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

Published by Patrick Mutisya · 14 days ago

Cambridge A-Level Physics 9702 – Fundamental Particles: Quarks

Fundamental Particles – Quarks

Learning Objective

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

What is a Fundamental Particle?

In the Standard Model of particle physics, a fundamental particle is an entity that is not known to be made up of smaller constituents. It is treated as point‑like with no internal structure. Examples include leptons (e.g. the electron), gauge bosons (e.g. the photon) and quarks.

Quarks as Fundamental Particles

Quarks are elementary constituents of hadrons (such as protons and neutrons). They cannot be isolated in free space because of a property called colour confinement. Nevertheless, their existence is inferred from deep‑inelastic scattering experiments and the patterns of particle decays.

Six Flavours of Quarks

Quarks come in six distinct types, called flavours. They are grouped into three generations, each containing an up‑type quark (charge \$+\tfrac{2}{3}e\$) and a down‑type quark (charge \$-\tfrac{1}{3}e\$).

FlavourSymbolElectric ChargeGenerationApprox. Mass (MeV/\$c^2\$)
Upu\$+\tfrac{2}{3}e\$I≈ 2.2
Downd\$-\tfrac{1}{3}e\$I≈ 4.7
Stranges\$-\tfrac{1}{3}e\$II≈ 96
Charmc\$+\tfrac{2}{3}e\$II≈ 1 280
Bottomb\$-\tfrac{1}{3}e\$III≈ 4 180
Topt\$+\tfrac{2}{3}e\$III≈ 173 100

Key Points to Remember

  • Quarks are elementary; they have no known sub‑structure.

  • There are exactly six flavours: up, down, strange, charm, bottom, and top.

  • Flavours are paired in generations: (up, down), (charm, strange), (top, bottom).

  • Up‑type quarks carry a charge of \$+\tfrac{2}{3}e\$; down‑type quarks carry \$-\tfrac{1}{3}e\$.

  • Quarks combine in groups of three (baryons) or a quark–antiquark pair (mesons) to form colour‑neutral particles.

Suggested Diagram

Suggested diagram: A schematic showing the three generations of quarks, their electric charges, and how they combine to form a proton (uud) and a neutron (udd).

Typical Examination Question

Explain why a proton is electrically neutral overall, given that it is composed of two up quarks and one down quark. Use the charges of the constituent quarks in your answer.

\$\text{Total charge} = 2\left(+\frac{2}{3}e\right) + \left(-\frac{1}{3}e\right) = +1e\$

Thus, the proton carries a net charge of \$+1e\$, while the neutron, composed of one up quark and two down quarks, has a net charge of \$0e\$.