The Standard Model of Elementary Particles

What is everything made of?

In Grade 9 you learned that atoms contain protons, neutrons and electrons. Through the twentieth century, physicists built accelerators to smash particles together and discovered hundreds of new, short-lived particles: a confusing "particle zoo." In the 1960s Murray Gell-Mann and George Zweig independently proposed that protons, neutrons and most of the zoo are made of a few simpler pieces, now called quarks. Experiments at the Stanford Linear Accelerator (late 1960s) found point-like objects inside protons, as Rutherford had found the nucleus inside the atom.

The theory that grew from this, completed in the 1970s, is the Standard Model. It is a quantum theory: every particle is a quantum of a field, and forces are carried by exchanging quanta (bosons), as you saw in the lesson Comparing Fields and the Fundamental Forces in the unit Gravitational, Electric and Magnetic Fields. It is one of the most precisely tested theories in science.

Matter particles: quarks and leptons

Quarks come in six types ("flavours"). Up, charm and top have charge +2/3 e; down, strange and bottom have −1/3 e. Quarks are never found alone: the strong force, carried by gluons, holds them inside hadrons:

• Baryons contain three quarks. The proton is uud; the neutron is udd.
• Mesons contain a quark and an antiquark, such as the pion π⁺ = u d̄ (a bar means antiquark, with opposite charge: d̄ has +1/3 e).

Leptons do not feel the strong force. They are the electron, its heavier cousins the muon and tau, and three nearly massless, neutral neutrinos.

Each column in the chart is a generation. Everyday matter uses only the first generation (up, down, electron, electron neutrino); the heavier ones appear in cosmic rays and accelerators and decay quickly.

Every particle has an antiparticle. You met the positron in lesson 7; antiquarks make up antimatter versions of hadrons.

Conservation laws decide what can happen

In every particle reaction observed, some quantities are always conserved:

• Electric charge.
• Baryon number (B): each quark counts +1/3 and each antiquark −1/3. Protons and neutrons have B = +1; mesons have B = 0; leptons have B = 0.
• Lepton number (L): each lepton counts +1 and each antilepton −1 (counted separately for the electron, muon and tau families).
• Energy and momentum, as always.

If a proposed reaction breaks any of these, it does not happen. This is how physicists check reactions, and it is the same logic Pauli used to predict the neutrino (see the lesson on the neutrino in Momentum and Collisions).

Evidence and Canadian contributions

• Neutrinos: at SNOLAB, 2 km underground in the Creighton Mine near Sudbury, Ontario, the Sudbury Neutrino Observatory used 1000 tonnes of heavy water to detect neutrinos from the Sun. It showed that neutrinos change from one type to another as they travel, which means they have mass. Arthur McDonald of Queen's University shared the 2015 Nobel Prize in Physics with Takaaki Kajita (Japan) for this discovery. In 2026, the Nobel Prize in Physics went to Francis Halzen for leading work at the IceCube Neutrino Observatory, a cubic kilometre of Antarctic ice wired with light detectors, which discovered high-energy neutrinos coming from deep space.
• The Higgs boson: predicted in 1964 and discovered at CERN's Large Hadron Collider in 2012, with Canadian physicists among the thousands in the ATLAS collaboration and TRIUMF (Canada's particle accelerator centre in Vancouver) contributing. The 2013 Nobel Prize went to François Englert and Peter Higgs for the prediction.
• Every quark and lepton in the chart has been observed; the top quark (1995), the tau neutrino (2000) and the Higgs (2012) were the last pieces.