The standard model of fermions (quarks and leptons) and force-mediating bosons, the classification of hadrons into baryons and mesons, and antimatter.

What this key area is asking

The SQA wants you to classify the fundamental particles of the standard model into fermions (quarks and leptons) and force-mediating bosons, describe how quarks combine to make hadrons (baryons and mesons), and explain what antimatter is.

Fermions: quarks and leptons

Bosons: the force carriers

The fundamental forces act through the exchange of bosons, sometimes called force-mediating or exchange particles. Each force has its own carrier.

• The electromagnetic force is carried by the photon.
• The strong force (which binds quarks) is carried by gluons.
• The weak force (responsible for some radioactive decay) is carried by the W and Z bosons.

The Higgs boson, discovered in 2012, is associated with the mechanism that gives particles their mass. The distinction to remember is simple: fermions are the matter, bosons carry the forces between them.

Hadrons: baryons and mesons

Quarks are never observed in isolation; the strong force confines them into composite particles called hadrons.

The charge of a hadron is the sum of its quark charges. The proton (uud) has charge $+\tfrac{2}{3}e + \tfrac{2}{3}e - \tfrac{1}{3}e = +1e$; the neutron (udd) has charge $+\tfrac{2}{3}e - \tfrac{1}{3}e - \tfrac{1}{3}e = 0$.

Antimatter

Every particle has a corresponding antiparticle with the same mass but the opposite charge (and opposite other quantum numbers). The antiparticle of the electron is the positron ($+1e$); the antiproton has charge $-1e$. When a particle meets its antiparticle they annihilate, converting their mass entirely into energy, usually a pair of photons, in line with $E = mc^2$.

Examples in context

PET scanners (positron emission tomography) in hospitals rely on antimatter: a radioactive tracer emits positrons that annihilate with electrons in the body, producing pairs of photons that the scanner detects to build an image. The Large Hadron Collider at CERN smashes protons together at high energy to create and study short-lived particles, and it confirmed the Higgs boson. The Sun produces neutrinos (leptons) by the billions in its fusion reactions, some of which pass through the Earth and are caught in deep underground detectors. The standard model underpins all of this: it is the most thoroughly tested theory of fundamental particles we have.

Try this

Q1. State the two families of fermions in the standard model. [1 mark]

• Cue. Quarks and leptons.

Q2. State how many quarks make up a baryon and how many make up a meson. [2 marks]

• Cue. A baryon has three quarks; a meson has one quark and one antiquark.

Q3. Describe what happens when a particle meets its antiparticle. [1 mark]

• Cue. They annihilate, converting their mass into energy (usually photons).