Describe the Standard Model's particles, the four fundamental forces and their carriers.

What this dot point is asking

The Standard Model is the best current theory of the fundamental particles and the forces between them. It is the culmination of Unit 4's exploration of modern physics.

Fermions: the matter particles

All ordinary matter is built from two families of fermions:

• Quarks: six types (up, down, charm, strange, top, bottom). They carry fractional electric charge and feel the strong force. Up-type quarks carry charge $+\tfrac{2}{3}$ and down-type carry $-\tfrac{1}{3}$.
• Leptons: six types, including the electron, muon and tau, each with a matching neutrino. The electron carries charge $-1$; neutrinos are neutral.

Each fermion also has a corresponding antiparticle with the opposite charge but the same mass.

Bosons: the force carriers

In the Standard Model, forces act through the exchange of particles called gauge bosons:

• The photon carries the electromagnetic force, acting between charged particles.
• Gluons carry the strong force, binding quarks together inside protons and neutrons.
• The W and Z bosons carry the weak force, responsible for beta decay and other particle transformations.

The Higgs boson, confirmed in 2012, is associated with the field that gives many particles their mass.

The four fundamental forces

There are four fundamental interactions in nature:

1. The strong force: the strongest, but very short range; holds nuclei and quarks together.
2. The electromagnetic force: acts between charges, infinite range, much weaker than the strong force.
3. The weak force: short range, governs certain decays such as beta decay.
4. Gravity: by far the weakest, infinite range, always attractive, and not yet part of the Standard Model.

How it fits together

Beta-minus decay is a neat illustration. A down quark inside a neutron changes into an up quark, turning the neutron (udd) into a proton (uud). This change is mediated by the weak force through a W boson, which then decays into an electron and an antineutrino. The Standard Model thus links nuclear decay to the deeper level of quarks and force carriers.

For exam answers, be able to classify a given particle as a quark, lepton or boson, name the force each boson carries, and rank the four fundamental forces by relative strength and range. Remember that gravity, although it dominates on astronomical scales, is the weakest force and remains outside the Standard Model.