Quarks

Smaller than small

Everything is made of atoms. Atoms are made of protons, neutrons and electrons. But are protons and neutrons the smallest pieces? No! They are made of even tinier particles called quarks.

The discovery of quarks

In 1964, physicists Murray Gell-Mann and George Zweig independently proposed that protons and neutrons are made of smaller parts. Gell-Mann named them "quarks," borrowing a word from James Joyce’s novel Finnegans Wake.

In 1968, experiments at the Stanford Linear Accelerator Center fired electrons at protons. Some bounced off as if they had hit small, hard points inside, the first evidence that quarks are real. Canadian-born physicist Richard Taylor shared the 1990 Nobel Prize for this work.

Six flavours of quarks

There are six types of quarks, called flavours:

Only up and down quarks are found in ordinary matter. The others are heavier and appear briefly in high-energy collisions in particle accelerators and cosmic rays. The top quark is about as heavy as a whole gold atom!

Quark charges are fractions of an electron’s charge, which is very unusual.

Building protons and neutrons

Particles made of quarks are called hadrons. There are two kinds:

• Baryons: three quarks (protons, neutrons)
• Mesons: one quark and one antiquark (pions, kaons)

The strong force and gluons

What holds quarks together? The strong force, the strongest of the four fundamental forces. It’s carried by particles called gluons (because they "glue" quarks together).

Strangely, the strong force gets stronger as quarks move apart, like a rubber band. If you try to pull a quark out, the energy you add creates a new quark–antiquark pair instead. That’s why quarks are never found alone. This is called confinement.

Quarks also have a property called colour charge (red, green or blue). It has nothing to do with real colours; it’s just a name for how they interact through the strong force.

Most of a proton’s mass doesn’t come from its quarks. It comes from the enormous energy of the gluons and quarks moving inside it, following E = mc².

The Standard Model

The Standard Model is the theory that describes all known fundamental particles, those not made of anything smaller.

Electrons are leptons, not quarks. As far as we know, they’re fundamental.

The Standard Model is extremely successful, but it’s incomplete. It doesn’t include gravity, and it doesn’t explain dark matter.

How scientists study quarks

Particle accelerators smash particles together at nearly the speed of light. The Large Hadron Collider at CERN, near Geneva, is a ring 27 km around, the largest machine ever built. Detectors record the shower of new particles produced. TRIUMF in Vancouver is Canada’s particle accelerator centre and contributes to these experiments.

Common mistakes

• Thinking protons and neutrons are fundamental.
• Thinking electrons are made of quarks.
• Thinking quark "colour" is visible colour.

Try it yourself

1. Show that two up quarks and one down quark give a charge of +1.
2. What would be the charge of a particle made of up, up and up quarks? (This particle, the delta-plus-plus, really exists briefly!)
3. Build a model of a proton and neutron using three coloured beads each, labelled u or d.

(Answer to 2: +2)

Quick recap

• Protons and neutrons are made of quarks.
• There are six quark flavours; ordinary matter uses up and down.
• Protons are uud; neutrons are udd.
• Gluons carry the strong force and confine quarks.
• The Standard Model describes all known fundamental particles.