In beta decay the changes must conserve electric charge and the relevant particle numbers, not just the number of visible charged particles.

1. Scattering reveals concentration of matter

Most alpha particles passing through a thin foil are only weakly deflected, while a small fraction turn through large angles. This supports a tiny concentrated positive nucleus within an otherwise largely empty atom. The observation does not mean electrons and nuclei are classical hard balls at the displayed simulation scale. Distinguish evidence about atomic arrangement from the later model of particles inside the nucleus.

2. Families organise particles

Hadrons are composite particles containing quarks: baryons contain three quarks and mesons contain a quark–antiquark pair. Leptons, such as electrons and neutrinos, are not hadrons. Up quarks have charge +2e/3 and down quarks −e/3. A proton is uud with total charge +e; a neutron is udd with total zero. Zero net charge does not mean a neutron has no charged constituents.

3. Antiparticles are not simply negative particles

An antiparticle has the same mass as its particle partner and opposite relevant additive quantum numbers, including electric charge when non-zero. The positron is the electron’s antiparticle. In pair production, available photon energy can become particle and antiparticle rest energy and kinetic energy under conditions that conserve momentum, commonly involving a nearby nucleus. In annihilation, particle and antiparticle energy can emerge as photons while total energy and momentum remain conserved.

4. Beta decay changes quark identity

In beta-minus decay, a neutron can become a proton while an electron and electron antineutrino are emitted. At quark level, a down quark changes into an up quark. The electron is produced by the interaction, not taken from an atomic orbit. Check charge, baryon number and lepton number on both sides. Invisible neutrinos are needed for consistent conservation accounting; a missing visible track does not establish that a process is impossible.

A method you can reuse

1. Identify whether each particle is a lepton or hadron.
2. Write constituent quark charges where relevant.
3. Add signed electric charges before and after the process.
4. Check baryon and lepton-number conservation as well as energy.
5. Distinguish an atomic electron from an electron created in nuclear decay.

Equations and relationships

Investigate and explain

Use labelled cards for particles and antiparticles to balance charge in proposed decay equations. No physical particle source is needed.

These are related scattering, nuclear-decay and wave models. They do not show the complete quark/lepton classification, weak-interaction mechanism or detector behaviour, so use diagrams and conservation reasoning too.

Open the interactive investigation

Check your understanding