Describe the Standard Model of particles, including quarks, leptons and the fundamental forces

What this dot point is asking

WACE wants you to describe the classification of fundamental particles, explain how quarks make up familiar particles, and name the fundamental forces and their carriers. This is the modern picture of what matter is made of.

Quarks and leptons

Fundamental matter particles divide into quarks and leptons, each coming in six types arranged in three generations. Quarks (up, down, charm, strange, top, bottom) carry fractional charge and feel the strong force. Leptons (the electron, muon and tau, each with a matching neutrino) do not feel the strong force; the charged ones feel electromagnetism and all feel the weak force. Everyday matter uses only the first generation: up and down quarks and the electron.

Building protons and neutrons

Quarks are never found alone; they combine to form composite particles. A proton is two up quarks and one down quark (uud), giving a charge of $+\tfrac{2}{3}+\tfrac{2}{3}-\tfrac{1}{3}=+1$. A neutron is one up and two down quarks (udd), giving $+\tfrac{2}{3}-\tfrac{1}{3}-\tfrac{1}{3}=0$. Particles made of three quarks are baryons; a quark and an antiquark together form a meson.

The fundamental forces

Four fundamental interactions govern all of physics. The strong force binds quarks into nucleons and nucleons into nuclei, acting only over nuclear distances. The electromagnetic force acts between charges over unlimited range. The weak force is responsible for beta decay and changing one type of quark into another. Gravity, by far the weakest, is not part of the Standard Model but acts on all mass and energy.

Force carriers

In the Standard Model forces act by exchanging carrier particles (gauge bosons). The photon carries the electromagnetic force, gluons carry the strong force between quarks, and the W and Z bosons carry the weak force. The Higgs boson, confirmed in 2012, is associated with how particles acquire mass. Gravity's hypothetical carrier, the graviton, has not been observed.

Antimatter

Every particle has a corresponding antiparticle with the same mass but opposite charge and other opposite quantum properties; the positron is the electron's antiparticle. When a particle meets its antiparticle they annihilate, converting their mass entirely into energy as photons, a direct demonstration of mass-energy equivalence.

Naming carriers and forces

When asked about a force, name both the force and its carrier (for example the strong force carried by gluons). When asked about a composite particle, give its quark content and check the charges add to the known value. Keep quarks and leptons in separate categories.