understand that a quark is a fundamental particle and that there are six flavours (types) of quark: up, down, strange, charm, top and bottom

Fundamental Particles – Quarks

Learning Objective

Students will be able to:

• Identify quarks as elementary (point‑like) particles.
• Recall the six quark flavours, their electric charges and generation grouping.
• Explain why protons and neutrons are not fundamental and describe their quark composition.
• Describe how quark flavour changes in β⁻ and β⁺ decay.
• Distinguish quarks from leptons (electron, neutrino) and recognise the role of the weak interaction.

What is a Fundamental Particle?

In the Standard Model a fundamental particle has no known sub‑structure; it is treated as point‑like. The three families are:

• Leptons (e⁻, μ⁻, τ⁻ and their neutrinos)
• Gauge bosons (γ, g, W⁺, W⁻, Z⁰)
• Quarks (up, down, …)

Quarks as Fundamental Particles

Quarks are the elementary constituents of hadrons (baryons and mesons). They are never seen free because of colour confinement: the strong force, carried by gluons, binds quarks into colour‑neutral combinations.

Intrinsic Properties of Quarks

• Spin: ½ ℏ (fermions)
• Colour charge: one of three “colours’’ – red, green, blue (anti‑quarks carry the corresponding anti‑colours).
• Electric charge: +2/3 e for up‑type, –1/3 e for down‑type.
• Mass: varies strongly between flavours (see table).

Anti‑quarks

Each quark has an anti‑quark with:

• Opposite electric charge.
• Opposite colour (anti‑red, anti‑green, anti‑blue).
• Same spin magnitude (½ ℏ).

Anti‑quarks combine with quarks to form mesons (q anti‑q) which are also colour‑neutral.

Six Flavours (Types) of Quarks

Quarks are grouped into three generations; each generation contains an up‑type (+2/3 e) and a down‑type (–1/3 e) quark.

Quarks in Hadrons

• Baryons: three quarks, one of each colour → colour neutral.

  • Proton (uud) Charge: +1 e
  • Neutron (udd) Charge: 0 e

• Mesons: quark + anti‑quark of complementary colour → colour neutral.

  • π⁺ (u anti‑d) Charge: +1 e
  • K⁰ (d anti‑s) Charge: 0 e

Quarks and the Weak Interaction

Flavour change occurs via the weak force (exchange of W⁺/W⁻ bosons). This underlies β‑decay.

• β⁻ decay (neutron → proton): d → u + e⁻ + ν̅ₑ
• β⁺ decay (proton → neutron): u → d + e⁺ + νₑ
• Higher‑generation examples: c → s + W⁺, t → b + W⁺ (subsequent W decay gives leptons or quarks).

Quarks vs. Leptons

Electrons and neutrinos are therefore not part of the proton or neutron structure; they appear only in weak‑interaction processes.

Key Points to Remember

• Quarks are elementary, spin‑½ fermions with no known sub‑structure.
• Six flavours exist: up, down, strange, charm, bottom, top.
• Flavours are paired in three generations: (up, down), (charm, strange), (top, bottom).
• Up‑type quarks: +2/3 e; down‑type quarks: –1/3 e.
• Each quark carries a colour charge; anti‑quarks carry the corresponding anti‑colour.
• Colour confinement forces quarks to combine into colour‑neutral hadrons (qqq or q anti‑q).
• Weak interactions can change a quark’s flavour, giving rise to β⁻ and β⁺ decay.
• Electrons and neutrinos are leptons, not components of nucleons.

Suggested Diagram

Typical Examination Question

Q: Explain, using quark composition, why a proton has a net charge of +1 e while a neutron is electrically neutral. Include the role of the weak interaction in β⁻ decay of a neutron.

A:

1. Proton = uud → 2(+2/3 e) + (–1/3 e) = +1 e.
2. Neutron = udd → (+2/3 e) + 2(–1/3 e) = 0 e.
3. In β⁻ decay a down quark in the neutron changes to an up quark by emitting a W⁻ boson: d → u + W⁻. The W⁻ subsequently decays into an electron and an electron‑antineutrino (W⁻ → e⁻ + ν̅ₑ). The resulting up quark makes the nucleon a proton.